Sexual reproduction in mammalsEdexcel A-Level Biology B: Revision notes

Gametogenesis makes haploid gametes from diploid germ cells in the gonads. In both sexes:

1. Mitosis of germ cells increases their number
2. Cells grow into primary gamete cells (diploid)
3. Meiosis I gives two secondary cells
4. Meiosis II gives haploid cells that develop into gametes

In males this is spermatogenesis in the testes. In females it is oogenesis in the ovaries.

Spermatogenesis occurs in the seminiferous tubules of the testes from puberty onwards.

• Diploid spermatogonia divide by mitosis
• Some grow into primary spermatocytes (2n)
• Meiosis I gives two secondary spermatocytes (n); meiosis II gives four spermatids (n)
• Spermatids differentiate into spermatozoa, supported by Sertoli cells

A sperm has a head with the haploid nucleus and an acrosome (hydrolytic enzymes), a midpiece packed with mitochondria for ATP, and a flagellum for swimming. Each primary spermatocyte gives four sperm, and millions are made every day.

• In the fetal ovary, oogonia divide by mitosis and form primary oocytes, which begin meiosis I but stop in prophase I and stay arrested until puberty
• Each month, hormones cause one follicle to mature. The primary oocyte completes meiosis I, giving a large secondary oocyte and a tiny polar body
• The secondary oocyte stops in metaphase II and is released at ovulation
• Meiosis II is completed only if a sperm enters, giving the ovum and a second polar body

The cytoplasm divides unequally, so the ovum keeps nearly all the food reserves. Each primary oocyte therefore gives one ovum, not four.

1. Sperm pass through the follicle cells and bind to receptors on the zona pellucida
2. Binding triggers the acrosome reaction: hydrolytic enzymes are released and digest a path through the zona pellucida
3. The sperm cell surface membrane fuses with the egg cell surface membrane, and the sperm nucleus enters the egg
4. Cortical reaction: cortical granules release enzymes that harden the zona pellucida, so no other sperm can enter. This prevents polyspermy
5. The secondary oocyte completes meiosis II; the sperm and egg haploid nuclei fuse to form a diploid zygote

The zygote travels along the oviduct and divides by mitosis, called cleavage. There is no growth between divisions, so the cells (blastomeres) get smaller.

• A solid ball of cells, the morula, forms
• A fluid-filled cavity develops, giving a hollow blastocyst, which has an outer trophoblast (later forms the placenta) and an inner cell mass (forms the embryo)
• After about 5 to 6 days the blastocyst reaches the uterus and implants in the endometrium