13.2 Cell Division in Sexual Reproduction

A free GCSE Biology lesson for AQA (8461) from OBA School: meiosis, fertilisation, chromosome numbers and variation. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain why gametes must have half the number of chromosomes of body cells
• Describe the stages of meiosis and where it happens
• Describe what happens at and after fertilisation
• Compare mitosis and meiosis
• Explain how meiosis and fertilisation lead to variation

Key words

• Chromosome: A long, coiled molecule of DNA carrying many genes. Found in the nucleus.
• Chromosome pair: Body cells hold chromosomes in pairs: one of each pair came from each parent.
• Meiosis: Cell division in the reproductive organs that makes four genetically different gametes with a single set of chromosomes.
• Fertilisation: Fusion of a male and a female gamete. It restores the full number of chromosomes.
• Zygote: The single cell formed at fertilisation (a fertilised egg).
• Embryo: The ball of cells that develops from the zygote by mitosis.

Halving the chromosome number

Human body cells have 46 chromosomes in 23 pairs. If gametes also had 46, a fertilised egg would have 92, and the number would double every generation.

So gametes have one chromosome from each pair: 23 in human sperm and eggs. At fertilisation 23 + 23 = 46, and the normal number is restored.

• Human; Body cell 46; Gamete 23
• Dog; Body cell 78; Gamete 39
• Chimpanzee; Body cell 48; Gamete 24
• Fruit fly; Body cell 8; Gamete 4

Worked example: chromosome numbers

A rabbit's skin cell has 44 chromosomes. How many chromosomes are in a rabbit sperm cell, and in a fertilised rabbit egg?
Sperm (gamete): half of 44 = 22. Fertilised egg: 22 from the sperm + 22 from the egg = 44.

Answer: sperm 22; fertilised egg 44

How meiosis works

Meiosis happens only in the reproductive organs: the testes and ovaries in animals, and the anthers and ovaries of flowers in plants.

• 1. Copy: the cell makes a copy of each chromosome (the genetic material is copied).
• 2. First division: the chromosome pairs separate. Each new cell gets one chromosome from each pair.
• 3. Second division: the copies separate. The cell divides again.
• 4. Result: four gametes, each with a single set of chromosomes. All of them are genetically different from each other.

Exam tip: Meiosis: one copy of the chromosomes, two divisions, four different cells, half the chromosome number.

Fertilisation and development

At fertilisation the gametes fuse to make a zygote with the normal number of chromosomes. The zygote divides by mitosis again and again. The number of cells increases and an embryo forms. As the embryo develops, its cells differentiate into specialised cells.

Exam tip: Gametes are made by meiosis; everything after fertilisation (growth of the embryo, then growth and repair) uses mitosis.

Mitosis or meiosis? And where variation comes from

• Where; Mitosis all over the body; Meiosis reproductive organs only
• Used for; Mitosis growth, repair, asexual reproduction; Meiosis making gametes
• Divisions; Mitosis one; Meiosis two
• Cells made; Mitosis two; Meiosis four
• Chromosomes; Mitosis full number (pairs); Meiosis half number (one of each pair)
• Genetically; Mitosis identical to the parent cell; Meiosis different from each other

Variation comes from two places: meiosis shuffles the chromosomes, so each gamete gets a different mix, and fertilisation is random: any sperm can fuse with any egg.

Worked example: how many mixes? (maths skill)

In meiosis each pair can go either way, so a cell with n pairs can make 2ⁿ different combinations of chromosomes.
A fruit fly has 4 pairs: 2⁴ = 16 combinations.
A human has 23 pairs: 2²³ = 8 388 608, about 8.4 million combinations, before even counting which sperm meets which egg.

Answer: 16 for a fruit fly; about 8.4 × 10⁶ for a human

Common mistakes

• Writing that meiosis makes two cells. It makes four; mitosis makes two.
• Saying gametes have 23 pairs of chromosomes. They have 23 single chromosomes.
• Thinking the embryo grows by meiosis. After fertilisation, cells divide by mitosis.
• Spelling: mitosis and meiosis look alike. Link meiosis with making eggs (and sperm).

Revision questions

Try these, then check your answers below each one.

Question 1. A dog body cell has 78 chromosomes. How many chromosomes are there in a dog sperm cell?

Answer: 39

Working: Gametes have half the number: 78 ÷ 2 = 39.

Question 2. How many cells are made when one cell divides by meiosis? (Choose from: 1, 2, 4, 8.)

Answer: 4

Working: The genetic material is copied once and the cell divides twice, giving four gametes.

Question 3. True or false? The cells of a growing embryo divide by meiosis.

Answer: False

Working: After fertilisation the zygote and embryo divide by mitosis. Meiosis only makes gametes.

Related lessons

• Previous lesson: 13.1 Types of Reproduction
• Next lesson: 13.3 The Best of Both Worlds
• In this chapter: 13.4 DNA and the Genome
• In this chapter: 13.5 DNA Structure and Protein Synthesis
• In this chapter: 13.6 Gene Expression and Mutation

Continue in the full course

This lesson is section 13.2 (AGCBH1302) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.