Meiosis and Genetic Variation

• Diploid body cells

  • Normal body cells have the diploid number (2n) of chromosomes - meaning each cell contains two of each chromosome, from the mum and one from dad
  • The chromosomes that make up each pair are the same size and have the same genes, although they could have different versions of those genes.
  • These pairs of matching chromosomes are called homologous pairs. Humans have 23 homologous pairs and so 46 chromosomes in total. Therefore the diploid number for humans is 46
• • Gametes are the sperm cells in males and egg cells in females
  • Gametes have a haploid (n) number of chromosomes - they only contain one copy of each chromosome in a homologous pair
  • The haploid number for humans is 23
  • In sexual reproduction two gametes join together at fertilisation to form a zygote, which divides and develops into a new organism
• Fertilisation

  • At fertilisation, a haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes.
  • Half these chromosomes are from the father and half are from the mother
• During sexual reproduction, any sperm can fertilise any egg - fertilisation is random

  Random fertilisation produces zygotes with different combinations of chromosomes to both parents

  This mixing of genetic material in sexual reproduction increases genetic diversity within a species
• Meiosis

  • Meiosis is a type of cell division which takes place in the reproductive organs of multicellular, eukaryotic organisms
  • Cells that divide by meiosis are diploid to start with, but the cells that are formed from meiosis are haploid - the chromosome number halves
  • Meiosis in humans and other mammals produces gametes directly
  • In other organisms it produces haploid cells which later divide by mitosis to become gametes
  • Without meiosis, you'd get double the number of chromosomes when the gametes fused
• 1. Before meiosis starts the DNA unravels and replicates so there are two copies of each chromosome called chromatids
  2. The DNA condenses to form double-armed chromosomes, each made from two sister chromatids . The sister chromatids are joined in the middle by a centromere
  3. Meiosis I - the chromosomes arrange themselves into homologous pairs
  4. These homologous pairs are then separated, halving the chromosome number
  5. Meiosis II - the pairs of sister chromatids that make up each chromosome are separated (the centromere is divided)
  6. Four haploid cells that are genetically different from each other are produced
• The two main events during meiosis that lead to genetic variation are: crossing over of chromatids and independent segregation of chromosomes
• Crossing over of chromatids:

  • During meiosis I, homologous chromosomes come together and pair up.
  • The chromatids twist around each other and bits of chromatids swap over
  • This crossing over and linkage is the chiasma and this forms a bivalent
  • The chromatids still contain the same genes but now have a different combination of alleles
  • The crossing over of chromatids in meiosis I means that each of the four daughter cells formed from meiosis II contain chromatids with different alleles
• Independent segregation of chromosomes

  • Each homologous pair of chromosomes in your cells is made up of one chromosome from your mum and one chromosome from your dad
  • When the homologous pairs are separated in meiosis I, it is completely random which chromosome from each pair ends up in which daughter cell.
  • So the four daughter cells produced by meiosis have completely different combinations of those maternal and paternal chromosomes
  • This is called independent segregation of the chromosomes
  • This shuffling of chromosomes leads to genetic variation in any potential offspring
• Crossing over

  1. Homologous pairs of chromosomes associate / form a bivalent;
  2. Chiasma(ta) form;
  3. (Equal) lengths of (non-sister) chromatids / alleles are exchanged;
  4. Producing new combinations of alleles