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abnormality: a condition that is not usual or typical.

abortion: a procedure where a pregnancy is ended by any means. The abortion usually occurs before the fetus is able to live independently.

absorption: a process in which a cell, tissue or organ takes up a substance. Compare secretion.

acetylcholine: a substance that carries signals between nerves.

acetylcholinesterase: the enzyme that breaks down the neurotransmitter acetylcholine at synapses.

acidosis: a condition where there is too much acid in the body.

acquired immune deficiency syndrome (AIDS): the final stage of infection that occurs when an individual is infected with the human immunodeficiency virus (HIV). The infected individual is more likely to develop opportunistic infections and malignancies.

acquired immunity: a state where the body is able to fight disease after it is exposed to antigens. The antigens then stimulate the body to produce antibodies.

action potential: the changes in electrical membrane potential that occur in muscle or nerve cells when they are stimulated.

active immunity: a type of acquired immunity where the body produces antibodies after infection or vaccination.

active ingredient: the active agent in a drug that is responsible for its medicinal properties.

active transport: a process where a molecule moves up an electrochemical gradient. This requires energy. Compare passive transport.

adaptive immunity: a type of immune response. It is a specific reaction to a particular antigen and becomes stronger when the individual is exposed to the antigen again. Compare innate immunity.

adenine: one of the bases. It contains nitrogen and participates in the formation of the nucleic acids DNA and RNA in animal and plant cells. It is often abbreviated to A.

adenosine 3', 5' cyclic monophosphate (cAMP): a molecule that is formed by the enzyme adenylate cyclase from adenosine triphosphate. It is important in controlling mechanisms within a cell.

adenosine triphosphate (ATP): an important source of cellular energy. Energy is stored in the chemical bonds of the three phosphate groups. It is released when ATP is split into its diphosphate and monophosphate forms, e.g. during muscle contraction.

adenylate cyclase: the enzyme that changes adenosine triphosphate (ATP) into adenosine 3', 5' cyclic monophosphate (cAMP). It has an important role in controlling mechanisms within a cell.

adrenal glands: a pair of glands above each kidney. The cortex of the glands secretes the stress hormones adrenaline and noradrenaline, and steroid hormones.

adrenaline: a hormone that is released by the adrenal medulla and acts on the alpha and beta adrenergic receptors. Adrenaline makes the heart beat faster. It also makes the bronchi open and reduces the flow of the blood to the arms and legs. It is also known as epinephrine.

adult haemoglobin (HbA): the main haemoglobin in children and adults. It is the most important haemoglobin produced after birth and throughout adult life. It is made up of two alpha globin chains and two beta globin chains.

aerobic: requiring oxygen to live and grow.

agarose gel electrophoresis: a method that is used to separate DNA fragments by molecular weight. Fragments move by electric current at different speeds through the agarose.

agonist: a molecule or drug that combines with a receptor and produces a biological response. Compare antagonist.

albumin: a protein that is found in the blood.

alimentary canal: the tube that begins at the mouth and ends at the anus. It is where food is taken in and digested. In vertebrates, it consists of the oesophagus, stomach and intestines.

alkali: a substance that contains hydroxide ions in solution. Alkaline solutions have a pH greater than 7 and turn litmus paper blue.

allergen: a substance that causes an allergic reaction in humans. It is usually a protein whose presence stimulates the body to form IgE antibodies. Common allergens are pollen, house dust and animal hairs.

allergy: a condition where an individual is hypersensitive to an allergen. It results in tissue damage.

allograft: a procedure where a graft is provided by one person and donated to another person.

alpha (α) globin: a protein chain that forms part of normal adult haemoglobin, haemoglobin Gower II, fetal haemoglobin, and haemoglobin A2. It is produced at low levels during early fetal life, and at increased levels during adulthood.

alpha (α) thalassaemia: a hereditary blood disease in which the production of alpha globin chains in haemoglobin is reduced.

amino acid: a compound that contains both an acidic group and an amino group. There are 20 biologically important amino acids that are present in proteins.

amnesia: a condition that affects an individual's ability to recall events, people, places etc.

amniocentesis: a procedure in which a needle is inserted into the amniotic fluid around the embryo in the womb. The needle is used either to remove a sample of amniotic fluid or to inject a substance.

amniotic fluid: the liquid that surrounds the embryo of reptiles, birds and mammals. The liquid is contained in a cavity or sac, which is lined with membrane.

amplification: in molecular biology, a process where a gene or fragment of DNA is duplicated many times.

amylase: an enzyme that breaks starch down into glycogen and other glucans.

amyloid: a complex protein substance that builds up in tissues in amyloidosis.

anabolism: the processes within a cell or organism by which simple substances are built up into more complex substances. Compare catabolism.

anaemia: a condition where there are fewer red cells in the blood or a lower haemoglobin concentration than normal.

animal model: a model of disease processes or drug effects in a laboratory animal. It helps us to understand the same processes or effects in humans.

antenatal: occurring before birth.

anterior: situated towards the front or near the head.

anteroposterior: describing the front to back axis of the body.

antibiotic: a substance that is produced by one microorganism and which inhibits or kills another. Antibiotics vary widely in structure. They can act either on a particular microorganism (ie. they are specific) or a group of microorganisms (ie. they are broad spectrum).

antibody: a protein that is produced by the lymphoid tissue to fight against antigens. All antibodies are immunoglobulins.

antigen presenting cell (APC): a type of cell in the immune system. It presents antigen to lymphocytes, e.g. T cells. Antigen presenting cells include macrophages and dendritic cells.

antigen: any substance that the body identifies as potentially dangerous, e.g.. a virus or bacterium. Its presence stimulates the body to produce antibodies.

antihistamine: a drug that acts against histamine. Antihistamines may be given during an allergic reaction, in which histamine is released by certain cells of the immune system.

antioxidant: a substance that can make oxygen free radicals neutral.

apoptosis: a process where a cell kills itself. It is a kind of programmed cell death.

arthritis: a condition where a joint becomes inflamed.

asexual reproduction: reproduction that does not involve the fusion of male and female cells. For example, budding and fission are methods of reproducing asexually.

asparagine: one of the 20 common amino acids that make up proteins. It is made by animals and is therefore not essential in the diet.

assay: a test that is used to measure the concentration of a substance.

asthma: an allergic condition where the bronchi and bronchioles become narrowed. This prevents airflow into and out of the lungs. The condition is triggered by allergens and is characterised by difficulty in breathing and wheezing.

atheroma: a condition where the walls of the arteries thicken and the arteries no longer work properly. The disorder occurs when lipid is deposited, or as a result of fibrosis or the growth of smooth muscle tissue.

atherosclerosis: a condition in which fat within the arteries develops over years into fatty plaques. These plaques can eventually cause complete obstruction. Atherosclerosis mainly affects the aorta, and coronary and cerebral arteries. It can lead to cardiovascular disease.

ATP: see adenosine triphosphate.

autoantibody: an antibody that reacts with antigens on the host tissues.

autoimmune disease: a condition in which the body produces an immune response against its own components. This can lead to chronic inflammation or tissue destruction.

autonomic nervous system: the nerves that control the activity of smooth muscle, cardiac muscle and glands.

autosome: a non-sex chromosome. Autosomes occur in pairs in the cell nucleus. Human somatic cells usually contain 22 pairs.


B cell/ B lymphocyte: a type of white blood cell that is formed by lymphoid tissue. It produces circulating antibodies. It is also known as a B lymphocyte.

bacterium (plural bacteria): a microorganism that is a single cell. Bacteria have a cell wall and multiply by cell division.

benign: harmless.

beta (β) globin: a protein chain that forms part of normal adult haemoglobin. It is produced at low levels from as early as six weeks of fetal life and at higher levels throughout adulthood.

beta thalassaemia: a blood abnormality where β globin chains are produced very slowly or not at all. The condition is inherited.

beta-adrenergic agonists: drugs that expand the airways of the lung, e.g. in asthma.

beta-blockers: drugs that slow the heart rate, reduce the work that is performed by the heart and lower high blood pressure. They are also known as beta-adrenergic antagonists.

bilateral: having or involving two sides. In bilateral blindness, for example, a person is blind in both eyes.

binary fission: a method of cell division where the cell divides into two similar or identical cells. Single-celled organisms commonly use this method to reproduce asexually.

biopsy: a procedure where tissue is removed from a living body to examine it under a microscope.

blood-brain barrier: a protective layer that prevents many ions and compounds of large molecular weight passing from the blood to brain tissue.

bone mineralisation: a process where new bone is formed as calcium phosphate is deposited in bone tissue.

brainstem: the core of the brain, which connects the cerebral hemispheres with the spinal cord. It consists of the medulla oblongata, pons and midbrain. The diencephalon may also be included.

bronchial tree: a branching system of tubes that carry air from the trachea to the lungs. The system includes the bronchi and their subdivisions and the bronchioles.

bronchodilator: a drug that relaxes smooth muscle in the bronchi and widens the airway. This makes breathing easier.

