Meiosis

Meiosis

1. Homologous chromosomes

   • At fertilisation 2 haploid gametes fuse
     • This gives a diploid zygote
       • The chromosomes passed to the zygote cannot be a random selection as essential genes may be missed out
         • 2 chromosomes with the same sequence of genes on them are called homologous
           • Within each pair one chromosome is from the father (paternal) and one from the mother (maternal)
             • Homologous: same genes at the same loci
               • Can be paired up and photographed
                 • 22 chromosomes are autosomes
                   • 23rd pair are sex chromosomes

2. Meiosis

   • Sexual reproduction
     • Maintain chromosome number in zygote
       • After fertilisation ---> diploid zygote
         • Genetic variation ---> ensures survival species
   • It results in 4, non-identical, haploid cells

3. Main stages of meiosis

   • Before meiosis, in the S phase of interphase each chromosome was replicated.
     • Each chromosome consists of 2 sister chromatids
       • There are 2 divisions in meiosis and in each division, there are 4 stages.
         • In the first meiotic division the phases are prophase 1, metaphase 1, anaphase 1, and telophase 1
           • There is then a short interphase (No DNA replication occurs)
             • Second meiotic division which has 2 stages prophase 2, metaphase 2, anaphase 2, and telophase 2
               • At the end of the 2nd meiotic division cytokinesis occurs

4. Prophase 1

   • Chromatin condenses and each chromosome supercoils
     • Can be seen with a light microscope at this stage
       • Nuclear envelope break down, spindle fibres form from the centriole
         • Chromosomes come together in homologous pairs
           • Each member of the pair consists of 2 chromatids
             • Crossing over occurs at this point
   • Prophase 1: Crossing over (recombination)
     • Homologous chromosomes pair up forming bivalents
       • The chromatids wrap around each other
         • Bits of the chromatids swap over
           • The chromatids still have the same combination of genes but now have a different combination of alleles