Question

If oocytes are already at metaphase 2 during ovulation and at fertilization meiosis 2 completes with help from the sperm, then how does the paternal DNA from the sperm “mix” with the maternal DNA if crossing over occurs during meiosis 1?

Answer 1

It is very important to note that The DNA of oocyte and DNA of sperm do not mix with each other like crossing over which occurs in meiosis I.

Each parent gives their haploid genome to the offspring in a zygote. Importantly, these haploid genome is a product of recombination taken place during meiosis I of gametogenesis.

Different proteins associated with the chromosome, like cohesins, keep chromosomes condensed in sperm. These proteins are neutralized by proteins in cytoplasm of oocyte. This causes decondensation of chromosomes given by sperm. Though mitochondria and histones are provided by oocyte mostly, centrioles required for chromosome assembly and next cell division are provided by sperm. Therefore, meiosis 2 completes after fertilization occurs.

Thus the oocyte DNA and sperm DNA do not undergo any crossover in the zygote but spem chromosomes aline with respective oocyte chromosomes with a mechanism not clear yet.