Cells divide


29/7/02. By Richard Twyman

The 'anchor points' that control the separation of chromosomes when cells divide.

Centromere diagram 1
Figure 1: Typical appearance of a human chromosome. Note that chromosomes only become visible when they are condensed, just before cell division. At this stage, replication is completed and there are two sister chromatids. These are joined at the centromere (arrow).

Each normal human chromosome has a single centromere, which appears as a constriction when viewed under the microscope. The centromere can be near the middle of the chromosome or near one end. It partitions the chromosome into long and short arms (see Figure 1).

The centromere is required for proper chromosome segregation. Every time a cell divides, the chromosomes first replicate so there are two identical copies (sister chromatids) and then line up across the middle of the cell. The chromatids are segregated so that each daughter cell receives one copy.

Segregation is achieved by building a cage-like structure called the spindle across the cell's nucleus. This attaches to the aligned chromosomes and draws sister chromatids to opposite poles of the dividing cell (see Figure 2). The spindle fibres attach to protein complexes called kinetochores that assemble on the centromere of each chromosome.

What are the DNA sequences that define a centromere? Human centromeres are large (several million base pairs) and consist predominantly of the same 171-bp sequence, known α-satellite DNA, repeated hundreds of thousands of times. This structure is recognised by the components of the kinetochore.

Centromere diagram 2
Figure 2: Function of the centromere. During cell division, the chromosomes line up across the cell. Fibres from the spindle apparatus attach to kinetochore complexes that have formed on the centromeres and draw the sister chromatids apart.

Chromosome derivatives with no centromere or multiple centromeres are occasionally formed when normal chromosomes are broken and incorrectly repaired. These aberrant structures are important because they do not segregate properly and are often lost from the dividing cell. If this happens in the germ cells, the resulting eggs or sperm can be unbalanced (have extra or missing chromosomes or chromosome segments). Many embryos formed from unbalanced gametes are spontaneously aborted but some survive to term producing individuals with birth defects such as Down syndrome.

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