centromere

The region at which the two sister chromatids are attached after a chromosome has replicated into two identical strands in preparation for cell division. During anaphase of mitosis and of the second meiotic division, the two chromatids detach at the centromere, each moving to opposite sides of the dividing cell. Before this occurs, the pairs of chromatids often look like the letter X; the cetromere is the point at which the two arms of the X cross.

The centromere serves a very important function in the chromosome during mitosis and meiosis. Without the centromere, segregation of sister chromatids would not separate properly. Not much is currently known about the controls on the centromere during the different phases of mitosis and meiosis, but some is known about their structure and function in general. A few technical points about the centromere:

1. The centromere does not have to occur at the center of the chromosome. The human Y chromosome, for instance, has its centromere at one end.
2. The DNA composition at the centromere can vary greatly from organism to organism. In humans, the centromere is usually either noncoding DNA or multiple copies of any of a number of enogenous retroviruses. In Saccharomyces cerevisiae, there is a short recognition sequence in a specific region of each chromosome called the CEN sequence; this sequence is the centromere region of the chromosome.
3. The centromere region actually has a number of proteins bound to it during mitosis/meiosis; some bind the sister chromatids together, and some bind the kinetochores.
4. In meiosis I, the centromeres of the homologous chromosomes do NOT touch. The homologous chromosomes are connected at chiasmata.