Centromere

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A centromere is present before the mitosis process begins. Its function is to hold together the two sister chromatids [1].

A protein known as the kinetochore is built on the centromere. The kinetochore is important as it attaches the sister chromatids to the mitotic spindle, allowing chromosome division [2].

Centromeres in higher eukaryotes usually consist of long repeated base sequences. In humans these repeated sequences are known as Alpha satellites. In eukaryotes such as S. cerevisiae the centromere consists of a much smaller sequence (126 bp) [3] and is not a repeat.

A chromosome without a centromere is known as acentric. The chromosome therefore cannot be equally distributed to the daughter cells as the mitotic spindles cannot attach. In contrast, the presence of two centromeres is know to be dicetric. This can also cause problems for daughter cells as the chromosomes can be inherited unpredictably.

Centromeres do not necessarily have to be in the centre of the chromosome, and chromosomes be classified according to position of the centromere. Metacentric chromosomes have their centromere in the centre of the chromosome, submetacentric have centromeres slightly off centre. Arcocentric chromosomes have long arms (q) and short arms (p) like submetacentric chromosomes, however the centromere is slightly higher leaving longer long arms and shorter short arms. Telocentric chromosomes have their centromere at the top of the chromosome.

A centromere is the part of a chromosome that joins two chromatids together, without them, cells cannot divide properly and mitosis could not occur, preventing cell replication.
Although part of the chromosome the centromere itself is a section of noncoding DNA and contains no genetic information.

Function

Centromeres contain kinetochores, these are the regions of the centromere that attach chromosomes to the spindles, seperating the chromosomes, during prophase of mitosis

Position

They can vary on position on the chromosome, being in the center (metacentric) or creating long and short arms if appearing slightly towards one end (submetacentric),
almost at the end (acrocentric) or joining the end of the chromatids (telocentric). The position of the centromere affects the shape of the daughter chromosomes during anaphase. Metacentric chromosomes produce V-shaped daughter chromosomes, submetacentric appear J-shaped and acrocentric form I-shapes.
When creating karyotypes, the position of the centromere is used to find matching pairs, as it can be very specific.
Chromosomes can also be holocentric, they do not have a single centromere position but instead have numerous kinetochores throughout their length, so mitosis can still take place. [4]

References

  1. Compendium for Medical Sciences (2008): pg 163, Banbury: Scion Publishing Limited,
  2. Bruce Alberts, Molecular Biology of the Cell (2008): pg 1082-1083, Fifth Edition, New York: Garland Science.
  3. Lodish et al.; Molecular Cell Biology; sixth edition 2008
  4. Dernburg, A. F. (2001). "Here, There, and Everywhere: Kinetochore Function on Holocentric Chromosomes" Available at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2192025/ (Last accessed 28/11/14)
 
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