Unit 7
Development Biology
Chapter 7.1
GametogenesisClass 12 Biology – Gametogenesis Notes PDF
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Introduction
In males, gamete formation is called spermatogenesis; in females it is called oogenesis. Both processes reduce chromosome number from diploid to haploid and prepare specialized cells for fertilization.
1. Spermatogenesis
Spermatogenesis occurs in the seminiferous tubules of the testes. It begins with diploid spermatogonia and ends with formation of haploid spermatozoa.
Main Stages
- Multiplication phase: spermatogonia divide mitotically.
- Growth phase: selected spermatogonia enlarge and become primary spermatocytes.
- Maturation phase: primary spermatocytes undergo meiosis I to form secondary spermatocytes; meiosis II produces spermatids.
- Spermiogenesis: spermatids differentiate into spermatozoa.
Fig. 1 – Stages of Spermatogenesis
2. Structure of Human Sperm
A sperm is a highly specialized motile male gamete. It consists of head, neck, middle piece and tail.
Fig. 2 – Labelled Structure of a Sperm
3. Oogenesis
Oogenesis is the formation of the female gamete in the ovaries. Unlike spermatogenesis, meiotic divisions are unequal and most cytoplasm is retained in one large cell.
Main Stages
- Multiplication: oogonia divide mitotically during early development.
- Growth: oogonia enlarge to form primary oocytes.
- Maturation: meiosis produces a large secondary oocyte and small polar body; completion of meiosis ultimately yields one functional ovum and polar bodies.
Fig. 3 – Stages of Oogenesis
4. Structure of the Ovum
Fig. 4 – Simplified Human Ovum
5. Spermatogenesis vs Oogenesis
| Feature | Spermatogenesis | Oogenesis |
|---|---|---|
| Site | Testes | Ovaries |
| Functional products from one primary cell | Four spermatozoa | One ovum |
| Cytokinesis | Approximately equal | Unequal |
| Gamete | Small and motile | Large and non-motile |
| Cytoplasm | Very little | Abundant |
Fig. 5 – Outcome of Male and Female Gametogenesis
6. Quick Revision & Exam Points
- Define gametogenesis.
- Explain the stages of spermatogenesis.
- Draw and label a sperm.
- Explain the stages of oogenesis.
- Draw and label an ovum.
- Differentiate spermatogenesis and oogenesis.
- What is spermiogenesis?
- Why does oogenesis produce only one functional ovum?
- Gametogenesis forms haploid gametes.
- Spermatogenesis occurs in testes.
- Oogenesis occurs in ovaries.
- Meiosis reduces chromosome number.
- One primary spermatocyte → four sperm.
- One primary oocyte → one functional ovum.
- Spermiogenesis = spermatid to spermatozoon.
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