Reproduction: How Life Continues · Lesson 8 of 12
How Reproductive Cells Are Made
“Meiosis carefully halves the chromosomes so fertilisation does not double the paperwork.”
• Define gametogenesis and connect it with meiosis. • Compare sperm and egg in size, number, nutrients and movement. • Explain ovulation and fertilisation in humans. • Distinguish fertilisation from implantation. • Describe the basic sequence of in-vitro fertilisation.
The male and female gametes carry the same number of chromosomes, but they are built very differently. One is small, mobile and produced in enormous numbers. The other is large, non-motile and contains stored nutrients. Their differences suit the parts they play before and during fertilisation.
Gametogenesis is the process by which gametes are formed in the testes and ovaries.
Gametes are produced through meiosis. Human body cells contain 46 chromosomes, but sperm and eggs contain 23. Reducing the number is essential because the sperm and egg later combine. Their 23 + 23 chromosomes restore the diploid number of 46 in the zygote.
| Feature | Sperm | Egg |
|---|---|---|
| Size | Very small | Large |
| Number produced | Millions | Few |
| Stored nutrients | Absent | Present |
| Movement | Actively motile | Non-motile |
| Chromosome number | 23 | 23 |
A sperm’s tail supports movement towards the egg, while its head carries genetic material. The egg does not swim through the tract; movement of the reproductive organs carries it. Its larger size and stored material support the beginning of development.
What happens when a sperm meets an egg?
Ovulation is the release of a mature egg from an ovary.
From puberty onwards, usually one mature egg is released during a cycle. Before release, the uterine lining begins thickening. The egg enters an oviduct. If sperm are present, some may travel through the vagina and uterus and reach the oviduct.
When one sperm fuses with the egg, a zygote forms. Fertilisation normally occurs in the oviduct, not in the uterus. The zygote begins mitotic divisions while moving towards the uterus. It becomes a growing group of cells without changing the chromosome number in each new cell.
Implantation is the attachment of the developing group of cells to the thickened inner lining of the uterus, where nourishment can be received.
Problem
A zygote has formed in an oviduct but has not yet attached to the uterus. Has implantation occurred?
- 1.Fusion of sperm and egg has occurred, so fertilisation is complete.
- 2.The zygote must still divide and travel towards the uterus.
- 3.Implantation requires attachment to the uterine lining.
- 4.Therefore, implantation has not yet occurred.
Problem
A sperm with 23 chromosomes fuses with an egg containing 23. What happens during later mitotic divisions?
- 1.The zygote begins with 46 chromosomes.
- 2.Before each mitotic division, genetic material is copied.
- 3.Daughter cells receive the same chromosome number.
- 4.The developing embryo therefore grows through mitosis while maintaining 46 chromosomes in its ordinary cells.
Meet a Scientist
In-vitro Fertilisation combines an egg and sperm outside the female body, usually in a laboratory dish. If fertilisation and early development occur, the resulting developing structure is placed in the uterus so that implantation may begin a pregnancy. The phrase “test tube baby” is informal because the important laboratory steps commonly occur in culture equipment rather than inside a literal test tube.
The chapter describes the pioneering work of Subhash Mukhopadhyay in Kolkata in 1978 and the birth of Kanupriya Agarwal, nicknamed Durga. The example connects knowledge of gametes, fertilisation and implantation with a medical method used when pregnancy cannot begin in the usual way.
Quiz
Which description best matches Gametogenesis?
Which description best matches Ovulation?
Which term matches this description: Gametogenesis is the process by which gametes are formed in the testes and ovaries.
Which term matches this description: Ovulation is the release of a mature egg from an ovary.
Which statement is a key takeaway from this lesson?
Practice Problems
- Define gametogenesis.
- Why must sperm and eggs contain only 23 chromosomes?
- Compare sperm and egg using four structural or functional features.
- What is ovulation?
- Where does human fertilisation usually occur?
- Distinguish fertilisation from implantation.
- Why does the zygote use mitosis rather than meiosis while developing?
- Outline the basic sequence of in-vitro fertilisation.
Key Takeaways
• Gametogenesis produces sperm in testes and eggs in ovaries. • Meiosis reduces the chromosome number to 23. • Fertilisation in the oviduct produces a 46-chromosome zygote. • Mitosis increases the number of cells. • Implantation occurs later in the uterine lining.