4.3 Meiosis

• What gametes are produced during meiosis cell division?

  Sperm and egg
• What is a haploid?

  It only contains one chromosome of each homologous pair.
• What is a diploid?

  It contains two chromosomes from each homologous pair.
• What's the purpose of meiosis?

  Genetically different gametes allow for natural selection which ensures the survival of species which is only possible with variation.

  The halved number of chromosomes in gametes allows for the restoration of the diploid number at fertilisation.
• What is the principle of independant segregation?

  It's a genetic principle stating that the alleles of two genes will segregate into daughter cells independantly of eachother
• What's the difference between mitosis and meiosis?

  • There's one division in mitosis and two in meiosis.
  • Cells are genetically identical in mitosis, cells are genetically different in meiosis.
  • Crossing over only occurs in meiosis.
  • Diploid to diploid/haploid to haploid in mitosis, diploid to haploid in meiosis.
• What happens during crossing over?

  Homologous pairs form chiasmata where chromatids twist together. The exchange of alleles between homologous pairs of chromosomes during crossing over produces new combinations of alleles.
• Why can't a mule (63 chromosomes) reproduce?

  If a cell has an odd number of chromosomes it must be haploid as diploid cells have 2 sets of chromosomes and so their total must be an even number of chromosomes. Due to this an organism that has an odd number of chromosomes must be sterile as during meiosis chromosomes pair up and then separate into different cells. In organisms with an odd number of chromosomes, it becomes impossible for each chromosome to find a partner during meiosis.
• What are the benefits of mitosis?

  • There are fewer stages so the process is quicker requiring less energy.
  • Only one parent is needed to colonise a new environment.
  • Produces clones so a successful genotype is maintained.
  • We can identify fertilisation as it's haploid to diploid.
• What are the benefits of meiosis?

  • It introduces genetic variation resulting in a greater chance of survival for a species.
  • We can identify meiosis as it's diploid to haploid.
• If the number of chromosomes goes from 8 to 16 what has occurred?

  Fertilisation has occurred as the number of chromosomes has doubled.
• If the number of chromosomes goes from 8 to 8 what has occurred?

  Mitosis has occurred as the number of chromosomes has stayed the same.
• If the number of chromosomes goes from 16 to 8 what has occurred?

  Meiosis has occurred as the number of chromosomes has halved.