A centromere is a region of DNA typically found near the middle of a chromosome where two sister chromatids come in contact. It is involved in cell division as the point of mitotic spindle attachment and the control of gene expression.
The centromere is, together with telomeres and origins of replication, one of the essential parts of any eukaryotic chromosome. The centromere usually contains specific types of DNA sequences which are in higher eukaryotes typically tandem repetitive sequences, often called "satellite DNA". These sequences bind specific proteins called "cen"-Proteins. During mitosis the centromeres can be identified in particular during the metaphase stage as a constriction at the chromosome. At this centromeric constriction the two mostly identical halves of the chromosome, the sister chromatids, are held together until late metaphase. During mitotic division, a transient structure called kinetochore is formed on top of the centromeres. The kinetochores are the sites where the spindle fibers attach. Kinetochores and the spindle apparatus are responsible for the movement of the two sister chromatids to opposite poles of dividing cell nucleus during anaphase. Usually the mitosis is immediately followed by a cell division cytokinesis. However, mitosis and cytokinesis are separate processes and can be uncoupled.
A centromere functions in sister chromatid adhesion, kinetochore formation, and pairing of homologous chromosomes 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 anaphase and telophase of mitosis.
Improperly functioning centromeres result in the chromosomes that do not align and separate properly, resulting in aneuploidy or daughter cells receiving the wrong number of chromosomes. Aneuploidy can cause conditions such as Down syndrome if the cells survive at all.
Point centromeres are smaller and more compact. DNA sequences are both necessary and sufficient to specify centromere identity and function in organisms with point centromeres. 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.
In the yeast Schizosaccharomyces pombe (and probably in other eukaryotes), the formation of centromeric heterochromatin is connected to RNAi. 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.
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