Biology · Reproduction and Meiosis
Meiosis and Fertilisation
A human sperm cell and a human egg cell each have only half of the chromosomes of an ordinary body cell. This lesson explains how meiosis halves the number of chromosomes in the gametes, how fertilisation restores the full number, and how the new cell develops into an embryo.
Learning Objectives
- 1Describe how cells in the reproductive organs divide by meiosis to form gametes.
- 2Explain how meiosis halves the number of chromosomes in the gametes, and why all the gametes are genetically different.
- 3Explain how fertilisation restores the full number of chromosomes.
- 4Describe how the fertilised egg divides by mitosis, and how the cells of the embryo differentiate.
Retrieval practice
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1
How many pairs of chromosomes are in a human body cell?
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23 pairs, which is 46 chromosomes.
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2
What is a gamete?
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A sex cell, such as a sperm cell or an egg cell.
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3
What is mitosis?
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Cell division that forms two genetically identical cells.
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4
What is a stem cell?
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An undifferentiated cell that can divide and form other types of cell.
Why a special division is needed
Most of the cells in your body have 23 pairs of chromosomes, which is 46 in all. If a sperm cell and an egg cell also had 46 each, the fertilised egg would have 92 chromosomes, and the number would double in every generation. This does not happen, because the gametes are made by meiosis, which halves the number of chromosomes. Each gamete has a single set of 23 chromosomes. When the gametes fuse at fertilisation, the full number of 46 is restored. The specification does not require knowledge of the stages of meiosis, but it does require the main events, and the result.
The events of meiosis
Meiosis takes place in cells in the reproductive organs, such as the testes and the ovaries.
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Copies are made
The cell makes copies of its genetic information, so the amount of DNA doubles.
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The cell divides twice
There are two divisions, one after the other.
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Four gametes
The result is four gametes, each with a single set of chromosomes, which is half the number of the parent cell.
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All different
The gametes are genetically different from each other, because the chromosomes of each pair are shared out at random, and the genes can be mixed.
How meiosis forms gametes
Meiosis makes four genetically different gametes, with half the number of chromosomes.
Fertilisation and development
The gametes join to restore the normal number of chromosomes.
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Fertilisation
A sperm cell fuses with an egg cell, and the nuclei join. The new cell, with 23 chromosomes from each gamete, has 46 chromosomes, in 23 pairs. This restores the normal number.
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Mixing
The new cell has genetic information from both parents, and is different from each of them.
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Mitosis
The fertilised egg cell divides by mitosis, and the cells produced are identical. The number of cells increases.
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Embryo
The cells form a ball of cells, which is the early embryo.
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Differentiation
As the embryo develops, the cells differentiate, which means that they become specialised, with different structures, such as blood cells, muscle cells and nerve cells.
From gametes to an embryo
Fertilisation restores the full number of chromosomes, and the fertilised egg divides by mitosis.
Gametes and body cells
Gamete (sperm or egg)
- Made by meiosis
- Half the number of chromosomes
- 23 chromosomes in humans
- Genetically different from other gametes
Body cell
- Made by mitosis
- Full number of chromosomes
- 46 chromosomes in humans
- Genetically identical to the parent cell
Counting chromosomes
A cell in the testes of a fruit fly has 8 chromosomes. State the number of chromosomes in each of the gametes made from it, and in the fertilised egg that is formed when one of these sperm cells fuses with an egg cell.
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- 1 Meiosis It halves the number, so 8 divided by 2 is 4 chromosomes in each gamete.
- 2 The egg cell It is also made by meiosis, so it has 4 chromosomes as well.
- 3 Fertilisation The nuclei join, so the numbers add: 4 plus 4 is 8.
- 4 Check The full number, 8, is restored in the fertilised egg.
AnswerEach sperm cell has 4 chromosomes. An egg cell also has 4, so the fertilised egg has 4 + 4 = 8 chromosomes.
Case study
When the number goes wrong
Meiosis does not always work perfectly. Sometimes a pair of chromosomes fails to separate, so one gamete gets two copies of a chromosome, and another gets none. If a gamete with an extra chromosome 21 is fertilised, the child has three copies of chromosome 21, which causes Down's syndrome. This shows how important it is that meiosis halves the number correctly, and that each gamete has exactly one set of chromosomes. Down's syndrome is more likely when the mother is older, but it can happen at any age.
Summary and exam focus
- Cells in the reproductive organs divide by meiosis to form gametes.
- Meiosis makes four gametes, each with a single set of chromosomes.
- All the gametes are genetically different from each other.
- Fertilisation restores the normal number of chromosomes.
- The fertilised egg divides by mitosis, and the cells differentiate.
Exam focus
Explain how meiosis and fertilisation keep the number of chromosomes the same from one generation to the next. (4 marks) (4 marks)
Use numbers from humans: gametes have 23, the fertilised egg has 46. Say that meiosis halves the number, and fertilisation restores it.
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Meiosis
- Cell division in the reproductive organs that forms four genetically different gametes.
- Gamete
- A sex cell with half the number of chromosomes of a body cell.
- Fertilisation
- The fusion of the nuclei of a gamete from each parent.
- Chromosome
- A structure in the nucleus that is made of DNA, and carries genes.
- Embryo
- A ball of cells, formed by the division of the fertilised egg.
- Differentiate
- To become specialised for a particular job.
- Reproductive organs
- The organs that make gametes, such as the testes and ovaries.
- Mitosis
- Cell division that forms two genetically identical cells.
- Zygote
- The cell formed when a male gamete fuses with a female gamete.
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