OOGENESIS 101: For everyone who asked!

tehsunshine:

Ok, so this whole business started when someone tongue-in-cheek asked nikosnature if menstruation was murder. He said no, because “eggs” only have 23 chromosomes, while embryos (or “babies,” if you’re an anti-science anti-choicer) have 46 chromosomes.

He’s wrong. Human “egg” cells go through two rounds of division, or meiosis, before being awesome enough to contribute to reproduction. They start with 46 chromosomes, double to 92, then divide back to 46. This is meiosis I.

Only one of the divided “egg” cells gets to continue on. This cell (which still has 46 chromosomes) chills happily in its ovarian home, awaiting a spermatozoon to come invade its personal space. If a sperm makes it through the outer layer of the egg and binds to a receptor, the egg starts meiosis II. In meiosis II, the egg nucleus divides its remaining 46 chromosomes in half. 23 are discarded, and 23 are combined with the sperm’s 23 chromosomes, creating a 46-chromosome zygote.

If a sperm does not get up in the egg’s business in time, the egg leaves the body with menstruation. Because it never made it to meiosis II, it still has 46 chromosomes. Nikosnature is wrong when he says that a menstruated-out egg has 23 chromosomes.

And always remember: if a sperm is wasted, God gets quite irate.

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Key terms:

meiosis: the division of cells that results in sex, or gametic, cells (egg and sperm). Meiosis produces 1 egg or 4 sperm, depending on who it happens in.

diploid: In humans, this refers to the number of chromosomes that normal, or somatic, cells have. This number is 46.

haploid: In humans, this refers to the number of chromosomes that sex, or gametic, cells have after completion of meiosis II. This number is 23.

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If you’re into molecular biology, you can read all about the signaling pathways that cause meiosis to happen. (MPF is a pretty cool cyclin-kinase, just so you know)

More wonderful information from the lovely tehsunshine.