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Meiosis and chromosome mutationsEdexcel A-Level Biology B: Revision notes

Section 1

Meiosis and haploid gametes

Body cells are diploid (2n): they contain homologous pairs of chromosomes, one from each parent, carrying the same genes (but possibly different alleles). Gametes are haploid (n): they contain one chromosome from each pair.

Meiosis is a reduction division. DNA is replicated once, then the cell divides twice, giving four haploid cells. Halving the number is essential: at fertilisation two gametes fuse, so the diploid number is restored and does not double each generation.

Key termsdiploidhaploidhomologous pairmeiosis

Section 2

The stages of meiosis

Meiosis I (homologous chromosomes separate)

  • Prophase I: chromosomes condense; homologous chromosomes pair to form bivalents; crossing over occurs at chiasmata
  • Metaphase I: bivalents line up on the equator in random orientation
  • Anaphase I: homologous chromosomes are pulled to opposite poles
  • Telophase I: two haploid cells form (each chromosome still has two chromatids)

Meiosis II (chromatids separate, like mitosis)

  • Metaphase II: chromosomes line up on the equator
  • Anaphase II: centromeres divide; chromatids are pulled apart
  • Result: four haploid cells
Key termsbivalentchiasmaanaphase Ianaphase II
Common mistake

In anaphase I whole chromosomes (homologous pairs) separate. In anaphase II it is chromatids. Mixing these up is a very common error.

Section 3

Genetic variation from meiosis

Two processes produce variation among gametes:

  • Crossing over: in prophase I non-sister chromatids of a homologous pair break and exchange equivalent sections at chiasmata. This recombines alleles, giving chromatids with new combinations
  • Independent assortment: in metaphase I each bivalent lines up randomly, so the maternal or paternal chromosome of each pair can go to either pole. The number of combinations is 2ⁿ, where n is the haploid number. For humans, 2²³ = 8 388 608, before crossing over is considered
Key termscrossing overindependent assortmentrecombination
Exam tip

Always say 'non-sister chromatids' for crossing over, and 'random orientation of bivalents at metaphase I' for independent assortment.

Section 4

Chromosome mutations: translocation

A chromosome mutation is a change in the structure or number of chromosomes. In a translocation a section of one chromosome breaks off and attaches to a non-homologous chromosome. The number of chromosomes is unchanged, but genes are in new positions, which can alter gene expression or create harmful fusion genes.

Example: the Philadelphia chromosome, formed by a translocation between chromosomes 9 and 22, is found in chronic myeloid leukaemia. Because it arises in body cells, it is not inherited.

Key termschromosome mutationtranslocation

Section 5

Non-disjunction, polysomy and monosomy

Non-disjunction is the failure of homologous chromosomes (anaphase I) or sister chromatids (anaphase II) to separate in meiosis. Some gametes then have one extra chromosome (n + 1) and some one fewer (n − 1). If these fuse with a normal gamete:

  • Polysomy: an extra chromosome in the zygote. Down's syndrome has three copies of chromosome 21 (47 chromosomes)
  • Monosomy: a chromosome is missing. Turner's syndrome has one X chromosome and no second sex chromosome (45 chromosomes, XO), giving a female with short stature and usually infertility
Key termsnon-disjunctionpolysomymonosomyDown's syndromeTurner's syndrome

Must know

  • Meiosis: one replication, two divisions, four haploid cells
  • Meiosis I separates homologous chromosomes; meiosis II separates chromatids
  • Variation: crossing over (prophase I) and independent assortment (metaphase I, 2ⁿ)
  • Translocation: section moves to a non-homologous chromosome; number unchanged
  • Non-disjunction gives polysomy (Down's, 47) or monosomy (Turner's, 45, XO)

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Meiosis and chromosome mutations

  1. A fruit fly, Drosophila, has 8 chromosomes in each of its body cells. Sperm are made by meiosis in the testes of the male fly.
    Explain why meiosis I, rather than meiosis II, halves the chromosome number.2 marks
  2. A baby girl is born with Down's syndrome. A karyotype of her cells shows 47 chromosomes, including three copies of chromosome 21. Both of her parents have a normal karyotype of 46 chromosomes.
    Explain how non-disjunction during meiosis could have caused this baby's condition.2 marks
  3. Many patients with chronic myeloid leukaemia have a Philadelphia chromosome in their white blood cell precursors. It forms when a piece of chromosome 22 breaks off and joins chromosome 9, while a piece of chromosome 9 joins chromosome 22. The change is found in the leukaemia cells but not in the patient's other body cells, such as skin cells.
    Explain why the Philadelphia chromosome is unlikely to be passed on to the patient's children.3 marks
See the full worksheet

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).