Burkitt's lymphoma: a malignant disease that occurs in lymphatic tissues. It usually affects children in Africa. The tumour grows very rapidly and is commonly found in the jaw or the abdomen.


cAMP (adenosine 3', 5' cyclic monophosphate): a molecule that is formed by the enzyme adenylate cyclase from adenosine triphosphate. It is important in controlling mechanisms within a cell.

cancer: a life-threatening illness that occurs when cells in an organ or tissue grow uncontrollably.

capillary: the smallest blood vessel (5-20 micrometres in diameter). It has a wall only one cell thick, so oxygen, carbon dioxide, salts etc. can be exchanged between the blood and body tissue.

carcinogen: a substance that causes cancer.

cardiovascular: relating to the heart and blood vessels.

carrier state: in genetics, the condition in which an individual carries a single abnormal gene that does not cause disease. The gene may be passed on to the carrier's children.

cartilage: a skeletal tissue that forms the embryonic skeleton. In adults, cartilage is found in the joints, the walls of the thorax, the larynx, trachea, bronchi, nose and ears.

case-control study: a technique that is used in epidemiology. The technique identifies individuals who have a disease (cases) and individuals who do not have the disease (controls). Something that is present in cases, but not in controls, is connected with the disease.

catabolism: the processes within a cell or organism by which complex substances are broken down into simple substances. Compare anabolism.

catalyst: a substance that speeds up a chemical reaction. The catalyst is unchanged at the end of the reaction.

cataract: a condition where the lens of the eye is cloudy. It is the most common cause of blindness in the world. The condition is painless.

catecholamines: compounds that include adrenaline, noradrenaline and some of their metabolites.

caudal: relating to the posterior part of the body.

cell cycle: the sequence of phases that occurs as a cell divides.

cell-mediated immunity (CMI): a form of immunity where antibodies have no role or play only a secondary role. It includes the processes where cytotoxic cells kill pathogens.

cellular immunity: a condition where lymphocytes and macrophages are able to fight against viruses, parasites, fungi and possibly cancer cells. They acquire the ability after they are exposed to antigens or pathogens.

central nervous system (CNS): the brain and spinal cord.

cephalic: relating to the head.

cerebellum: the part of the brain that deals with finely controlled movements, e.g. picking up a small object. It is found at the back of the brain.

cerebral hemisphere: the right or left half of the brain. Each cerebral hemisphere consists of cerebral cortex and basal ganglia.

cerebrospinal fluid (CSF): the fluid that fills and surrounds the brain and spinal cord.

chemokine: a small protein that binds to specific receptors on the surface of immune system cells, particularly phagocytic cells and lymphocytes. Chemokines play a crucial role in inflammatory responses.

chemotherapy: the use of chemicals or drugs to treat disease.

cholesterol: a fat molecule that is biologically important. It is found in the cell membranes of animals. It is a precursor of bile acids and steroid hormones.

chorionic villus: a finger-like projection in the outer layer of the membrane that surrounds the embryo. The membrane, or chorion, forms part of the placenta. The villus is important, because it is involved in exchanges between the mother and the embryo. Cell samples that are taken from the chorionic villus are used to diagnose genetic disease before birth.

chromatid: a strand formed by lengthwise division of a chromosome. Chromatids remain joined by the centromere until metaphase. At this stage the centromere divides, and the chromatids separate and become chromosomes.

chromatin: a complex of nucleic acids and proteins, which makes up the genetic material of the cell.

chromosome: a thread-like structure in a cell nucleus made up of highly condensed DNA and associated proteins. It carries genetic information in the form of genes.

circadian rhythm: a biological rhythm or cycle of behaviour that is repeated every 24 hours.

cirrhosis: a disease of the liver in which the normal lobular structure is replaced by fibrosis and regenerating nodules.

classical complement pathway: one arm of the complement pathway. It is a reaction to antigen, which is bound to antibody. Compare alternative complement pathway.

clinical diagnosis: a process where a disease is identified from history taking and physical examination.

clinical trial: an experiment that is carried out on human beings to evaluate scientifically the efficacy and safety of a therapy.

clinical: based on, or involving, direct examination and care of patients.

coagulation factor: a protein used by the body to prevent bleeding.

codominance: a property that is shown by some alleles. When the alleles are present in a heterozygous state they produce a different phenotype from that produced by either allele in the homozygous state.

codon: a group of three DNA or RNA nucleotide bases. Together they specify the choice of an amino acid residue during peptide synthesis. If one or more of the bases changes, a different amino acid may be specified.

coevolution: the process where species that interact evolve in response to each other.

cognition: the mental processes by which an individual acquires knowledge. It includes perception, intuition and reasoning.

cohort study: in analytical epidemiology, a study where a population that is exposed to a presumed cause of disease is followed over a set period of time to monitor the appearance of the disease. A control group is also monitored to identify the cause and effect of infection. It is also known as a prospective study.

collagen: a structural protein found in connective tissue.

complement: a group of 20 proteins found in blood. They are involved in the destruction of foreign cells, after the cells have been coated with antibodies.

computerised tomography: a technique that is used to examine the soft tissues of the body. X-ray images of sections or 'slices' of the body are integrated by a computer to form a cross-sectional image.

congenital abnormality: any abnormality that is present at birth. It may be inherited or caused by other external factors.

connective tissue: the tissue that binds together and supports the parts of the body.

coronary arteries: the main arteries that supply blood to the heart.

coronary heart disease (CHD): a group of conditions where the supply and demand for oxygenated blood to the heart is not balanced. It is usually caused by reduced blood supply to the heart.

coronary vein: the vein that carries blood away from the heart.

cortisol: a steroid hormone that has anti-inflammatory properties. It is also known as hydrocortisone.

crossing-over: a process occurring during meiosis or mitosis. It results in the exchange of segments of DNA between two homologous chromosomes.

cutaneous: relating to the skin.

cystic fibrosis: a condition where the exocrine glands do not work properly. Excess mucus is produced in the respiratory tract, which can lead to chronic lung disease.

cytochrome p450: a group of enzymes that are found in the liver. They are responsible for processing drugs.

cytokine: a soluble molecule that is involved in signalling between cells during an immune response, e.g. tumour necrosis factor.

cytoplasm: a part of the cell. The cytoplasm is the part that lies outside the nucleus.

cytosine: one of the bases. It contains nitrogen and participates in the formation of the nucleic acids DNA and RNA in animal and plant cells. It is often abbreviated to C.

cytoskeleton: the network of structural proteins inside a cell. It gives the cell its shape, flexibility and motility.

cytosol: the liquid part of the cytoplasm. It is where many chemical reactions take place.


deformity: a condition where a part of the body is not formed properly. The condition can be acquired or congenital.

deletion: a process where nucleotides are lost from DNA. It results in codon changes.

dementia: a condition where an individual has suffered a significant loss in cognitive functioning and ability to think due to physical brain disease. However, consciousness is not affected. The individual cannot then work or function socially.

demographic data: information used in descriptive epidemiology to describe the characteristics of a population, such as number of people and age groups. The data are useful in evaluating and anticipating the occurrence of a disease in that population.

demyelination: a process in which the myelin sheath of nerve fibres is destroyed.

dendrite: a branch of a nerve cell. It carries nerve impulses towards the cell body.

dendritic cell: a type of cell that takes up antigen and presents it to other cells of the immune system. It is a phagocyte, which is present in lymphoid and other tissues. Dendritic cells in lymphoid tissues stimulate T cell responses.

deoxyribonucleic acid (DNA): the genetic material of most organisms, which controls heredity. In eukaryotic cells DNA is located in the nucleus. In the prokaryotes (bacteria) it occurs in the cytoplasm, and it forms a core in many viruses.

diabetes mellitus: a condition where the pancreas does not produce enough insulin. The body cannot therefore change glucose into energy. The condition is characterised by the production of large amounts of urine and excessive thirst.

diagnosis: a process where a disease is identified by signs and symptoms, a history and laboratory tests.

differentiation: 1. in cell biology, a process in which cells and tissues become specialised for specific roles. For example, stem cells in the small bowel crypts differentiate into enterocytes. 2. in a neoplasm, the extent to which cells have the structure and function of the mature normal cells of the tissue of origin. The degree of differentiation is sometimes used to grade tumours.

disaccharide: a type of sugar. It contains two molecules of a simple sugar joined together. Examples are lactose and sucrose.

disseminated: spread throughout an organ or the body, e.g. cancer or an infection.

dizygotic: coming from two separate fertilised eggs, e.g. non-identical twins. Compare monozygotic.

