Editing Centromere (section)

{{short description|Specialized DNA sequence of a chromosome that links a pair of sister chromatids}}
{{distinguish|Centisome|Centrosome}}
[[Image:Chromosome.svg|thumb|In this diagram of a duplicated chromosome, (2) identifies the centromere—the region that joins the two [[sister chromatids]], or each half of the chromosome. In prophase of mitosis, specialized regions on centromeres called [[kinetochore]]s attach chromosomes to spindle fibers.]]

The '''centromere''' links a pair of sister [[chromatid]]s together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During [[mitosis]], [[Spindle apparatus|spindle fibers]] attach to the centromere via the [[kinetochore]].

The physical role of the centromere is to act as the site of assembly of the [[kinetochores]] – a highly complex multiprotein structure that is responsible for the actual events of [[chromosome segregation]] – i.e. binding [[microtubules]] and signaling to the cell cycle machinery when all chromosomes have adopted correct attachments to the [[Spindle apparatus|spindle]], so that it is safe for [[cell division]] to proceed to completion and for cells to enter [[anaphase]].

There are, broadly speaking, two types of centromeres. "Point centromeres" bind to specific [[protein]]s that recognize particular [[DNA]] [[Nucleic acid sequence|sequences]] with high efficiency. Any piece of DNA with the point centromere DNA sequence on it will typically form a centromere if present in the appropriate species. The best characterized point centromeres are those of the budding yeast, ''[[Saccharomyces cerevisiae]]''. "Regional centromeres" is the term coined to describe most centromeres, which typically form on regions of preferred DNA sequence, but which can form on other DNA sequences as well. The signal for formation of a regional centromere appears to be [[Epigenetics|epigenetic]]. Most organisms, ranging from the fission yeast ''[[Schizosaccharomyces pombe]]'' to humans, have regional centromeres.

Regarding mitotic chromosome structure, centromeres represent a constricted region of the chromosome (often referred to as the primary constriction) where two identical [[sister chromatids]] are most closely in contact. When cells enter mitosis, the sister chromatids (the two copies of each chromosomal DNA molecule resulting from [[DNA replication]] in chromatin form) are linked along their length by the action of the [[cohesin]] complex. It is now believed that this complex is mostly released from chromosome arms during prophase, so that by the time the chromosomes line up at the mid-plane of the mitotic spindle (also known as the metaphase plate), the last place where they are linked with one another is in the chromatin in and around the centromere.