Trying to Avoid Your Sister

Connections between chromosomes are essential for their accurate segregation during cell division. In somatic cells dividing by mitosis, newly replicated sister chromatids are connected by cohesin proteins. When the sister chromatids become attached to microtubules emanating from opposite poles of the spindle, cohesins resist the pulling forces and the ensuing tension stabilizes the chromatid–microtubule attachments. In this way, each pair of sister chromatids achieves a stable bipolar attachment to the spindle. Consequently, when cohesion is destroyed at the onset of anaphase, sister chromatids are pulled to opposite poles and each new cell receives a full complement of maternal and paternal chromosomes. In the germline, meiosis employs two successive rounds of nuclear division to produce gametes containing half the number of chromosomes as the original precursor cell. During the first division, sister chromatids remain connected while the paternal and maternal homologs are segregated (one homolog comprises a pair of sister chromatids). Homolog segregation during meiosis is governed by the same mechanical principles as sister segregation during mitosis and, as such, homologs must be connected. These connections are called chiasmata, and they are established via a process called homologous recombination, a DNA repair process that involves interaction between a broken chromosome and a homologous template chromosome. To ensure that each pair of homologs is connected by at least one chiasma, homologous recombination during meiosis is regulated at several levels. A key aspect of this regulation is the choice of recombination template. The sister chromatid is the preferred template for recombinational repair in cells dividing by mitosis. However, during meiosis this bias must be overcome so that homologs recombine and become connected by chiasmata. How template choice is regulated remains unclear, but studies of meiotic recombination in budding yeast have suggested a number of possible mechanisms.