Meiosis in human females takes place over decades. At any point in this process, an incorrect number of chromosomes can be transferred to daughter cells, resulting in aneuploid gametes, the most common cause of miscarriage and the root of certain developmental disorders, such as Down syndrome.
During gestation, primordial germ cells replicate their DNA and pair up homologous chromosomes for homologous recombination. Meiosis is then arrested until ovulation many years later.
© 2016 MICA DURAN
Just before ovulation, the oocyte resumes meiosis, building a meiotic spindle of microtubules to segregate homologous chromosomes. Upon fertilization, the egg undergoes a second round of division, segregating sister chromatids. (These divisions are asymmetrical, resulting in one large oocyte and two or three small polar bodies, shown here as equal size.)
© 2016 MICA DURAN
