Biology · Reproduction, Meiosis and Variation
Meiosis and Gametes
Your body cells have 46 chromosomes, but a sperm or an egg has only 23. This lesson explains how a special type of cell division, meiosis, halves the chromosome number and makes every gamete genetically different.
Learning Objectives
- 1Explain the difference between a diploid body cell and a haploid gamete.
- 2Describe meiosis as a cell division that produces four genetically different haploid gametes.
- 3Explain how fertilisation restores the diploid number.
- 4Compare meiosis and mitosis.
Retrieval practice
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1
How many chromosomes are in a human body cell?
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46, in 23 pairs.
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2
What type of cell division produces two genetically identical cells?
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Mitosis.
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3
What is the nucleus?
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The part of a cell that contains the chromosomes and controls its activities.
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4
What do we call a cell that has not yet specialised?
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A stem cell (undifferentiated).
Cells that are not like the others
Almost all of the cells in your body divide by mitosis, to give identical copies. The cells that make the next generation are different. The sex cells, or gametes, are made in the reproductive organs: sperm in the testes and eggs in the ovaries in humans, and pollen and ovules in plants. A gamete has to carry half of the genetic information of its parent, so that when two gametes join, the new organism has a complete set. The type of cell division that makes gametes is called meiosis.
Diploid and haploid cells
Two terms describe how many sets of chromosomes a cell has.
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Diploid
A cell with two sets of chromosomes, one from each parent, which are in pairs. All human body cells are diploid, with 46 chromosomes.
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Haploid
A cell with only one set of chromosomes. Human gametes are haploid, with 23 chromosomes.
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Why halve the number
If gametes were diploid, the number of chromosomes would double with every generation. Halving them keeps the number steady.
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Zygote
The cell formed when a sperm and an egg fuse at fertilisation. It is diploid again, with 46 chromosomes, and it divides by mitosis to form an embryo.
What happens in meiosis
You do not need the named stages of meiosis, but you must know the main events and the result.
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1
Start
A diploid cell in the ovary or testis has pairs of chromosomes.
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2
Copying
The DNA is copied, so each chromosome has two identical copies.
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3
First division
The chromosome pairs separate, and the cell divides into two. Each new cell has one chromosome of each pair, in a random mix.
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4
Second division
The copies of each chromosome separate and the cells divide again.
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5
Result
Four haploid cells are formed, each with half the number of chromosomes, and all genetically different.
Meiosis: four different gametes from one cell
Meiosis makes four cells, each with half the chromosomes and a different mix of genes.
Why meiosis makes variation
The four gametes from one cell are not identical to each other or to the parent cell.
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Random mixing
In the first division each pair of chromosomes splits at random, so each gamete gets a different mixture of the mother's and the father's chromosomes.
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Huge number of combinations
With 23 pairs of chromosomes, one person can make over 8 million different gametes, since \(2^{23} = 8\,388\,608\).
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Fertilisation adds to it
Which sperm fertilises which egg is also random, so the number of possible children from one couple is enormous.
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Genetically different
The result is that gametes, and the children made from them, are genetically different from each other.
Mitosis and meiosis
Mitosis
- Makes two daughter cells
- Daughter cells have the same number of chromosomes as the parent (diploid)
- Daughter cells are genetically identical
- Used for growth, repair and asexual reproduction
Meiosis
- Makes four daughter cells
- Daughter cells have half the number of chromosomes (haploid)
- Daughter cells are genetically different
- Used to make gametes, in the ovaries and testes
Chromosome numbers in a life cycle
A dog body cell has 78 chromosomes. How many chromosomes are in a dog sperm cell, and how many are in the zygote after fertilisation?
Show the solutionHide the solution
- 1 Gametes are haploid They have half the number of chromosomes of a body cell.
- 2 Halve it \(78 \div 2 = 39\) chromosomes in a sperm cell.
- 3 Fertilisation The nucleus of a sperm (39) fuses with the nucleus of an egg (39), so \(39 + 39 = 78\) chromosomes in the zygote.
AnswerA sperm cell has 39 chromosomes, and the zygote has 78.
From gametes to a new organism
Meiosis is only one step in the life of a sexually reproducing organism.
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Gametes made
Meiosis in the reproductive organs makes haploid gametes.
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Fertilisation
A male gamete fuses with a female gamete, producing a diploid zygote.
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Mitosis
The zygote divides by mitosis many times to form an embryo.
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Growth and differentiation
The embryo grows by mitosis, and its cells differentiate to make a whole organism.
Case study
Why brothers and sisters are not identical
Two children of the same parents can be very different, even though they inherit their genes from the same two people. Each egg and each sperm is a different mix of the parents' chromosomes, and any one of millions of sperm could fertilise any one of the eggs. The number of possible combinations is about 70 trillion for one couple. The exception is identical twins, which come from one fertilised egg that splits in two, so they share the same genes.
Summary and exam focus
- Meiosis makes four genetically different haploid gametes, each with half the number of chromosomes.
- Gametes are made in the reproductive organs: the ovaries and testes in humans.
- Fertilisation fuses two haploid gametes, to form a diploid zygote.
- Mitosis makes two identical diploid cells, for growth and repair.
- Meiosis and fertilisation both produce genetic variation.
Exam focus
Describe how meiosis differs from mitosis, and explain why meiosis is important in sexual reproduction. (6 marks) (6 marks)
Compare like with like, using the same features for both: number of cells made, chromosome number and genetic makeup. Then explain the importance: haploid gametes keep the chromosome number constant after fertilisation, and the variation gives a mixture of different offspring.
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Meiosis
- A type of cell division that produces four genetically different haploid gametes.
- Gamete
- A sex cell, such as a sperm or an egg, with half the number of chromosomes of a body cell.
- Haploid
- Having one set of chromosomes, as in a gamete.
- Diploid
- Having two sets of chromosomes, as in a body cell.
- Fertilisation
- The fusion of a male gamete and a female gamete.
- Zygote
- The cell formed when two gametes fuse. It is diploid.
- Sperm
- The male gamete in animals.
- Egg cell
- The female gamete in animals.
- Genetic variation
- Differences in the genes of individuals of the same species.
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