DNA (deoxyribonucleic acid): the genetic material of most organisms. DNA is located in the cell nucleus and controls heredity.

DNA fingerprinting: Studies of variations in the lengths of pieces of DNA that are used to establish identity. These fragments are obtained from different individuals due to genetic polymorphism of repeated DNA – minisatellites or microsatellites – in the genome.

DNA polymerase: an enzyme that creates DNA from nucleotide building blocks.

DNA restriction fragment length polymorphism (RFLP): variation in the lengths of restriction fragments of DNA. These fragments are obtained from different individuals due to genetic polymorphism of the restriction sites in the chromosomes.

DNA sequencing: a laboratory technique that is used to find out the sequence of nucleotide bases in a DNA molecule.

dominant: describing a phenotypic character state or an allele, hiding an alternative character state or allele when both are present in the heterozygous state.

double helix: the structural form of the DNA molecule. It consists of two coiled polynucleotide chains held together by hydrogen bonds.

dysplasia: a condition where cells grow more than normal. The growth can become malignant.


ecology: the study of the relationships between living organisms and their environment.

ectopic: occurring in an abnormal position, e.g. organs, pregnancy.

egg: an ovum; a female gamete.

electrocardiogram (ECG): a recording of the electrical activity of the heart, made on a moving strip of paper.

electrochemical gradient: a difference between the two sides of a cellular membrane. The gradient is made up of a difference in the concentration of a molecule (a chemical gradient) and/or a difference in electrical potential (an electrical gradient).

electroencephalogram (EEG): a graphic trace showing variations in the electrical activity of the brain. The test is performed with electrodes attached to the intact scalp.

electrolyte: a substance that separates into ions when it is in the form of a solution. For example, sodium chloride solution contains free sodium and free chloride ions. The term electrolyte in medicine usually refers to the ion itself, e.g. the sodium ion.

electron micrograph (EM): the image produced by an electron microscope. The microscope uses a beam of electrons to reveal structures that are too small to see with a light microscope.

electrophoresis: a laboratory procedure that separates molecules according to their electric charge.

embryo: an early stage in the development of an animal. The embryo is contained in either an egg, egg membranes, or the mother. The human embryo develops into a fetus.

encephalopathy: any disease of the brain.

endemic: 1. generally, present at all times. 2. of a disease of humans, always present within a geographical area or population. Compare epidemic.

endocrine: relating to cells or glands that secrete a substance directly into the bloodstream. The substance is delivered by the blood to anywhere in the body it is needed.

endocytosis: a process by which a cell takes up material from its environment. The cell membrane folds inwards to take in material. It includes pinocytosis and phagocytosis.

endogenous: developing within an organism. Compare exogenous.

endoplasmic reticulum: a structure within a plant or animal cell. It is a highly folded arrangement of membranes within the cytoplasm. There are two types, called rough and smooth endoplasmic reticulum. The rough endoplasmic reticulum is the site where secreted and membrane proteins are produced.

endosome: a structure that forms inside the cytoplasm during endocytosis. It is a vesicle that is made when the infolding cell membrane breaks off inside the cell.

endothelium: the single layer of cells that lines the cavities of the heart, blood vessels, and lymph vessels.

environment: the external conditions and surroundings in which people and organisms live and work.

enzyme: a protein found inside cells. It speeds up chemical reactions. Enzymes usually work in highly specific ways.

epidemic: 1. affecting a large proportion of the population at the same time. 2. a sudden outbreak of disease affecting a large proportion of a population at the same time. Compare endemic.

epidemiology: the study of the frequency, distribution and cause of disease in order to find ways of prevention and control. It includes social and environmental factors that influence the disease, e.g. occupation.

epidermis: the outer layer(s) of the skin.

epilepsy: a neurological disorder characterised by fits. The fits occur repeatedly and suddenly. The cause may be known or unknown.

epinephrine: a hormone that is released by the adrenal medulla and acts on the alpha and beta adrenergic receptors. Epinephrine makes the heart beat faster. It also makes the bronchi open and reduces the flow of the blood to the arms and legs. It is also known as adrenaline.

epithelial cell: a cell that is part of the outer layer of the skin and mucous membranes.

epitope: the region of an antigen that binds to a T cell or antibody.

erythroblast: an immature red blood cell. It has a nucleus.

erythrocyte: a red blood cell.

essential fatty acid: a type of fatty acid that cannot be synthesised by the human body and must be obtained from the diet. Linoleic and linolenic acids are examples of essential fatty acids.

exogenous: developing outside an organism. Compare endogenous.

exon: the coding sequence of a gene.

expression library: a collection of DNA that includes only the DNA that is expressed by the organism. Compare genome library.

expression: of a gene, activity. It is shown by production of RNA or protein.

extracellular matrix: material that is secreted by cells into the tissue around them. It usually refers to non-cellular matter such as fibrous material and glycosaminoglycans.


fascicle: a bundle of nerve fibres.

fatty acids: a group of molecules that have a chain structure and contain a carboxyl (COOH) group. They make up lipid molecules and are an essential component of cell membranes.

ferritin: a protein that is used by the body to store iron. It occurs in the liver, bone marrow and spleen.

fertilisation: the process during which a sperm enters an egg to form a zygote.

fetal haemoglobin (Hb F): the main type of haemoglobin in the fetus. It contains two alpha globin chains and two gamma globin chains.

foetus: the unborn child in the womb during the later part of pregnancy.

fibrin: the protein that causes blood to clot.

fibrinogen: a plasma protein. It is changed to fibrin in the blood by thrombin.

fibrosis: a process during which connective tissue becomes thicker and causes a scar. The thickening often occurs as a result of inflammation or injury.

folic acid: a vitamin of the B complex. Folic acid deficiency causes anaemia. It is also known as folate.

frameshift: a process where nucleotides in DNA are inserted or deleted. As a result there is a change in the sequence in which triplets of nucleotides are read as codons.

free radical: an atom or group of atoms that has an unpaired electron. Free radicals are short-lived and very reactive, and can cause damage to cells and tissues. Examples include the superoxide and hydroxyl radicals.

fructose: a simple sugar. It combines with glucose to form sucrose.


G protein: a protein that has a role in controlling mechanisms within a cell. G proteins interact with adenylate cyclase in controlling the level of adenosine 3', 5' cyclic monophosphate (cAMP) within cells.

galactose: a simple sugar. It combines with glucose to form lactose in milk.

gamete: a mature sex cell; the gametes in humans are the egg and sperm. A gamete has half of the normal chromosome set (ie. is haploid). In sexual reproduction, two gametes join to form a zygote.

gametocyte: a cell that produces a gamete ie. a sex cell; an oocyte or spermatocyte.

gamma (γ) globin: a protein chain forming part of fetal haemoglobin. It is mainly produced in the fetus, but also at low levels after birth.

gamma interferon (IFNγ): a cytokine that is produced by T cells in response to antigen. It makes macrophages active.

ganglion (plural ganglia): 1. a group of nerve cell bodies that are outside the central nervous system. 2. a swelling that occurs in the peripheral nervous system.

gas exchange barrier: the cellular barrier in the lung where gas exchange takes place. It has three tissue layers - an endothelium lining the capillaries; an epithelium that lines the air spaces and an interstitial layer of connective tissue. The gas exchange barrier has minimum thickness and maximum surface area so that the respiratory gases can be exchanged efficiently. It is also known as the respiratory membrane.

gastrin: a hormone that stimulates the stomach to secrete gastric acid.

