<Tr> <Td> spermatozoids </Td> <Td> sperm </Td> <Td> haploid (N) / 23 </Td> <Td> 1C / 23 </Td> <Td> spermiation </Td> <Td> 64 days (total) </Td> </Tr> <P> Spermatocytes regularly overcome double - strand breaks and other DNA damages in the prophase stage of meiosis . These damages can arise by the programmed activity of Spo11, an enzyme employed in meiotic recombination, as well as by un-programmed breakages in DNA, such as those caused by oxidative free radicals produced as products of normal metabolism . These damages are repaired by homologous recombination pathways and utilize RAD1 and γ H2AX, which recognize double strand breaks and modify chromatin, respectively . As a result, double strand breaks in meiotic cells, unlike mitotic cells, do not typically lead to apoptosis, or cell death . Homologous recombinational repair (HRR) of double - strand breaks occurs in mice during sequential stages of spermatogenesis but is most prominent in spermatocytes . In spermatocytes, HRR events occur mainly in the pachytene stage of meiosis and the gene conversion type of HRR is predominant, whereas in other stages of spermatogenesis the reciprocal exchange type of HRR is more frequent . During mouse spermatogenesis, the mutation frequencies of cells at the different stages, including pachytene spermatocytes, are 5 to 10-fold lower than the mutation frequencies in somatic cells . Because of their elevated DNA repair capability, spermatocytes likely play a central role in the maintenance of these lower mutation rates, and thus in the preservation of the genetic integrity of the male germ line . </P> <P> It is known that heterozygous chromosomal rearrangements lead to spermatogenic disturbance or failure; however the molecular mechanisms that cause this are not as well known . It is suggested that a passive mechanism involving asynaptic region clustering in spermatocytes is a possible cause . Asynaptic regions are associated with BRCA1, kinase ATR and γ H2AX presence in pachytene spermatocytes . </P> <P> The gene Stimulated By Retinoic Acid 8 (STRA8) is required for the retinoic - acid signaling pathway in humans, which leads to meiosis initiation . STRA8 expression is higher in preleptotene spermatocytes (at the earliest stage of Prophase I in meiosis) than in spermatogonia . STRA8 - mutant spermatocytes have been shown to be capable of meiosis initiation; however, they cannot complete the process . Mutations in leptotene spermatocytes can result in premature chromosome condensation . </P>

Formation of primary and secondary spermatocytes and spermatids