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B12.2 Cell Division in Sexual Reproduction

GCSE Combined Science · AQA · Chapter B12: Reproduction

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: meiosis, haploid gametes, fertilisation and where variation comes from. 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 steps of meiosis and say where it happens
  • Describe what happens at and after fertilisation
  • Compare mitosis and meiosis
  • Explain how meiosis and random fertilisation produce variation

Key words

  • Diploid: A cell with the full number of chromosomes, in pairs (46 in human body cells).
  • Haploid: A cell with half the number of chromosomes, one from each pair (23 in human gametes).
  • Meiosis: Cell division in the reproductive organs that makes four genetically different haploid gametes.
  • Fertilisation: Fusion of a male and a female gamete. It restores the full (diploid) 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 are diploid: they have 46 chromosomes in 23 pairs, one of each pair from each parent. If gametes also had 46, a zygote would have 92, and the number would double every generation.

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

  • Human; Body cell (diploid) 46; Gamete (haploid) 23
  • Cat; Body cell (diploid) 38; Gamete (haploid) 19
  • Horse; Body cell (diploid) 64; Gamete (haploid) 32
  • Pea plant; Body cell (diploid) 14; Gamete (haploid) 7

Worked example: chromosome numbers

A goat's muscle cell has 60 chromosomes. How many chromosomes are in a goat egg cell, and in a goat zygote?
Egg (gamete): half of 60 = 30. Zygote: 30 from the sperm + 30 from the egg = 60.

Answer: egg 30; zygote 60

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, so each one becomes an X shape.
  • 2. First division: the chromosome pairs line up and separate. Each new cell gets one chromosome from each pair.
  • 3. Second division: the copies are pulled apart and each cell divides again.
  • 4. Result: four haploid gametes, all 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 diploid zygote. The zygote divides by mitosis again and again, so the number of cells increases and an embryo forms. As the embryo develops, its cells differentiate into different types of specialised cell.

  • 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 diploid cells; Meiosis four haploid cells
  • Genetically; Mitosis identical to the parent cell; Meiosis different from each other

Where variation comes from

In meiosis, which chromosome of each pair goes into a gamete is random. So every gamete gets a different mix. Then fertilisation is random too: any sperm can fuse with any egg.

Worked example: how many mixes? (maths skill)

A cell with n pairs of chromosomes can make 2ⁿ different combinations in its gametes.
A pea plant has 7 pairs: 2⁷ = 128 combinations.
A human has 23 pairs: 2²³ = 8 388 608, about 8.4 million, before counting which sperm meets which egg.

Answer: 128 for a pea plant; 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.
  • Mixing up haploid and diploid. Gametes are haploid; body cells and the zygote are diploid.

Revision questions

Try these, then check your answers below each one.

Question 1. A horse body cell has 64 chromosomes. How many chromosomes are there in a horse egg cell?

Answer: 32

Working: Gametes are haploid, so they have half the number: 64 ÷ 2 = 32.

Question 2. The diagram shows a body cell with two pairs of chromosomes, and three other cells. Which cell, A, B or C, could be a gamete made from the body cell by meiosis? (Choose from: A, B, C.)

Answer: A

Working: Cell A has one long and one short chromosome: one from each pair. B has both long chromosomes (two from one pair, none from the other), and C still has all four.

Question 3. True or false? After fertilisation, the cells of a growing embryo divide by meiosis.

Answer: False

Working: An embryo grows by mitosis, which makes identical diploid cells. Meiosis only makes gametes.

Related lessons

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This lesson is section B12.2 (AGCSC1202) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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