Centromere

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Chromosome.
(1) Chromatid. One of the two identical parts of the chromosome after S phase.
(2) Centromere. The point where the two chromatids touch, and where the microtubules attach.
(3) Short arm
(4) Long arm.

The centromere is a region of chromosomes with a special sequence and structure. The centromere plays a role in cellular division and the control of gene expression.

Function

Centromere- The strongest and thinnest region of a chromosome, to which the spindle fiber is attached during mitosis. the centromere is the point on the chromosome at which chromatids are held together and to which the spindle fibers attach during cell division. the centromere is visible during cell division as a constriction along the length of a contracted chromosome.

A centromere functions in sister chromatid adhesion, kinetochore formation, pairing of homologous chromosomes and is involved in the control of puberty.

A centromere is the region where sister chromatids join in the double chromosomal structure during mitosis, prophase and metaphase. The centromere is also where kinetochore formation takes place: proteins bind on the centromeres that form an anchor point for the spindle formation required for the pull of chromosomes toward the centrioles during the anaphase and telophase of mitosis.

Aberrant centromeric function can lead to improper chromosomal alignment and segregation, resulting in aneuploidy and conditions such as Down syndrome..

Centromere Positions

Each chromosome has two arms, labeled p (for petite, or short) and q (for queue, or long). They can be connected in either metacentric, submetacentric, acrocentric or telocentric manner. (The p arm does stand for petite, however the q arm is named q just because it follows p in the alphabet.)

Metacentric

If both arms are equal in length, the chromosome is said to be metacentric.

Submetacentric

If arms' lengths are unequal, the chromosome is said to be submetacentric

Acrocentric

If p arm is so short that is hard to observe, but still present, then the chromosome is acrocentric (The "acro-" in acrocentric refers to the Greek word for "peak.").

There are five acrocentric chromosomes in the human genome: 13, 14, 15, 21 and 22. These five chromosomes are the site of genes encoding rRNAs.

Telocentric

A telocentric chromosome's centromere is located at the terminal end of the chromosome. Telomeres may extend from both ends of the chromosome. All mice chromosomes are telocentric[1]; humans do not possess any telocentric chromosomes.

The centromeric sequence

In most eukaryotes, the centromere has no defined DNA sequence. It typically consists of large arrays of repetitive DNA (e.g. satellite DNA) where the sequence within individual repeat elements is similar but not identical. In humans, the primary centromeric repeat unit is called α-satellite (or alphoid), although a number of other sequence types are found in this region. However, in budding yeasts the centromere region is relatively small (about 125 bp DNA) and contains two highly conserved DNA sequences that serve as binding sites for essential kinetochore proteins.

Inheritance

Epigenetic inheritance plays a major role in specifying the centromere in most organisms. The daughter chromosomes will assemble centromeres in the same place as the parent chromosome, independent of sequence. However, there must still be some original way in which the centromere is specified, even if it is subsequently propagated epigenetically.

Structure

The centromeric DNA is normally in a heterochromatin state, which is essential for the recruitment of the cohesin complex that mediates sister chromatid cohesion after DNA replication as well as coordinating sister chromatid separation during anaphase. In this chromatin, the normal histone H3 is replaced with a centromere-specific variant, CENP-A in humans (Lodish et al. 2004). The presence of CENP-A is believed to be important for the assembly of the kinetochore on the centromere. CENP-C has been shown to localise almost exclusively to these regions of CENP-A associated chromatin.

In the yeast Schizosaccharomyces pombe (and probably in other eukaryotes), the formation of centromeric heterochromatin is connected to RNAi.[2] In nematodes such as Caenorhabditis elegans, some plants, and the insect orders Lepidoptera and Hemiptera, chromosomes are "holocentric", indicating that there is not a primary site of microtubule attachments or a primary constriction, and a "diffuse" kinetochore assembles along the entire length of the chromosome.

Centromeric aberrations

In rare cases in humans, neocentromeres can form at new sites on the chromosome. This must be coupled with the inactivation of the previous centromere since chromosomes with two functional centromeres (Dicentric chromosome) will result in chromosome breakage during mitosis. In some unusual cases human neocentromeres have been observed to form spontaneously on fragmented chromosomes. Some of these new positions were originally euchromatic and lack alpha satellite DNA altogether.

Centromere proteins are also the autoantigenic target for some anti-nuclear antibodies such as anti-centromere antibodies

Related links

References

Further reading

  • Lodish et al.; Molecular Cell Biology; fifth edition; 2004; ISBN 0716743663


External links

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