You need to keep track of meiosis in oogenesis. Remember that in human females, the primary oocyte is arrested with paired homologous chromosomes in prophase I before the female is even born, with meiosis only being completed in the monthly cycle of oogenesis after puberty. Subsequent to sperm penetration (the acrosome reaction facilitating passage through the jelly coat and vitelline layer), and upon fusion of the sperm and egg nuclei, the completion of egg meiosis occurs. Frequently the first meiotic division occurs at ovulation and the second after fusion of egg and sperm. Upon formation of a diploid set, DNA synthesis can begin. The fertilized egg is a zygote preparing to divide mitotically.
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