gene deletion: a process where nucleotides change or are rearranged. As a result, the codon changes.

gene expression: the activity of a gene. It is shown by production of RNA or protein.

gene marker: a gene that determines an easily recognisable phenotype. It is used as a reference point in the study of inherited characteristics.

gene: a unit of DNA containing information for coding proteins. It is found at a particular place on a specific chromosome.

genetic code: a system that describes how nucleotide sequences in DNA and amino acid sequences in proteins relate to each other.

genetic counselling: a service that is performed by a trained counsellor. The counsellor combines information from individuals' medical and family history with knowledge of human genetics. This then provides information about whether a hereditary condition is possibly present. The counsellor is able to provide information about the treatment available and how to prevent the condition.

genetic recombination: 1. any process that results in offspring inheriting a different combination of genes to both parents. 2. a process that occurs during meiosis when homologous chromosomes pair up and exchange segments of DNA.

genetic susceptibility: an inherited condition that is carried on the genetic material DNA. It makes an individual more likely to develop an illness.

genetics: 1. the study of the structure and function of genes. 2. the genetic features occuring in individuals, families and populations.

genome library: a collection of the complete DNA of an organism. Compare expression library.

genome: the total number of genes in a single set of chromosomes, ie. in a haploid nucleus.

genotype: the genetic composition of an individual. Compare phenotype.

gestation: the period during which the embryo develops in the womb before delivery.

gland: a cell or group of cells that secrete substances.

globin: the protein part of haemoglobin.

glomerulus (plural glomeruli): a network of blood capillaries. For example within the kidney the glomerulus is the site of filtration of waste products from the blood into the kidney.

glucagon: a hormone. It is important in controlling the level of sugar within the blood. It stimulates liver cells to release glucose.

glucose: a simple sugar. It combines with galactose to form lactose in milk.

glycogen: a polysaccharide found mainly in muscle and liver. It is used by the body to store carbohydrate. It is easily broken down to produce glucose.

glycolipid: a lipid containing a carbohydrate group.

glycoprotein: a protein that has carbohydrate molecules attached to it.

gonad: a male or female sex organ.

graft: tissue that is moved from one place to another on the same organism, or from one organism to another.

growth plate: a layer of specialised cartilage. It is found in a growing bone between the shaft and the extremity. It allows the bone to grow to its characteristic size.

guanine: one of the bases. It contains nitrogen, and participates in the formation of the nucleic acids DNA and RNA in animal and plant cells. It is often abbreviated to G.

guanosine 3', 5' cyclic monophosphate (cGMP): a molecule that is formed by the enzyme guanylate cyclase from guanosine triphosphate. It is important in controlling mechanisms within cells.

guanosine triphosphate (GTP): a source of cellular energy. Energy is stored in the chemical bonds of the three phosphate groups. It is released when GTP is split into its diphosphate and monophosphate forms.

guanylate cyclase: the enzyme that changes GTP into cGMP.


haem: a pigment which contains iron and porphyrin.

haematology: the study of the blood and blood forming tissues.

haemoglobin: a protein consisting of a pigment (haem) and polypeptides (globin). Haemoglobin gives blood its red colour. It takes up oxygen as the blood passes through the lungs and carries it to the body's tissues.

haemoglobinopathy: a inherited disorder occurring in the globin chain part of haemoglobin. It includes structural abnormalities such as sickle haemoglobin and reduced production of globin chains as in the thalassaemias.

haemophilia: an inherited condition where the blood's ability to clot is reduced. Affected individuals tend to bleed spontaneously and when they suffer minor trauma. A person with haemophilia is called a haemophiliac.

haemopoietic: relating to the production of blood cells.

haploid: describing cells, nuclei or organisms that have one set of unpaired chromosomes. Compare diploid.

haplotype: the set of alleles that are carried on one of a pair of homologous chromosomes.

HbS (sickle haemoglobin): an abnormal type of haemoglobin found in sickle cell disease and sickle cell trait. The abnormality occurs when there are changes in the beta globin gene. These changes then produce an abnormal beta globin chain.

health education: a method of education providing information to persuade people to change their life style and so improve their health.

heart failure: a condition where the heart is unable to pump enough blood to body tissues. Blood pressure builds up in the veins, and the lungs and liver become congested, which leads to breathlessness, swollen ankles, etc. The cause is any condition that damages heart muscle or reduces its ability to work normally. When the heartbeat stops suddenly and permanently, the result is death.

heavy chain: a part of an antibody. A typical antibody such as IgG contains two heavy chains and two light chains.

helix: in genetics, a chain or coil that carries the genetic material in DNA. It is composed of nucleotides. DNA has two chains/coils that are wound around each other. This is called a 'double helix'.

helper cell: a functional subclass of T cells that can help to create cytotoxic T cells and cooperate with B cells to produce an antibody response. They usually recognise antigens in association with class 2 major histocompatibility complex (MHC) molecules.

hepatic: relating to the liver.

hepatocyte: the main type of cell that is found in the liver.

hereditary persistence of fetal haemoglobin (HPFH): an inherited disorder of the blood in which fetal haemoglobin is not replaced by adult haemoglobin after birth.

heredity: transmission of genetic material or characteristics from parent to offspring.

heterozygote: an individual who has different alleles at a particular point on the chromosome map.

heterozygous: inheriting different alleles at a particular point on the chromosome. Compare homozygous.

hippocampus: a part of the brain. It is at the floor of the lateral ventricle of the brain. It is concerned with long-term memory.

histamine: a molecule released from mast cells during inflammation. It makes blood vessels more permeable.

homeostasis: a system that keeps the environment inside an organism stable. The system is controlled by negative feedback. For example, a high level of carbon dioxide in the blood increases breathing. This, in turn, causes a decrease in carbon dioxide.

homozygote: an individual who has the same alleles at a particular point on the chromosome map. Compare heterozygote.

homozygous: inheriting the same alleles at a particular point on the chromosome map. Compare heterozygous.

hormone: a substance produced by a gland. Hormones are important in controlling mechanisms within the body.

human leukocyte antigen (HLA): a system of cell surface antigens that induce and regulate the immune response. HLAs are used to match body tissues for organ transplants.

humoral immunity: a form of immunity where antigens stimulate antibodies circulating in the blood.

hybrid: an offspring whose parents belong to two different strains, varieties or species.

hybridisation: a laboratory process where a double-stranded molecule is produced from two complementary strands of DNA or a strand of DNA and a complementary strand of RNA.

hypertension: a condition where the blood pressure is abnormally high. Compare hypotension..

hyperthyroidism: a condition where the thyroid gland produces and secretes more thyroxine or triiodothyronine than normal.

hypotension: a condition where the blood pressure is abnormally low. Compare hypertension.

hypothesis: a theory that can be proved or disproved by experiments or the collection of data.


immune response: the way in which the body defends itself against pathogens or foreign bodies.

immune system: the body's defences against pathogens or foreign substances. It includes non-specific and highly specific defence mechanisms, e.g. antibodies and cytotoxic T cells.

immunity: a state where an organism is able to fight disease with circulating white blood cells and antibodies.

immunodeficiency: a condition where an individual has a decreased ability to fight disease because there are not enough white blood cells or antibodies.

immunosuppressive drug: a drug that makes a patient less able to develop an immune response.

in utero: within the womb.

in vitro: within a test tube.

in vivo: within a living organism.

infection: 1. a disease caused by a pathogenic organism. 2. The process during which pathogenic organisms multiply or damage tissue in the body of the host.

inflammation: a reaction whereby the body protects itself against injury. It is characterised by painful swelling and redness.

initation sequence: the first codon of the coding region of mRNA or DNA. A change in the initiation sequence can affect the expression of that gene.

innate immunity: a type of immune response. It is not specific to a particular antigen and does not become stronger when an individual is exposed to the antigen again. Compare adaptive immunity.

insulin dependent diabetes mellitus (IDDM): a type of diabetes that usually begins in childhood or adolescence. It is associated with little or no endogenous insulin production.

insulin: a hormone. It is important in controlling the level of sugar within the blood.

integrase: an enzyme that catalyses the insertion of DNA into another stretch of DNA. The HIV integrase inserts the viral DNA into the host cell genome.

interferon: a type of cytokine produced by the body in response to an infection. It also protects the body from other infections.

