Gametogenesis in Humans
Gametogenesis is the formation of gametes by meiosis. Spermatogenesis in the testis produces four functional sperm from each parent cell, while oogenesis in the ovary produces one functional egg and smaller polar bodies.
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Spermatogenesis
Spermatogenesis takes place in the seminiferous tubules of the testis. A diploid germ cell divides by mitosis to increase numbers, then one such cell undergoes meiosis to produce four haploid spermatids, which develop a tail and become mature sperm.
Because meiosis halves the chromosome number, each sperm carries only half the genetic material of a normal body cell, so that fertilisation restores the full number in the zygote. Once it begins at puberty, spermatogenesis continues throughout a man's life, producing very large numbers of sperm continuously under the control of hormones such as testosterone.
Oogenesis
Oogenesis takes place in the ovary. A diploid germ cell undergoes meiosis, but the cytoplasm divides unequally: one large secondary oocyte forms, along with a small polar body that eventually degenerates.
Meiosis in the human egg cell is not completed until after a sperm penetrates it, at which point the second polar body is released and the mature egg nucleus is ready to fuse with the sperm nucleus. Unlike spermatogenesis, oogenesis begins before birth, when a large fixed number of immature germ cells is already present in the ovaries, and only a small fraction of these ever mature and are released across a woman's reproductive years.
Comparing the two processes
| Feature | Spermatogenesis | Oogenesis |
|---|---|---|
| Site | Seminiferous tubules of testis | Ovary |
| Cell division type | Meiosis (equal cytoplasm division) | Meiosis (unequal cytoplasm division) |
| Number of functional gametes | 4 sperm per parent cell | 1 egg per parent cell |
| Timing | Continuous from puberty | One egg matures roughly per monthly cycle |
How it is examined
You may be asked to state where each process occurs, to explain why meiosis is essential for forming gametes, or to compare the number and size of the cells produced by spermatogenesis and oogenesis.
Worked exam-style question
Question. A diagram shows a diploid germ cell in the testis undergoing meiosis to produce sperm, and a diploid germ cell in the ovary undergoing meiosis to produce an egg. (a) State the number of functional gametes produced from one germ cell by spermatogenesis and by oogenesis.
(b) Explain why oogenesis produces only one large functional cell instead of four equal-sized ones. (c) Explain, in terms of chromosome number, why gametes must be formed by meiosis rather than mitosis.
(d) State what happens to the second polar body in the human egg, and explain when this occurs.
Model answer. (a) Spermatogenesis produces four functional sperm; oogenesis produces one functional egg (plus polar bodies that degenerate). (b) During oogenesis, the cytoplasm divides unequally at each division, so almost all the cytoplasm and nutrient reserves go to one large secondary oocyte, giving the future zygote enough resources to support early development, while the rest forms small polar bodies that do not develop further.
(c) Meiosis halves the chromosome number, producing haploid gametes; if gametes were diploid (formed by mitosis), fusion at fertilisation would double the chromosome number every generation, whereas meiosis keeps the chromosome number constant across generations. (d) The second polar body is released only after a sperm penetrates the egg; it eventually degenerates, while the mature egg nucleus fuses with the sperm nucleus to complete fertilisation.
Practice question
Try this. State one similarity and one difference between spermatogenesis and oogenesis in terms of the type of cell division used and the number of functional gametes produced.
Exam tip
Key terms
These Chapter 15 terms are central to understanding gametogenesis:
- Meiosis, the cell division that halves the chromosome number to produce haploid gametes.
- Diploid, having the full, paired set of chromosomes, as germ cells do before meiosis.
- Haploid, having only one set of chromosomes, as gametes do after meiosis.
- Fertilisation, the fusion of a sperm and an egg, which restores the diploid number.
- Zygote, the diploid cell formed by fertilisation, which develops into an embryo.
Source:SRC-DSKP-EN
Frequently asked questions
Why does meiosis, not mitosis, produce gametes?
Why does oogenesis produce only one functional egg instead of four?
When exactly does oogenesis in a human egg cell finish?
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