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4.3.3Genetics, Variation and Interdependence

Meiosis — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 18 of 46 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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4.3.3 Meiosis — AQA A Level Biology 7402 notes: Describe the stages of meiosis and explain how it produces haploid cells that differ genetically from each other.Meiosis is the type of nuclear division that produces four genetically different haploid cells from one diploid cell.It…

  • Prophase I
  • Metaphase I
  • independent assortment
  • Anaphase I
  • Telophase I
  • Crossing over

Section 1 of 9The role of meiosis

Meiosis is the type of nuclear division that produces four genetically different haploid cells from one diploid cell.

The role of meiosis
It happens in the reproductive organs (testes, ovaries, anthers and ovules) and produces gametes in animals.
If gametes were made by mitosis, the chromosome number would double at each generation,
  • so meiosis halves the number and fusion at fertilisation restores the diploid number.
Meiosis also creates variation, which provides the material for natural selection.

It consists of two divisions, meiosis I and meiosis II, preceded by a single replication of DNA in interphase, so that each chromosome consists of two sister chromatids.

Section 2 of 9Meiosis I: separation of homologous chromosomes

1Prophase I
Prophase I chromosomes condense and the nuclear envelope breaks down.
  • Homologous chromosomes pair up to form bivalents.
  • Non-sister chromatids cross over at points called chiasmata, exchanging sections of DNA.
2Metaphase I
Metaphase I bivalents line up on the equator of the cell.
  • The orientation of each pair is random (independent assortment), determining which chromosome of each pair goes to which pole.
3Anaphase I
Anaphase I the spindle fibres pull the homologous chromosomes apart to opposite poles; the centromeres do not divide.
4Telophase I
Telophase I the nuclear envelopes may reform and the cell divides (cytokinesis),
  • giving two haploid cells, each with chromosomes still consisting of two chromatids.

Section 3 of 9Meiosis II: separation of chromatids

1Prophase II
Prophase II a new spindle forms.
2Metaphase II
Metaphase II chromosomes line up singly on the equator.
3Anaphase II
Anaphase II centromeres divide and the chromatids are pulled to opposite poles. Because of crossing over, the chromatids are no longer identical.
4Telophase II
Telophase II nuclear envelopes form around four haploid nuclei, and cytokinesis gives four haploid cells.

In humans, meiosis I and II in males gives four sperm, whereas in females only one egg cell results from each meiosis, together with polar bodies that degenerate.

Section 4 of 9How meiosis produces genetic variation

Crossing over
Crossing over in prophase I, non-sister chromatids of a bivalent break and rejoin at the same places, exchanging alleles.
  • This produces new combinations of alleles on each chromatid.
Independent assortment
Independent assortment homologous pairs line up randomly in metaphase I,
  • so the maternal and paternal chromosomes are distributed into gametes in many combinations.
  • For n pairs of chromosomes there are 2n possible combinations;
    • for humans, with 23 pairs, that is over eight million (223 = 8,388,608) combinations in the gametes without counting crossing over.
Random fertilisation
Random fertilisation any male gamete can fuse with any female gamete, so the number of possible zygotes is the product of the possibilities.

Combined, these produce enormous genetic diversity within a species.

Section 5 of 9Links

Meiosis is linked to the inheritance of alleles, to dihybrid crosses (independent assortment), to linkage (alleles on the same chromosome stay together unless crossing over separates them), and to chromosome mutations when it goes wrong. Make sure that you can describe what happens to chromosome number and DNA content at each stage.

Section 6 of 9Following chromosome and DNA numbers through meiosis

A common exam task is to follow the numbers through the stages of meiosis.

1
Take a cell with 2n = 6. During the S phase of interphase, the DNA content doubles, but the number of chromosomes remains 6, each with two chromatids.
2
After meiosis I, each cell has 3 chromosomes, each with two chromatids, so the DNA content per cell is half that of the original.
3
After meiosis II, each of the four cells has 3 chromosomes, each with a single chromatid,
  • and the DNA content is a quarter of that at the start of meiosis.
    • (after replication)
4
If you are given a graph of DNA content against time, it shows a doubling in S phase, a first halving at the end of meiosis I and a second halving at the end of meiosis II.
5
In the case of mitosis, there is only one halving.

You should also be able to explain why the daughter cells from meiosis I are already haploid even though each chromosome contains two chromatids. It is the number of centromeres, i.e. the number of chromosomes, that determines the chromosome number, not the number of chromatids.

Remember, too, that if two genes are on different chromosomes then alleles are shuffled by independent assortment, whereas crossing over breaks up combinations of linked alleles. Combining these ideas, you can explain why siblings, apart from identical twins, are not genetically identical, even though they have the same parents.

Section 7 of 9Exam-style questions

  1. Describe how crossing over leads to genetic variation. [3]3 marks

  2. A species has 2n = 8. Calculate the number of different combinations of chromosomes possible in gametes from independent assortment alone. [2]2 marks

  3. Explain why meiosis I is called the reduction division. [2]2 marks

Section 8 of 9Key terms

TermMeaning
MeiosisCell division that produces four haploid genetically different cells.
BivalentA pair of homologous chromosomes held together in prophase I.
ChiasmaA point where non-sister chromatids have crossed over.
Independent assortmentRandom orientation of homologous pairs at metaphase I.
HaploidContaining one set of chromosomes.
GameteA haploid sex cell.
Crossing overExchange of DNA sections between non-sister chromatids.

Section 9 of 9Exam technique

1
Saying that the chromatids separate in anaphase I. It is the homologous chromosomes that separate in meiosis I.
2
Confusing crossing over (within a bivalent, in prophase I) with independent assortment (metaphase I).
3
Writing that the daughter cells from meiosis are identical.
4
Stating that meiosis I results in four cells.
5
Forgetting that DNA replication takes place before meiosis, in interphase, and not between the two divisions.

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