Concept preview

What you will be able to do

1. 01Describe spermatogenesis and explain why it produces four sperm per meiosis.
2. 02Describe oogenesis and explain why it produces one egg (plus polar bodies) per meiosis.
3. 03Explain why meiosis must halve the chromosome number when gametes are made.
4. 04Describe the ovarian and uterine phases of the menstrual cycle and the roles of FSH, LH, estrogen and progesterone.

A human body cell carries 46 chromosomes, arranged as 23 pairs — a condition biologists call diploid, written $2n = 46$. If a sperm with 46 chromosomes fertilized an egg with 46 chromosomes, the new individual would have 92, and the number would double again in every generation, which is impossible. The solution is that gametes are haploid: each carries only a single set of 23 chromosomes, written $n = 23$. When a haploid sperm ($n = 23$) fuses with a haploid egg ($n = 23$) at fertilization, the full diploid number $2n = 46$ is restored, and the offspring receives exactly one set of chromosomes from each parent. Making these haploid gametes is the job of a special kind of cell division called meiosis, and the whole process of building gametes — spermatogenesis in the male and oogenesis in the female — is built around it. Every other cell in the body keeps the full diploid set; only the gametes are halved, and only meiosis can do the halving, so it is the single most important division to understand in the whole of reproduction.