Reproduction

Gametogenesis: the production of gametes (reproductive cells)

- In males: gametogenesis continues through puberty

- In females: gametogenesis takes place before birth

Primodial germ cells: divide in the gonads by mitosis and differentiate into gametes by meiosis

1. (Diploid) Primodial germ cells divide by mitosis and form diploid spermatogonia

2. The spermatogonia grow without dividing until they become primary spermatocytes

3. The primary spermocytes undergo meiosis. Meiosis I creates two haploid cells called secondary spermocytes.

4. Meiosis II creates four haploid cells called spermatids.

5. The spermatids differentiate into spermatozoa

1. Primodial germ cell divides by mitosis to form a diploid oogonia.

2. Most oogonia do not develop further and degenerate. Only one grows very large and is referred to as a primary oocyte.

3. The primary oocyte undergoes meiosis. Meiosis I forms two haploid cells; one secondary oocyte and a smaller polar body that sticks to the secondary oocyte.

4. The secondary oocyte undergoes meiosis II and forms one haploid ovum and another polar body.

5. The polar bodies degenerate and die, leaving the ovum.

Spermatozoa:

- Many: millions produced

- Mini: extremely small

- Motile: can move

Ova:

- Few: only a few thousand in a lifetime

- Fat: very large

- Fixed: do not move before fertilisation

External fertilisation:

- Occurs outside the body.

- Male gamete released into the environment.

- Fertilisation occurs by chance.

Internal fertilisation:

- Male gametes transferred directly to the female.

- Fertilisation more likely than external fertilisation.

- Only occurs on land.

1. Around 100 million sperm are released into the cervix, only a few thousand reach the oviduct. The acrosome region of the sperm prepares to release a digestive enzyme.

2.The front of the sperm touches the zona pellucida of the oocyte and the accrosome releases digestive enzymes to digest the follicle cells and the zona pellucida.

3. Fertilisation occurs when one sperm touches the surface membrane of the oocyte and the membranes fuse.

4. The sperms nucleus decondenses and releases its chromosomes into the oocyte. The zona pellucida react and form a fertilisation membrane that stops other sperm from fertilising the egg (known as polyspermy).

5. The sperms chromosomes fuse with those of the ovum and they form a diploid zygote completing fertilisation.