interleukin (IL): one of a group of peptides that signals between the cells of the immune system. It is a cytokine.

interspecific variability: the difference in characteristics between different species. Compare intraspecific variability.

intracellular: occurring or functioning within cells.

intragenic: 1. relating to a segment of DNA within a gene locus. 2. relating to a genetic mutation within a gene locus.

intranuclear: within the nucleus of a cell.

intraocular pressure: the pressure within the eye. It is controlled by the aqueous fluid. If the pressure is too high, it causes glaucoma.

intrauterine: within the womb

intravenous (IV): within or into a vein.

intron: a section of DNA occurring in a gene. A change in the intron does not affect the structure of that gene. It is also known as an intervening sequence.

invagination: folding inward.

invasion: a process where an infection or tumour spreads into or destroys surrounding tissues.

invertebrate: 1. without a backbone. 2. an organism that does not have a backbone.

isoenzyme: one of a group of enzymes that catalyses the same reaction, but have different physical properties. They are also known as isozymes.


jaundice: a yellow discoloration of the skin, mucous membranes and the whites of the eyes. This occurs when there is too much bile pigment in the blood and the cells of the discoloured tissue.

jejunum: the part of the small intestine that lies between the duodenum and the ileum.

juxtaglomerular apparatus: a collection of cells found in the distal convoluted tubule and the afferent arteriole of the kidney. The cells secrete renin when blood pressure begins to fall.


karyotyping: a technique in which a photomicrograph is taken of a cell undergoing meiosis. The chromosomes are ordered according to their size. Karyotyping is often used for prenatal diagnosis of conditions such as Down syndrome.

keratin: a protein substance found in horny tissues, e.g. nails, hair.

kilobase (kb): a unit that is used to measure the length of a nucleic acid sequence. For example, 7 kb indicates a sequence 7000 nucleotides in length.

kinins: a group of substances. They have different actions. Some make smooth muscle contract and blood vessels dilate.

Kupffer cell: a cell that is found in the liver. It is a phagocyte.

kyphosis: a condition where an individual has a spine that is more curved than normal. The person has a humpbacked appearance.


lactase: an enzyme. It breaks down lactose from milk and milk products into galactose and glucose. An individual who does not have full lactase activity can suffer from lactose intolerance.

lactose: the main sugar in milk. It is broken down into galactose and glucose by the enzyme lactase.

larynx: the upper part of the wind pipe, containing the vocal cords. It is tubular and has a framework of cartilage rings.

latent infection: an infection that does not produce evident symptoms but may appear at a later time.

lateral: of or relating to the side or sides of a structure. Compare medial.

LDL cholesterol: low density lipoprotein cholesterol. This type of cholesterol makes up about 75 per cent of plasma total cholesterol. A high level of plasma LDL cholesterol is a risk factor for coronary heart disease (CHD).

lectin: a type of protein that binds specifically to sugars and glycoproteins on the surface of many mammalian cells.

lens: the structure behind the iris of the eye. It changes shape to focus objects on the retina.

leucocyte: a mature blood cell containing a nucleus. Leucocytes, e.g. monocytes, lymphocytes and granulocytes, are found in the blood or tissue fluid. They are part of the body's immune system. It is also known as a white blood cell.

leukaemia: a group of cancers in which the bone marrow and other blood-forming organs over-produce abnormal white blood cells.

leukocyte: a white blood cell.

ligament: a fibrous band of connective tissue connecting bones or cartilages and strengthens the joint.

ligand: a small molecule that binds to a larger molecule, for example a molecule that binds to a receptor or an antigen that binds to an antibody.

light chain: a part of an antibody. A typical antibody such as IgG contains two light chains and two heavy chains.

lipase: an enzyme that helps to break down fat.

lipid bilayer: the layers of phospholipid molecules found in a cell membrane. The hydrophobic parts of the molecules are in the middle of the bilayer, and the hydrophilic parts are on the inner and outer surfaces.

lipid: a type of organic compound that is an ester of a fatty acid or a closely related substance. Examples include fats and phospholipids.

lipoprotein: a compound that contains both lipid and protein. Plasma lipoproteins are used to transport fats throughout the body.

liver: a large organ found in the abdomen. The major functions of the liver are - detoxification; secretion of bile; metabolism of proteins, carbohydrates and fats; synthesis of plasma proteins, and the storage of glycogen.

lymph: tissue fluid that is pale yellow and contains lymphocytes. It occurs in the lymphatic system.

lymphocyte: the type of white blood cell that is formed by lymphoid tissue. B lymphocytes produce circulating antibodies; T lymphocytes are involved in cell-mediated immunity (CMI).

lymphoma: a malignant disease of the lymph nodes, for example Burkitt's lymphoma and Hodgkin's lymphoma.

lysine: one of the essential amino acids that are incorporated into proteins.

lysis: 1. a process where a substance is broken down. 2. a process where a cell is destroyed by rupture of the outer cell membrane.

lysosome: a body within a cell, containing contains digestive enzymes. These enzymes break down foreign bodies.

lysozyme: an enzyme. It is found in saliva, milk and tears.


macrophage: a type of white blood cell. It is a large scavenger cell derived from a blood monocyte. Macrophages are present in connective tissue and many organs where they destroy bacteria and other foreign particles.

macular degeneration: a progressive disorder that is common among the elderly. The disorder affects the central part of the retina and causes gradual loss of central vision. It usually affects both eyes, but is painless.

magnetic resonance angiography (MRA): a procedure that is used to show up blood vessels in the body. It is used to diagnose conditions such as strokes.

magnetic resonance imaging (MRI): a procedure that is used to show and examine tissues in the body. Tissues are placed in a strong magnetic field. Images are then made as radio waves are absorbed and transmitted.

major histocompatibility complex (MHC): a group of genes. They produce proteins that help to control signalling between antigen presenting cells and lymphocytes.

malignant: of a tumour, uncontrollable; tending to become worse and resulting in death. Compare benign.

mammal: one of a group of highly evolved vertebrates. The females of this group secrete milk, which is used to feed their newborn.

medial: situated in or toward the middle. Compare lateral.

Meibomian glands: small glands lying under the conjunctiva in the eyelid. They secrete an oil that forms a barrier to stop the tears overflowing onto the cheeks.

meiosis: a form of nuclear division in which the content of the nucleus is halved (from diploid to haploid), e.g. in gamete formation. Compare mitosis.

melanin: black or dark brown pigments that are present in the hair, skin, eyes and various tumours, e.g. melanoma.

melanoma: a tumour containing cells that form melanin.

membrane: a double layer of lipid molecules surrounding cells and some of their internal structures.

memory cell: a type of T or B cell that has been exposed to antigen. The cell remembers the antigen and will respond more effectively to it if it is exposed again.

Mendelian inheritance: the inheritance of genes or characteristics according to Mendel's laws.

messenger RNA (mRNA): a molecule that carries genetic information to the site of protein synthesis. It specifies the order of amino acid residues.

metabolism: the physiological and chemical processes that enable the body to function and to grow.

metabolite: a substance that is produced or takes part in a chemical process within a living organism.

metastasis: the spread of disease form one part of the body to another. It usually occurs through the bloodstream or lymphatic system.

metazoan: an organism that develops from an embryo and consists of many cells.

micrometre (υm): one-millionth of a metre.

microtubule: a hollow tube found in the cytoplasm of some cells. It is mainly composed of the protein tubulin. Microtubules help cells keep their shape and divide. They also help cells, organelles and other structures inside cells to move.

mitochondrial DNA: the genetic material occurring within mitochondria.

mitochondrion (plural mitochondria): a small, round or rod-like body covered by a double membrane. It contains ATP and enzymes responsible for energy production and cell metabolism. The mitochondrion is present in the cytoplasm of all cells with a nucleus.

mitogen: a substance that causes cells to divide.

mitosis: a form of nuclear division that produces two identical nuclei. It is usually followed by division of the cytoplasm. Compare meiosis.

monoclonal antibody: an antibody that is produced artificially from a cell clone. It consists of only one immunoglobulin, which comes from its parent lymphocytes.

monosaccharide: the simplest type of sugar. It contains one molecule, e.g. glucose. Two monosaccharides join to form a disaccharide, e.g. lactose, and many monosaccharides join to form a polysaccharide, e.g. starch.

monozygotic: coming from one fertilised egg, e.g. identical twins. Compare dizygotic.

