Centromeres




[You must be registered and logged in to see this link.]29/7/02. By Richard Twyman


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


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.

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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