Reproduction II: Sexual Reproduction – SS2 Biology Lesson Note (First Term)

Sexual reproduction is the production of new organisms from two parents of different sexes. Each parent produces gametes, and meiosis halves the chromosome number from diploid to haploid. A male and a female gamete fuse in fertilisation to form a diploid zygote, which divides and develops into a young organism similar to its parents.

Behavioural objectives

By the end of the lesson, students should be able to:

• describe how sexual reproduction takes place;
• state the two types of sexual reproduction and where each occurs;
• describe conjugation in spirogyra;
• list differences between asexual and sexual reproduction.

How does sexual reproduction take place?

Sexual reproduction involves two parents of different sexes (male and female). Each parent produces gametes: the male produces male gametes and the female produces female gametes. Meiosis takes place during the formation of gametes, and the number of chromosomes is halved, from diploid to haploid.

The egg cell (female gamete) is large and non-motile. The sperm (male gamete) is small and motile.

During fertilisation, the two haploid gametes (male and female) fuse to form a diploid zygote. The zygote undergoes repeated cell division and forms an embryo. The embryo undergoes repeated cell division and differentiation and develops into a young organism similar to the parents.

What are the types of sexual reproduction?

There are two types of sexual reproduction:

1. Conjugation, which occurs in paramecium and spirogyra.
2. Fertilisation, which occurs in higher plants and animals.

Conjugation in spirogyra

In spirogyra, two filaments lie close to each other and outgrowths appear on the walls of the cells lying opposite one another. The outgrowths meet and their walls break, and a conjugation tube is formed. One of the gametes passes through the conjugation tube and merges with the gamete in the other cell, and their nuclei unite. This results in the formation of a zygospore.

The zygospore can withstand dry conditions and can germinate into a new individual when moisture is available. This process of reproduction is called conjugation. It also occurs in mucor.

What are the differences between asexual and sexual reproduction?

Evaluation

1. What happens to the chromosome number during the formation of gametes?
2. Describe the egg cell and the sperm.
3. Name the two types of sexual reproduction and state where each occurs.
4. Describe how a zygospore is formed in spirogyra.
5. State three differences between asexual and sexual reproduction.

Assignment

1. Describe what happens from the fusion of gametes to the development of a young organism.
2. Explain why offspring from sexual reproduction show genetic variation, using the table above.