4.7.9Additional lessons
Gametogenesis — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 63 of 101
Section 1 of 5Spermatogenesis in the Testis
1
Germ cells called spermatogonia in the wall of the seminiferous tubule divide repeatedly by mitosis.
2
Some grow into primary spermatocytes, which undergo meiosis I to form two secondary spermatocytes.
3
Meiosis II produces four haploid spermatids from each primary spermatocyte.
4
Spermatids differentiate into sperm with a head, acrosome, midpiece and tail, helped by Sertoli cells.
5
Sperm production begins at puberty and continues throughout adult life.
Section 2 of 5Oogenesis in the Ovary
1
Oogonia divide by mitosis in the fetal ovary and grow into primary oocytes, which begin meiosis I and pause in prophase I.
2
At puberty, each cycle one follicle completes meiosis I, forming a large secondary oocyte and a small first polar body.
3
The secondary oocyte begins meiosis II and stops in metaphase until ovulation.
4
Meiosis II is completed only if a sperm penetrates the egg, producing an ovum and a second polar body.
5
Unequal cytoplasm division gives one large egg full of nutrients per primary oocyte.
Section 3 of 5Comparison of Gametogenesis
Start
- Sperm production starts at puberty, whereas egg development starts before birth and pauses until puberty.
Number
- One primary spermatocyte gives four sperm, but one primary oocyte gives one egg and polar bodies.
Duration
- Sperm are made continuously, but eggs are released one per cycle until menopause.
Cytoplasm
- Division is equal in sperm formation and unequal in egg formation.
| Feature | Spermatogenesis | Oogenesis |
|---|---|---|
| Site | Seminiferous tubules | Ovary |
| Gametes per cell | 4 | 1 |
| Polar bodies | None | 2 or 3 |
| Begins | Puberty | Before birth |
Section 4 of 5Gamete Structure and Function
Sperm
- Small and motile, with many mitochondria and an acrosome of enzymes, adapted to reach and penetrate the egg.
Egg
- Large with cytoplasmic food reserves, a zona pellucida and cortical granules, adapted to nourish the embryo.
Polar bodies
- Small cells with little cytoplasm that disintegrate, and they allow the egg to keep most nutrients.
Section 5 of 5Exam Checklist & Tips
4.7.9 – Key Points for the Exam
- Spermatogenesis The production of sperm in the testis.
- Oogenesis The production of eggs in the ovary.
- Polar body A small cell produced during oogenesis that contains a nucleus but little cytoplasm.
- Primary oocyte A diploid cell in the ovary that begins meiosis I.
Common Mistakes
- Saying four eggs are produced from each primary oocyte.
- Stating that oocytes finish meiosis before birth.
- Claiming that meiosis II of the egg is completed at ovulation; it is completed at fertilisation.
- Forgetting that spermatogonia and oogonia multiply by mitosis.
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