Meiosis

• gamete - sex cells i.e. sperm & egg cells. Fuse to form zygote (fertilisation)
• diploid - a cell with 2 copies of each chromosome
• haploid - a cell with 1 copy of each chromosome
• meiosis - a type of cell division which results in 4 non-identical cells being produced.
• chromosome - in eukaryotes, is composed of DNA wrapped around histone proteins, contains the genes of the organism
• chromatid - one half of a chromosome when it is a 'double structure'
• homologous chromosomes - a pair of chromosomes containing the same gene but different alleles.
• centromere - joins 2 chromatids together
• centriole - attach to centromeres at the poles of each cell
• spindle fibres - attach centrioles to centromeres and draw centromere/chromatids apart.
• Meiosis

  • initially, DNA is replicated causing chromosomes to form as 'double structures' i.e., 2 sister chromatids joined together by a centromere.
  • the long, thin chromosomes condense and become visible, revealing these double structures.
• Meiosis I Metaphase

  • the homologous chromosomes line up in pairs in double file at the equator. The order, within the pair is random - independent segregation. This is a source of variation.
  • During crossing over, the arms of adjacent homologous chromosomes wrap around each other, snap off and alleles are exchanged. This is a source of variation.
  • This gives new allelic combinations.
• Meiosis I Anaphase

  • The spindle fibres contract and the homologous chromosomes are pulled to either pole
• Meiosis I telophase and cytokinesis

  • the chromosomes have reached either pole and the cytoplasm divides. Meiosis I ends with 2 daughter cells - each containing half the number of chromosomes
• Meiosis II Metaphase - chromosomes line up on the equator in single file
• Meiosis II Anaphase - the centromeres are pulled apart as the spindle fibres contract and the sister chromatids are drawn to opposite poles.
• Meiosis II telophase and cytokinesis - the chromatids reach the poles and the cytoplasm divides. These are now 4, non-identical daughter cells.