1. Meiosis and Genetic variation

• Normal body cell have diploid number (2n) of chromosomes- each cell contains 2 of each chromosome (a pair), one from mother and one from father
• 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 (Alleles). These pairs of matching chromosomes are called homologous pairs. Humans have 23 homologous pairs so 46 chromosomes in total.
• Gametes- sex cells, have haploid (n) number of chromosomes- they only contain 1 copy of each chromosome in homologous pair. Haploid number is 23 (humans)
• In sexual reproduction 2 gametes join together at fertilisation to form zygote, which divides and develops into a new organism
• At fertilisation, haploid sperm fuses with haploid egg, making cell with normal diploid number of chromosomes. Half these chromosomes are from father and half from mother
• Meiosis: Takes place in reproductive organs of multicellular, eukaryotic organisms. Cells that divide by meiosis are diploid to start with, but cells that are formed from meiosis are haploid- chromosome number halves.
• Meiosis Stage:

  1. Before meiosis starts, DNA unravels and replicates so there are 2 copies of each chromosome called chromatids
  2. The DNA condenses to form double- armed chromosomes, each made from 2 sister chromatids. Sister chromatids are joined in middle by centromere
  3. Meiosis I (first division)- chromosomes arrange themselves into homologous pairs
  4. These pairs are then separated, halving chromosome number
• Meiosis stage 2:

  1. Meiosis II (second division)- pairs of sister chromatids that make up each chromosome are separated (centromere divided)
  2. 4 haploid cells that are genetically different from each other are produced
• Crossing over chromatids:

  During meiosis I , homologous chromosomes come together and pair up. Chromatids twist around each other and bits of chromatids swap over. Chromatids still contain same gene but now have different combinations of alleles
• Independent segregation of chromosomes:

  When homologous pairs are separated in meiosis I, its completely random which chromosomes from each pair ends up in which daughter cells. So 4 daughter cells produced by meiosis have completely different combinations of those maternal and paternal chromosomes
• Comparison between mitosis and meiosis:

  Mitosis:

  • Produces same number of chromosomes as parent cell
  • Daughter cell genetically identical from one another and parent cell
  • Produces 2 daughter cell
  • Meiosis:
  • Produces halve number of chromosomes as parent cell
  • Daughter cell genetically different from another and parent cell
  • Produces 4 daughter cells