Cell Division

Cell Division

1. Mitosis

   • The cell cycle
     • Interphase
       • The longest part of the cell cycle
       • Cell increases in size
       • Produces new organelles
       • DNA replicates
     • Prophase
       • The chromosomes condense ( get shorter and thicker
       • Centrioles divide and move to opposite ends of the cell
       • Protein microtubules form each centriole and the spindle develop extending from pole to pole
       • Nuclear membrane disintegrates and the nucleolus disappears
     • Metaphase
       • The chromosomes arrange themselves at the centre of the spindle and become attached to the spindle fibres at the centromere
     • Anaphase
       • The centromere splits and the spindle fibres contract pulling he separated chromatids to opposite poles, centromere first
       • This stage is very rapid
     • Telophase
       • Chromosomes uncoil and lengthen
       • Spindles breaks down
       • cytokinesis occurs
       • centrioles replicate
       • Nucleolus reappears
       • Nuclear membrane reforms
   • 2 daughter cells made
   • Cell is genetically identical to the parent cell
   • Used for growth, tissue repair and replacement of dead cells
   • Each chromosome is an exact replica of one of the originals

2. Meiosis

   • Gametes must be haploid since at fertilisation half of the chromosomes come from the male sperm and half from the female egg to produce a diploid zygote
   • Meiosis produces four genetically different haploid cell and involves two consecutive divisions
     • Meiosis I
       • Chromosome number is reduced and crossing over takes place
     • Meiosis II
       • Two haploid nuclei divide again in a division identical to mitosis
   • Prophase I
     • Very similar to that in mitosis but when the chromosomes condense they associate in their homoloous pairs
       • This pairing is called synapsis
       • Each pair is called a bivalent
   • Prophase I crossing over
     • The chromatids in each bivalent wrap around each other then partially repel each other but remained joined at points called chiasmata
     • The chromatids may break and recombine at these points swapping a piece of chromosome
       • This is a source of genetic variation
   • Metaphase I
     • The bivalents arrange themselves randomly on the equator of the spindle
     • This is determined by chance so when the chromosome pairs separate different combinations of chromosomes are produced
       • This random distribution and consequent independent assortment of chromosomes produces new genetic combinations