morbidity: 1. a condition where a patient is suffering from a disease. 2. in epidemiology, the measure of the prevalence or incidence of a disease in a population.

morphology: the study of the form and structure of organisms.

mortality: 1. death. 2. in epidemiology, the rate of death in a population.

mRNA: see messenger RNA.

mutation: in genetics, any change in the amount or chemical structure of the DNA, which causes changes in the characteristics of an organism.

myelin: a material that forms the sheath around the axons of certain nerve cells. These axons conduct electrical impulses faster than those without a myelin sheath.

myofibril: a protein fibril of muscle cells that is able to contract. It is made of many threads of protein.

myoglobin: a protein that is present within muscle cells. It is made of a single molecule of haem and a single globin polypeptide.

myopia: a condition where an individual cannot see objects 6 metres from the eye clearly. It is also known as short-sightedness.

myosin: the main protein found in the thick filaments of muscle.


natural killer (NK) cell: a type of cell that is formed by lymphoid tissue. It kills cells that are infected with pathogens.

necrosis: the death of some or all of the cells in an organ or tissue.

nematode: a roundworm.

neonatal: 1. newborn. 2. relating to infants within the first 28 days of life.

neoplasm: an abnormal swelling or growth. It can be benign or malignant. It is also known as a tumour.

nephron: the part of the kidney that produces urine. It has three functions. It filters waste substances from the blood. It also secretes additional waste substances and reabsorbs useful materials, which are returned to the blood.

nervous system: the collection of nerve fibres that carries electrical impulses between parts of the body.

neural tube: the structure that develops into the brain and spinal cord of a fetus during pregnancy.

neurone: a nerve cell.

neurotransmitter: a chemical substance that is released from nerve endings. Such substances carry impulses across the spaces between nerves.

neutrophil: a white blood cell containing a lobed nucleus and fine granules. These cells fight bacterial infection.

noradrenaline: an adrenal hormone that is a neurotransmitter. It acts at alpha adrenergic receptors. Its main action is to vasoconstrict arterioles and increase blood pressure.

nuclear membrane: a thin layer of tissue surrounding the nucleus.

nucleic acid: an organic acid present in the nucleus, and occasionally in the cytoplasm, of living cells. There are two types - DNA and RNA. Their main functions are in inheritance of genetic information and in protein synthesis.

nucleotide: a compound that has a base that contains nitrogen. The base is linked to a sugar-phosphate backbone. Nucleic acids (DNA and RNA) are made of long chains of linked nucleotides.

nucleus: 1. the central part of a cell containing the genetic material. The structure is surrounded by a membrane. 2. the central part of an atom about which the electrons orbit.


obesity: a condition where an individual is above normal weight-for-height or body mass index.

ocular: of or relating to the eye.

oestrogen: a group of steroid hormones that are secreted mainly by the ovary and placenta. They promote the growth and function of the female sex organs and secondary sexual characteristics. Oestrogens also stimulate changes in the female sex organs during the menstrual cycle.

oligonucleotide: a short chain of nucleotides.

oligosaccharide: a carbohydrate made up of a small number of monosaccharides (usually between two and ten).

oncogenic: causing cancer.

oocyte: the female germ cell, which matures into an ovum by meiosis.

opsin: a protein that is found in the retina of the eye. It is joined to a form of vitamin A to make rhodopsin.

optic nerve: the second cranial nerve. It sends images from the eye to the brain.

organelle: a structure that is always present within a cell and has a particular function or functions. Organelles include the mitochondrion and the Golgi body.

organic: 1. relating to chemical substances that come from living organisms and contain carbon. 2. relating to an organ. Compare inorganic.

organism: any living animal or plant, protozoan, bacterium or virus.

osmosis: the movement of water through a membrane from a solution of low concentration to a solution of high concentration. It is the main mechanism by which water is absorbed by the bowel.

osmotic pressure: the pressure needed to prevent the passage of solvent molecules through a semipermeable membrane separating the solvent from the solution. Osmotic pressure increases as the concentration of the solution increases.

ossification: a process where tissues are converted to bone.

osteoblast: a cell that forms bone.

osteoporosis: a condition where an individual loses bony tissue. As a result, the bones become brittle and are likely to fracture.

ovary: the female reproductive organ that produces egg cells and hormones. There are two ovaries, one on each side of the womb.

oxidative stress: damage to an organism caused by oxidating substances such as free radicals.


pancreas: an organ in the abdomen. It makes secretions thatwhich help digest food. It also produces the hormones insulin and glucagon.

parasite: an organism that lives in or on a living organism of another species, i.e. the host. It can cause harm to the host.

passive transport: a process where a molecule moves down its electrochemical gradient. This does not require energy. Compare active transport.

pathogen: a microorganism or other substance that causes disease, e.g. a bacterium.

pathogenesis: 1. the origin of a disease. 2. the way a disease develops and its effects.

PCR: see polymerase chain reaction.

peptide: a short segment of a protein. It is formed from two or more amino acids are linked by a peptide bond.

peripheral nerve: a nerve that is outside the central nervous system. It is found in the skin or other surface parts of the body

pH: a measure of the acidity or alkalinity of a solution. Pure water has a pH of 7; acidic solutions have a pH less than 7, and alkaline solutions a pH greater than 7. pH is measured on a logarithmic scale from 0 to 14.

phagocyte: a cell that can surround and destroy foreign particles, e.g. cell debris or bacteria.

pharmacokinetics: the study of how a drug is distributed in, and excreted from, the body.

phase II trial: a pilot study investigating the efficacy and safety of a drug. It involves 200-500 volunteers who are randomly given either control or study therapy.

phase III trial: a study that aims to generate statistically relevant data about a drug. It can be multicentre, and usually involves thousands of volunteers who are randomly given either control or study therapy.

phenotype: the visible or measurable physical and biochemical characteristics of an organism. These characteristics are the result of the interaction of the genetic composition of the organism (the genotype) and the environment in which it develops. Compare genotype.

pheromone: any chemical that is secreted externally by an animal or insect and that influences the behaviour of other members of the species.

phosphatase: an enzyme that breaks down certain compounds containing phosphate to release inorganic phosphate.

phospholipid: a molecule containing phosphate and lipid. It is an important part of biological membranes.

phosphoprotein: a protein containing phosphorous. The phosphate group is bound to the amino acids serine, threonine or tyrosine.

physiology: the study of how living organisms work.

pituitary gland: a gland at the base of the hypothalamus. It has two parts. The neurohypophysis secretes the hormones oxytocin and antidiuretic hormone. Hormones secreted from the adenohypophysis control the secretion of hormones from glands such as the thyroid, the ovaries and testes, and the adrenal cortex.

placebo: a substance that is identical in appearance to a drug or medication. It is not an active substance.

placenta: an organ that attaches the fetus to the wall of the womb. It is bathed in maternal blood.

plasma membrane: a very thin membrane that is composed of lipids and protein. It surrounds the cytoplasm of a cell and controls the passage of substances into and out of the cell.

plasma: the straw-coloured fluid in which the blood cells are found. It is made of a solution of inorganic salts and protein.

plasmid: a structure in bacteria that contains genetic material. Bacterial cells pass on the structure separately from the chromosome. Sometimes the plasmid passes on resistance to antibiotics.

platelet: a cell fragment that does not have a nucleus. Platelets help blood to form a clot.

pluripotent: having the capacity to develop in a number of ways, e.g. into one of several cell types.

point mutation: a change in a single nucleotide in the genomic DNA.

polyacrylamide gel: a substance that is used for electrophoresis of proteins. It is used in the Western blot diagnostic test.

polyclonal antibody: specific antibodies that have been obtained by immunisation of an animal. They are the products of different clones of antibody-producing cells.

polymerase chain reaction (PCR): a procedure where a piece of DNA is copied many times to do genetic analysis. It can be used to amplify very small amounts of DNA.

polymorphism: in genetics, variability in a DNA sequence.

polypeptide: a molecule that is made up of linked amino acids.

polyploidy: the state of a cell or cell line in which more than two complete haploid sets of paired homologous chromosomes are present.

polysaccharide: a type of carbohydrate. It contains many molecules of a simple sugar, e.g. glucose, which are joined together. When a polysaccharide is digested, the smaller sugar units are released. Polysaccharides include starch and glycogen.

