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

Sexual reproduction is the production of new organisms from the fusion of a male gamete and a female gamete.

Behavioural objectives

1. Describe how sexual reproduction takes place.
2. Compare the egg cell and the sperm.
3. State the two types of sexual reproduction and where each occurs.
4. Describe conjugation in spirogyra.
5. Explain why sexual reproduction produces variation.
6. List differences between asexual and sexual reproduction.

What are the key terms in sexual reproduction?

These nine terms are used throughout the topic.

• Gamete: a sex cell, either male (sperm, pollen nucleus) or female (egg cell).
• Diploid: having two sets of chromosomes, one from each parent.
• Haploid: having one set of chromosomes, as in gametes.
• Meiosis: cell division that halves the chromosome number from diploid to haploid.
• Mitosis: cell division that gives two identical cells with the same chromosome number as the parent cell.
• Fertilisation: the fusion of a male gamete and a female gamete.
• Zygote: the diploid cell formed when two gametes fuse.
• Embryo: the young organism in its early stage of development from the zygote.
• Differentiation: the change of cells so that they become specialised for different jobs.

How does sexual reproduction take place?

Sexual reproduction takes place when a haploid male gamete fuses with a haploid female gamete to form a diploid zygote.

• Two gametes: a male gamete and a female gamete are needed. In humans and goats they come from two different parents.
• Plants: in many flowering plants, both kinds of gamete are made in the same flower or on the same plant.
• Gamete formation: the male parent makes male gametes and the female parent makes female gametes.
• Meiosis: it takes place when gametes form in most animals and flowering plants.
• Halving of chromosomes: gametes are haploid, so fusion restores the diploid number in the zygote.
• Result: the zygote divides and develops into a young organism similar to its parents.

How does the egg cell differ from the sperm?

• Egg cell: the female gamete. It is large, cannot move and carries food reserves.
• Sperm: the male gamete. It is small, swims with a tail and carries very little food.

What happens after fertilisation?

After fertilisation, the zygote divides repeatedly and develops into a young organism.

1. The two haploid gametes (male and female) fuse during fertilisation to form a diploid zygote.
2. The zygote undergoes repeated cell division (mitosis) and forms an embryo.
3. The embryo continues to divide and its cells differentiate.
4. The embryo develops into a young organism similar to the parents.

What are the types of sexual reproduction?

There are two types of sexual reproduction: conjugation and fertilisation.

What is conjugation in paramecium?

Conjugation in paramecium is the temporary joining of two individuals to exchange nuclear material.

1. Two paramecia come together and join at their mouth grooves.
2. They exchange nuclear material through the joined region.
3. They separate, and each now has a new combination of genetic material.

• Number of individuals: two paramecia join and two separate, so no extra individual is made at this stage.

What is conjugation in spirogyra?

Conjugation in spirogyra is the fusion of gametes from cells of two neighbouring filaments through a conjugation tube.

• Spirogyra: a green alga made of long, thread-like filaments.
• Conjugation tube: a channel formed between two cells lying opposite each other.
• Zygospore: the thick-walled zygote formed after the gametes fuse.

1. Two filaments lie close to each other.
2. Outgrowths appear on the walls of the cells lying opposite one another.
3. The outgrowths meet, their walls break and a conjugation tube forms.
4. The gamete from one cell passes through the tube into the other cell.
5. The two gametes merge and their nuclei unite to form a zygospore.
6. The zygospore develops a thick wall and can withstand dry conditions.
7. When moisture is available, the zygospore germinates into a new individual.

• Chromosome number: spirogyra cells are haploid, so the zygospore undergoes meiosis when it germinates.
• Other organisms: conjugation also occurs in mucor, a fungus that grows on stale bread.

What are the types of fertilisation in animals?

There are two types of fertilisation in animals: external fertilisation and internal fertilisation.

• External fertilisation: the gametes meet outside the body, in water, as in fish and frogs.
• Internal fertilisation: the male gamete enters the female’s body and fuses with the egg there, as in humans, birds and goats.

How does fertilisation take place in flowering plants?

In flowering plants, a male nucleus from a pollen grain fuses with the egg cell in the ovule.

1. Pollen grains are carried to the stigma (pollination).
2. A pollen tube grows down from the pollen grain.
3. A male nucleus travels down the pollen tube and fuses with the egg cell in the ovule.
4. The fertilised ovule becomes a seed.
5. The ovary becomes the fruit.

Why does sexual reproduction produce variation?

Sexual reproduction produces variation because each offspring receives a new mixture of genes from two parents.

• Two sets of genes: each offspring receives half of its chromosomes from each parent, so it carries a mixture of characters.
• Meiosis: it shuffles the genes, so no two gametes from the same parent are exactly alike.
• Random fusion: any male gamete may fuse with any female gamete, which gives many possible combinations.
• Value to the species: some individuals are likely to survive when the environment changes, for example in disease outbreaks or drought.

What are the advantages and disadvantages of sexual reproduction?

Sexual reproduction gives variation and better survival, but it is slower and costs more energy.

Advantages

• Variation: offspring differ from one another and from their parents.
• Survival: some offspring are likely to cope with changes in the environment.
• Adaptation: useful new combinations of characters can build up in a species over time.

Disadvantages

• Slower: offspring are produced at a slower rate than in asexual reproduction.
• Gametes needed: male and female gametes must meet, and many gametes are wasted.
• Energy cost: making gametes and finding a mate uses a lot of energy.

What are the differences between asexual and sexual reproduction?

• Asexual reproduction: the production of new organisms from one parent without gametes or fertilisation.
• Sexual reproduction: the production of new organisms from the fusion of male and female gametes.

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, and explain why the offspring of sexual reproduction show genetic variation.