• 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
• Both?crossing over?and?independent assortment?result in?different combinations of alleles?in gametes

Crossing over

• Crossing over is the process by which?non-sister chromatids exchange alleles
• Process:
  • During?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

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?meiosis I
• The different combinations of chromosomes in daughter cells increases 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