positron emission tomography (PET): a method that is used to record internal body images. Gamma rays are detected from body tissues that are isotopically labelled.

postnatal: relating to the period of infancy that begins one month after birth and ends on the first birthday.

precursor: anything that develops into a specified thing. An example is an undifferentiated cell from which a certain type of cell develops.

primer: a short sequence of nucleotides. Primers are used in PCR to start copying of the DNA that is under analysis.

probe: in molecular genetics, a labelled fragment of cloned DNA or RNA that is used to detect other DNA or RNA sequences.

progesterone: a hormone that stimulates the uterus to prepare for a fertilised egg and has a crucial role in the development of the fetus.

prognosis: a forecast that is made of the probable outcome of a patient's disease. It is based on all the knowledge related to a specific case, e.g. the course of the disease in other patients, age, sex.

promoter region: a region of genomic DNA that binds RNA polymerase. It is the site that regulates the synthesis of RNA mRNA.

prophase: the first stage of meiosis or mitosis.

prospective study: in analytical epidemiology, a study where a population that is exposed to a presumed cause of disease is followed over a set period of time to monitor the appearance of the disease. A control group is also monitored to identify the cause and effect of infection. It is also known as a cohort study.

protease: an enzyme that can break down proteins into peptides by splitting the peptide bonds. It is also known as a proteinase.

protein kinase: a type of enzyme. It transfers a phosphate group from ATP to a protein. Protein kinases are important in controlling mechanisms within cells.

protein synthesis: a process within a cell in which amino acids are joined together to make a protein. It requires a messenger RNA and takes place on ribosomes.

protein: a type of organic compound that contains carbon, hydrogen, oxygen, nitrogen and sulphur. Proteins are found in all viruses, bacteria, plants, animals and fungi. They include enzymes, hormones and antibodies.

proteolytic enzyme: a substance that breaks down proteins.


qualitative: relating to characteristics that are based on quality. Compare quantitative.

quantify: to determine the quantity of something.

quantitative: relating to characteristics that are based on numbers. Compare qualitative.


random sampling: a technique to select a sample that represents a population. People are chosen from the population at random, for example using randomly generated numbers. Compare systematic sampling and cluster sampling.

randomised controlled trial: a type of trial. The technique divides individuals at random into a group that receives an intervention, e.g. a treatment, and a group which does not. The impact of the intervention is assessed by comparing the outcome in these two groups.

reagent: a substance that takes part in a chemical reaction.

receptor: a molecule on the surface of a cell that binds to certain chemicals or proteins.

recessive: relating to alleles that are not reflected in the phenotype when present as one member of a heterozygous pair. They determine the phenotype only when they are present in the homozygous state.

recombinant: 1. of a vaccine, produced from two different sources. 2. of a chromosome, cell or organism, the result of recombination, either natural or artificial.

recombination: a process by which genetic information is rearranged during meiosis. This process also occurs during the somatic rearrangements of DNA that occur in the formation of genes that encode antibody molecules.

red blood cell (RBC): a cell with no nucleus, containing haemoglobin. It carries oxygen in the blood.

renal: relating to the kidney.

renin-angiotensin-aldosterone system: a system of hormones that controls blood pressure, blood volume and sodium balance.

replication: a process where an exact copy of genetic material (DNA or RNA) is made.

respiration: 1. the exchange of gases between the atmosphere, blood and cells. It involves three processes, pulmonary ventilation, external respiration and internal respiration. 2. the metabolic process in animals and plants in which organic substances such as carbohydrates are broken down to simpler products with the release of energy. In most plants and animals, respiration requires oxygen, and carbon dioxide and water are end products.

respiratory system: the system of interconnected passages, organs and cells that allow people to breathe. It permits oxygen to come into the body, and carbon dioxide to be removed. The main organs involved are the nose, pharynx, larynx, trachea, bronchi and lungs. It is also known as the respiratory tract.

restriction enzyme: a bacterial enzyme that breaks DNA at a short, specific sequence. It is also known as a restriction endonuclease.

restriction fragment length polymorphism (RFLP): variation in the lengths of restriction fragments of DNA. These fragments are obtained from different individuals due to genetic polymorphism of the restriction sites in the chromosomes. It is also known as DNA fingerprinting.

retina: the innermost part of the eye. It is composed mostly of nerve fibres. Images are focused on to the retina. They are then transmitted to the brain by the optic nerve.

retrospective study: in analytical epidemiology, a study that is used to identify individuals who have a disease (cases) and individuals who do not have the disease (controls). The data are taken from past records or survey data. Something that is present in cases but not in controls is connected with the disease. See also case-control study.

rhodopsin: a protein found in the retina of the eye. It contains opsin and retinal, a form of vitamin A. It is also known as visual purple.

ribonucleic acid (RNA): a type of nucleic acid containing ribose, phosphate, and bases that contain nitrogen. Cellular forms include ribosomal RNA, messenger RNA and transfer RNA. It is also the genetic material in some viruses, e.g. HIV.

ribosome: a particle in a cell, consisting of RNA and protein. It is the site where protein synthesis occurs in the cell. Ribosomes can be attached to the endoplasmic reticulum membrane inside the cell or can be free in the cell cytoplasm.

RNA (ribonucleic acid): a type of nucleic acid containing ribose, phosphate, and bases that contain nitrogen. Cellular forms include ribosomal RNA, messenger RNA and transfer RNA. It is also the genetic material in some viruses, e.g. HIV.

RNA polymerase: an enzyme that catalyses the production of RNA from a DNA template.


salbutamol: a drug that relaxes smooth muscle in the airways and makes them open wider. It is used to treat asthma and chronic bronchitis.

scanning electron micrograph (SEM): an image produced by an electron microscope. A beam of small particles (electrons) is directed on to the object. The electrons that are then given off from the surface of the object are analysed.

secretion: a process in which a gland selectively secretes a substance. Compare absorption.

sequence analysis: a technique in which the sequences of molecules, e.g. DNA, from different organisms are compared. More closely related organisms have more similar sequences.

serine: one of the 20 common amino acids that make up proteins. It is made by animals and is therefore not essential in the diet.

serology: the study of blood serum and its constituents, and how they help to protect the body against disease.

severe combined immunodeficiency (SCID): a condition where there is a severe lack of T and B lymphocytes. SCID patients usually have a very weak immune system.

sibling: a brother or sister.

sickle cell anaemia: a group of inherited blood diseases. Individuals with one of these diseases produce (abnormal) sickle haemoglobin and experience a wide range of clinical problems. It is also known as sickle cell disease.

side-effect: a secondary effect of a drug, which is produced as well as the therapeutic effect. The term is usually used to refer to undesirable or toxic effects.

signal transduction: the processes by which an extracellular signal (typically a hormone or neurotransmitter) interacts with a receptor at the cell surface. This causes a change in the level of a second messenger (for example cyclic AMP) and ultimately effects a change in the cell's functioning.

single photon emission tomography (SPET): an imaging technique. It takes images of physiological and metabolic functions.

single point substitution: a mutation of DNA in which one base is substituted with another. It is also known as single point or base mutation.

smooth muscle: muscle that is not under voluntary control. It is found in the walls of blood vessels, the gastrointestinal tract and the urinary tract, and other organs. It does not include cardiac muscle. It is also known as involuntary muscle.

solute: a substance that dissolves in another substance.

somatic: relating to the nonreproductive parts of the body.

somite: one of the series of block-like masses of mesoderm, arranged as segments alongside the neural tube of the embryo.

species: the smallest unit in biological classification. Members of the same species are able to breed among themselves successfully. Similar species are grouped within one genus.

spermatocyte: a male sex cell. It is formed during the production of sperm.

spermatozoon (plural spermatozoa): a mature male sex cell. It has a tail so that it can swim to reach and fertilize the female ovum.

spina bifida: a congenital abnormality that occurs when the spinal cord is not completely formed during pregnancy. It is a type of neural tube defect.

spleen: a large organ that is located on the left side of the body below and behind the stomach. The spleen is an important part of the reticuloendothelial system. It produces lymphocytes in the newborn and contains phagocytes that remove worn out red blood cells and other foreign bodies from the bloodstream.

splice: to remove noncoding sequences from a strand of RNA and then to rejoin the strand. This is part of the process of transcription.

spliceosome: a structure within a cell in which splicing of messenger RNA takes place.

