Meiosis and Fertilisation

1. 1 State [1 mark]

   State the number of chromosomes in a human egg cell.

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   Model answer

   23.

   Mark scheme

   • 23 — 1 mark
2. 2 Describe [3 marks]

   Describe what happens when a cell divides by meiosis to form gametes.

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   Model answer

   Copies of the genetic information are made. The cell then divides twice, to form four gametes. Each gamete has a single set of chromosomes, and all the gametes are genetically different from each other.

   Mark scheme

   • Copies of the genetic information are made — 1 mark
   • Divides twice, to make four gametes — 1 mark
   • Single set of chromosomes, all different — 1 mark
3. 3 Explain [3 marks]

   Explain why gametes must have half the number of chromosomes of a normal body cell.

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   Model answer

   When the gametes fuse at fertilisation, their chromosomes are added together. If they had the full number, the fertilised egg would have double the normal number. With half each, the fertilised egg has the normal number.

   Mark scheme

   • Chromosomes are added at fertilisation — 1 mark
   • Full number would double the number — 1 mark
   • Half each restores the normal number — 1 mark
4. 4 Calculate [2 marks]

   A body cell of a horse has 64 chromosomes. State the number of chromosomes in a horse sperm cell, and in a fertilised horse egg.

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   Model answer

   A sperm cell has 32 chromosomes, and a fertilised egg has 64.

   Mark scheme

   • 32 — 1 mark
   • 64 — 1 mark
5. 5 Describe [3 marks]

   Describe what happens to a fertilised egg cell as an embryo develops.

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   Model answer

   The fertilised egg cell divides by mitosis, so the number of cells increases and the cells are identical. As the embryo develops, the cells differentiate, which means that they become specialised.

   Mark scheme

   • Divides by mitosis — 1 mark
   • Number of cells increases — 1 mark
   • Cells differentiate — 1 mark
6. 6 Evaluate [6 marks]

   Explain how meiosis and fertilisation produce offspring that are genetically different from their parents, but that have the normal number of chromosomes.

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   Model answer

   In the reproductive organs, a cell divides by meiosis to form gametes. The cell makes copies of its genetic information, and divides twice, to form four gametes. Each gamete has a single set of chromosomes, which is half the normal number, and the gametes are all genetically different from each other. At fertilisation, a male and a female gamete fuse, and their chromosomes are added together, so the fertilised egg has the normal number of chromosomes. The fertilised egg has genetic information from both parents, and this is a new mixture, so the offspring is genetically different from each parent, which gives variety. The fertilised egg then divides by mitosis, and the cells differentiate as the embryo develops.

   Mark scheme

   • Meiosis forms gametes — 1 mark
   • Half the number of chromosomes — 1 mark
   • Gametes are genetically different — 1 mark
   • Fertilisation restores the normal number — 1 mark
   • Mixture of information from both parents, so variety — 1 mark
   • Mitosis and differentiation of the embryo — 1 mark

Quick check

1. 1

   How many cells are formed when one cell divides by meiosis?

   1. AOne
   2. BTwo
   3. CThree
   4. DFour
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   D: Four

   Four.

2. 2

   How many chromosomes are in a human sperm cell?

   1. A46
   2. B23
   3. C92
   4. D12
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   B: 23

   23.

3. 3

   What does fertilisation restore?

   1. AThe normal number of chromosomes
   2. BThe number of gametes
   3. CThe amount of cytoplasm
   4. DThe number of nuclei
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   A: The normal number of chromosomes

   The normal number of chromosomes.

4. 4

   What type of cell division follows fertilisation?

   1. AMeiosis
   2. BFertilisation
   3. CMitosis
   4. DDigestion
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   C: Mitosis

   Mitosis, which makes identical cells.

5. 5

   What happens to the cells of an embryo as it develops?

   1. AThey all stay the same
   2. BThey differentiate
   3. CThey turn into gametes
   4. DThey stop dividing straight away
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   B: They differentiate

   They differentiate.

6. 6

   Where does meiosis take place?

   1. AIn the skin
   2. BIn the liver
   3. CIn the brain
   4. DIn the reproductive organs
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   D: In the reproductive organs

   In the reproductive organs.

7. 7

   How do the four gametes made by one cell compare?

   1. AThey are all genetically different
   2. BThey are all identical
   3. CTwo are identical, two are different
   4. DThey are all clones
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   A: They are all genetically different

   They are all genetically different.

8. 8

   A cell has 20 chromosomes. How many are in each gamete made by meiosis?

   1. A20
   2. B5
   3. C10
   4. D40
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   C: 10

   Half of 20 is 10.

9. 9

   What is an embryo?

   1. AA type of gamete
   2. BA ball of cells formed from a fertilised egg
   3. CA sex chromosome
   4. DA type of hormone
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   B: A ball of cells formed from a fertilised egg

   A ball of cells formed by the division of the fertilised egg.