A nuclear division in which one diploid cell gives usually four haploid cells that are genetically different from each other.

How many nuclear divisions are there in meiosis, and how many cells result?

Two nuclear divisions, giving usually four cells from one parent cell.

What happens in meiosis I?

Homologous chromosomes pair up and separate into two cells, so the chromosome number is halved.

What happens in meiosis II?

Sister chromatids of each chromosome separate, giving four haploid cells.

Why is meiosis I called the reduction division?

Because the chromosome number is halved from diploid to haploid when homologous chromosomes separate.

What does homologous mean?

Chromosomes of a pair, one maternal and one paternal, that carry the same genes in the same positions.

Why must gametes be haploid?

So that fertilisation restores the diploid number; otherwise the chromosome number would double each generation.

A parent cell has 2n = 10. How many chromosomes are in each cell after meiosis I and after meiosis II?

Five in each cell after both divisions (two cells after I, four cells after II).

What is the difference between a chromosome after meiosis I and after meiosis II?

After meiosis I each chromosome still has two chromatids; after meiosis II each is a single chromatid.

State two differences between the outcomes of mitosis and meiosis.

Mitosis: two diploid, identical cells. Meiosis: four haploid cells that are genetically different from each other.

How can you recognise meiosis in an unfamiliar life cycle?

Look for the step where the chromosome number halves from diploid to haploid.

In a life cycle, how are gametes made if the parent organism is haploid?

By mitosis, because the cells are already haploid.

MeiosisAQA A-Level Biology: Flashcards

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What is meiosis?

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What is meiosis?: A nuclear division in which one diploid cell gives usually four haploid cells that are genetically different from each other.
How many nuclear divisions are there in meiosis, and how many cells result?: Two nuclear divisions, giving usually four cells from one parent cell.
What happens in meiosis I?: Homologous chromosomes pair up and separate into two cells, so the chromosome number is halved.
What happens in meiosis II?: Sister chromatids of each chromosome separate, giving four haploid cells.
Why is meiosis I called the reduction division?: Because the chromosome number is halved from diploid to haploid when homologous chromosomes separate.
What does homologous mean?: Chromosomes of a pair, one maternal and one paternal, that carry the same genes in the same positions.
Why must gametes be haploid?: So that fertilisation restores the diploid number; otherwise the chromosome number would double each generation.
A parent cell has 2n = 10. How many chromosomes are in each cell after meiosis I and after meiosis II?: Five in each cell after both divisions (two cells after I, four cells after II).
What is the difference between a chromosome after meiosis I and after meiosis II?: After meiosis I each chromosome still has two chromatids; after meiosis II each is a single chromatid.
State two differences between the outcomes of mitosis and meiosis.: Mitosis: two diploid, identical cells. Meiosis: four haploid cells that are genetically different from each other.
How can you recognise meiosis in an unfamiliar life cycle?: Look for the step where the chromosome number halves from diploid to haploid.
In a life cycle, how are gametes made if the parent organism is haploid?: By mitosis, because the cells are already haploid.

Exam questions on Meiosis

A grasshopper has 24 chromosomes in each of its body cells. Sperm cells are produced in the testes by meiosis, starting from diploid cells that have already replicated their DNA.
Explain why the first division of meiosis is known as a reduction division.2 marks
In a species of seaweed, the main plant body is haploid. It releases gametes, and two gametes fuse to form a diploid zygote. The zygote grows by mitosis into a diploid plant body, which releases spores. Each spore grows by mitosis into a new haploid plant body. Cells of the diploid plant body contain 18 chromosomes.
State the number of chromosomes in a spore of this seaweed and explain how this number is produced from the diploid plant body.2 marks
A biologist studies cells in the testis of a mouse, where each body cell has 40 chromosomes. Some testis cells divide by mitosis to produce more of the same cell type, while others divide by meiosis to form sperm.
Describe what happens to the chromosomes in the first division of meiosis and how this differs from what happens in the second division.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).