Meiosis

Five mouse nuclei showing chromosomes at the pachytene stage of meiosis. The green stain highlights the synaptonemal complex, a structure that holds the two chromosomes of a pair together prior to recombination. The red stain shows the tumour suppressor protein, BRCA1, binding to the sex chromosomes and to all the telomeres. The top left nucleus is at late pachytene when the synaptonemal complexes are beginning to break down and BRCA1 staining is very intense on the telomeres. BRCA1 is thought to be involved in inactivation of the X-chromosome during meiosis, DNA repair, and the regulation of cell division especially in cells lining the milk ducts in the breast. Mutations in the BRCA1 gene are associated with an increased risk of breast, ovarian, prostate and colon cancer.