splicing site: the join between a noncoding sequence and a coding sequence of RNA. This is where the noncoding sequences come apart from one another during the process of transcription.

stem cell: a precursor cell that can develop into more than one kind of cell. For example, early bone marrow cells can develop into red blood cells, white blood cells or platelets.

stop codon: a termination codon. It is made up of three bases and is located in a strand of DNA. Its presence signals that translation should stop at that point.

strain: a group of organisms. They come from a particular source that shares a set of distinguishing characteristics.

stroke: a sudden attack of weakness that often affects one side of the body. It occurs when the flow of blood to the brain is interrupted, e.g. by thrombosis or embolism.

subcutaneous: beneath the skin.

substrate: the specific substance upon which an enzyme acts.

sucrose: common sugar. It is obtained from sugar cane and sugar beet.

susceptibility: the likelihood a person has of being affected by a disease or condition.

sympathetic nervous system: part of the autonomic nervous system. It has many functions. For example, it makes the pupils dilate, slows the digestive system, increases the heart rate, widens the bronchi, and constricts peripheral blood vessels and skeletal muscle.

symptom: any evidence of a disease or disorder that is experienced by a patient. It is then often reported as a subjective observation, such as pain.

synapse: the gap between one nerve cell and another. Nerve impulses are carried by neurotransmitters across the gap.

syncytium (plural syncytia): a collection of cells with more than one nucleus.

syndrome: a collection of signs and symptoms. The cause may be known or unknown.

synergistic: describing the effect of two agents in combination. The combined effect is greater than that of each on its own.

synthesis: a process where a substance is formed by a chemical reaction. The chemical reaction builds up a complex molecule from simpler molecules.

systemic: affecting the whole body rather than parts of the body.


T cell: a type of white blood cell that is formed by lymphoid tissue. It is involved in cell-mediated immunity and humoral immunity. It is also known as a T lymphocyte.

tendon: a fibrous cord found at the end of a muscle. It attaches the muscle to a bone.

teratogen: an agent that causes the fetus to develop abnormally.

TGF-β: transforming growth factor beta.

thalassaemia: a group of conditions where there is a reduction in the rate at which globin chains of haemoglobin are formed. The conditions are inherited.

thrombin: an enzyme that converts fibrinogen into fibrin.

thrombocyte: a platelet.

thrombosis: a condition in which the blood changes from a fluid to a solid and produces a blood clot.

thymine: one of the bases. It contains nitrogen, and it participates in the formation of the nucleic acid DNA in animal and plant cells. It is often abbreviated to T.

thymus: a gland found at the base of the neck. It is responsible for the development of immunity in childhood. In adults it shrinks in size.

thyroid gland: a gland in the neck that collects iodine from the blood. When an individual does not have enough iodine in the diet, the gland becomes larger and forms a goitre.

thyroid hormone: a collective term for a group of hormones that are secreted by the thyroid gland. It includes thyroxine and triiodothyronine.

TNF: see tumour necrosis factor.

toxicity: the unwanted effects of a drug that cause harm or death.

trait: 1. a characteristic that is determined by genetics. 2. in genetics, it usually describes a condition, where an individual carries an abnormal gene, but has no symptoms, e.g. sickle cell trait.

transcription: the process by which the information in the genetic code is transferred from DNA to RNA. This is the first step in the manufacture of proteins in cells.

transfer RNA (tRNA): pieces of RNA that are small and are found in all cells. Each different tRNA carries a specific amino acid to the ribosomes in the cell so that protein synthesis can take place. Each piece of tRNA lines up with a specific piece of messenger RNA to ensure that the amino acids are arranged in the correct sequence.

transferrin: a protein that is used by the body to transport iron. It occurs in blood and extracellular fluid. It is a glycoprotein.

transforming growth factor beta (TGF-β): a cytokine that is made by many cells, including platelets and T cells. It has anti-inflammatory effects.

transfusion: a procedure where blood or blood products are passed from one individual to another.

translation: the manufacture of proteins in a cell. It takes place at the ribosomes. The information that determines the sequence of amino acids in the protein is carried to the ribosome by messenger RNA. The amino acids are brought to their correct position in the protein by transfer RNA.

translocation: 1. the process by which protein is moved across a cell membrane. 2. the process by which DNA fragments move between chromosomes.

transmission electron micrograph (TEM): an image produced by an electron microscope. A beam of small particles (electrons) is directed through a very thin object.

transplant: an implant of an organ or tissue from one part of the body to another or from one person to another.

trophoblast: the outer layer of cells that grows at the blastocyst stage as the fertilised ovum develops. It becomes part of the placenta.

trypsin: an enzyme that is produced by the pancreas. It is a protease.

tryptophan: one of the 20 common amino acids that make up proteins. It cannot be made by animals and is essential in the diet.

tubule: a small tube. In the kidney, one of the tubes that carries filtrate from the glomerulus to the renal pyramid.

tumour necrosis factor (TNF): a cytokine that is released from cells as part of the immune response. It has many actions.

tumour: an abnormal swelling or growth. It can be benign or malignant. It is also known as a neoplasm.


ulcer: a break in the skin or a mucous membrane. It may be slow to heal and is associated with inflammation.

ultrasound: high frequency sound waves. They are used for making images of the organs within the body.

ultraviolet (UV) light: invisible radiation of short wavelength.

undifferentiated: of a cell, not changed in any way from its original form.

unilateral: occurring on one side only.

uninucleate: having a single nucleus; mononuclear.

up-regulate: to increase the activity of a gene.

uracil: one of the bases. It contains nitrogen, and it participates in the formation of the nucleic acid RNA in animal and plant cells in place of thymine. It is often abbreviated to U.


vaccination: a process where individuals are given a substance, such as a vaccine, to make them resistant or less susceptible to a disease. The vaccine can be given by injection or orally

vacuole: a space or cavity within a cell. It is formed when the cell membrane folds inwards and takes in material.

valine: an essential amino acid.

variant: a group of organisms in a species that differ very slightly from the other organisms in the same species.

vasculature: the network of blood vessels in an organ.

vasoconstrict: to cause the blood vessels to narrow.

vasodilation: widening of the blood vessels. It produces an increase in blood flow.

ventral: situated towards the belly surface of the body. Compare dorsal.

ventricle: 1. a chamber of the heart. The ventricles have thick muscle walls and pump blood to the lungs and systemic circulation. Compare atrium. 2. in the brain, part of a system of cavities that are filled with cerebrospinal fluid. It includes the lateral ventricles, and the third and fourth ventricles.

vertebrate: a subphylum of animals that have a well formed backbone that encloses the spinal cord. They also have a brain that is enclosed in a skull.

vesicle: a membrane-lined sac in the cytoplasm of a cell.

virulence: the ability of a microorganism to cause disease.

virus: a small intracellular parasite with a single type of nucleic acid (either DNA or RNA) that is usually within a protein shell. Viruses are wholly dependent on the host cell for their replication.

vitamin A: a fat soluble vitamin needed by the body to maintain the epithelia, to help the immune system work and to produce rhodopsin in the eyes. Vitamin A refers to a group of naturally occurring compounds, the main one of which is retinol. The vitamin is present only in foods of animal origin.

vitamin D: one of a group of hormones that are essential for forming bones. It is also known as cholecalciferol.

vitamin K: a group of fat-soluble vitamins. Vitamin K is necessary for the production of prothrombin and other clotting factors.

vitamin: a part of the diet that the body needs in small amounts, e.g. vitamin A, vitamin B12 and vitamin D. Vitamins are organic. Compare mineral.


wheeze: the sound of disturbed airflow in the bronchi, e.g. in bronchitis or asthma. The noise is caused by. 1. too much mucus in the bronchi 2. narrowing of the bronchi.

white blood cell: a mature blood cell that contains a nucleus. White blood cells, e.g. monocytes, lymphocytes and granulocytes, are found in the blood or tissue fluid. They are part of the body's immune system. It is also known as a leucocyte.

white matter: the part of the brain and spinal cord that is composed of myelinated nerve fibres.


X-ray: a negative image produced on a photographic film. It is made by passing radiation through tissue. It is also known as a radiograph.


yeast: a single-celled fungus that reproduces by budding or fission. It is not a taxonomic term.


zoonosis: a disease of animals that may be passed to humans.

zygote: a diploid cell formed in the fusion of haploid gametes; a fertilised ovum.

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