Centromeres (pronunciation: SEN-truh-meers). A centromere is the region on a eukaryotic chromosome where sister chromatids are attached to each other (see diagram at right). The position of the centromere is visible on a condensed metaphase chromosome as a constricted region where the sister chromatids come into close contact. The two kinetochores (one for each chromatid), which serve to attach the chromosome to the spindle apparatus during cell division, are also present in the centromere.
An "arm" of a linear chromosome is one of the two portions of the chromosome that extend from the centromere to one of the two ends of the chromosome. Thus, the position of the centromere determines the length of a chromosome's arms. The shorter of the two arms is called the p arm and the longer, the q arm (from French "petit" and "queue"). Normally, a chromosome has a single centromere. A fragment of a chromosome lacking a centromere is called an "acentric fragment."
A eukaryotic chromosome is classified as metacentric, submetacentric, acrocentric, or telocentric, depending on the relative lengths of its p and q arms. It is metacentric if its two arms are about the same length; submetacentric, if its arms are both well-developed but unequal in length; acrocentric if the p arm is very short; and telocentric if the centromere lies very near, or at one end of the chromosome.
Thus, centromeres are important not only in their function in cell division, but also in the role they play in the cytogenetic classification of the chromosomes themselves.
In a holocentric chromosome, the entire chromosome acts as the centromere. Although this sort of chromosome is not typical, it does occur in certain types of organisms, for example, in nematodes.
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The centromere of a chromosome is the place where
the two sister chromatids are attached to each other.
Here, one chromatid is shown in yellow, the other
in blue.
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