Centromere Dynamics in Male and Female Germ Cells.

In sexually reproducing organisms the germ line is the cellular lineage that gives rise to gametes. All germ cells originate from germline stem cells that divide asymmetrically to generate gonial pre-cursors, which are amplified in number by mitotic divisions, undergo meiosis and eventually differentiate into mature gametes (haploid eggs and sperm). Information transmitted with gametes is inherited by offspring, and potentially by subsequent generations, instructing in organismal development and beyond. Meiosis comprises one round of DNA replication, followed by two rounds of chromosome segregation; homologous chromosomes segregate in the first division (meiosis I) and sister chromatids segregate in the second division (meiosis II). Important mechanistic features of meiosis occur in substages of prophase I and are critical for genetic recombination, including pairing and synapsis of homologous chromosomes (at leptotene and zygotene), crossing-over (at pachytene), and the appearance of chiasmata (at diplotene/diakinesis). Another unique feature of meiosis is the altered centromere/kinetochore geometry at metaphase I, such that sister kinetochores face the same spindle pole (mono-orientation) and stay together at anaphase I. This chapter reviews centromere dynamics in germ cells, focusing on centromere function and assembly in meiotic cell cycles, as well as centromere inheritance in zygotes. Centromeres are functionally defined by the presence of the histone H3 variant CENP-A, the epigenetic determinant of centromere identity. In most eukaryotes, it is well established that CENP-A function is essential for chromosome segregation in mitosis. CENP-A function in meiosis is less well understood and emerging insights into the differential regulation of meiotic and mitotic CENP-A are discussed.