Edexcel International A Level Biology

Revision Notes

3.10 Meiosis & Variation

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Independent Assortment

  • Meiosis gives rise to cells that are genetically different from each other and is the type of cell division used to produce gametes (sex cells)
  • During meiosis, the nucleus of the original 'parent' cell undergoes two rounds of division. These are:
    • Meiosis I
    • Meiosis II

Meiosis I

  • The nucleus of the original 'parent' cell is diploid (2n) i.e. it contains two sets of chromosomes
  • Before meiosis I, these chromosomes replicate
  • During meiosis I, the homologous pairs of chromosomes are split up, to produce two haploid (n) nuclei
    • At this point, each chromosome still consists of two chromatids
  • Note that the chromosome number halves (from 2n to n) in the first division of meiosis (meiosis I), not the second division (meiosis II)

Meiosis II

  • During meiosis II, the chromatids that make up each chromosome separate to produce four haploid (n) nuclei
    • At this point, each chromosome now consists of a single chromatid

Meiosis I and II

During meiosis, one diploid nucleus divides by meiosis to produce four haploid nuclei

  • Having genetically different offspring can be advantageous for natural selection
  • Meiosis has several mechanisms that increase the genetic diversity of gametes produced. The two main mechanisms are:
    • Independent assortment
    • Crossing over
  • Both independent assortment and crossing over result in different combinations of alleles in gametes

Independent assortment

  • Independent assortment is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I of meiosis I
  • The different combinations of chromosomes in daughter cells generate an increase in the genetic variation between gametes
  • In meiosis I, homologous chromosomes pair up and are pulled towards the equator of the spindle
    • Each pair can be arranged with either chromosome on top, this is completely random
    • The orientation of one homologous pair is independent/unaffected by the orientation of any other pair
  • The homologous chromosomes are then separated and pulled apart to different poles
  • The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
  • To work out the number of different possible chromosome combinations the formula 2n can be used, where n corresponds to the number of chromosomes in a haploid cell
  • For humans, this is 223 which calculates as 8,324,608 different combinations

Independent assortment (1)Independent assortment (2)

Independent assortment of homologous chromosomes leads to different genetic combinations in daughter cells

Crossing Over

  • Crossing over is the process by which non-sister chromatids exchange alleles
  • Process:
    • During prophase I of meiosis I homologous chromosomes pair up and are in very close proximity to each other
    • The paired chromosomes are known as bivalents
    • The non-sister chromatids can cross over and get entangled
    • These crossing points are called chiasmata
    • The entanglement places stress on the DNA molecules
    • As a result of this, a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
  • This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes
  • There is usually at least one, if not more, chiasmata present in each bivalent during meiosis
  • Crossing over is more likely to occur further down the chromosome away from the centromere

Genetic Variation Crossing Over

Crossing over of non-sister chromatids leads to the exchange of genetic material

Exam Tip

Several sources of genetic variation have been outlined above. It is also worth remembering that genetic variation can occur on an even smaller scale than chromosomes. Mutations can occur within genes. A random mutation that takes place during DNA replication can lead to the production of new alleles and increased genetic variation.

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Author: Alistair

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.