Meiosis & Variation (Edexcel International AS Biology)

• 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

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 of homologous chromosomes leads to different genetic combinations in daughter cells

• 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

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