Cell division, diversity and organisation

• Why is mitosis important?

  For growth, repair and asexual reproduction
• In meiosis, genetic variation occurs at

  prophase 1 via crossing over where chromatids entangle and sections of DNA break off and recombine on the other parent strand

  Metaphase 1 and 2 via random assortment where chromosome pairs are randomly orientated at the metaphase plate so a different proportion of each parent DNA ends up at either pole of the cell

  anaphase 1 via DNA exchange at the chiasmata which is the same as crossing over but forms to make recombinant DNA
• What is the purpose of checkpoint in the cell cycle?

  G1: Checks cell size,if enough proteins and organelles have been made and checks if the DNA has been damaged

  S: DNA replication occurs

  G2: Checks the DNA after replication for any errors and if there are too many then the cell undergoes apoptosis
• Apoptosis: programmed cell destruction
• mitotic index: number of cells that are undergoing mitosis/number of cells counted
• Stages of mitosis:

  Prophase:

  The nuclear membrane breaks down.

  Chromosomes condense (become visible) as tightly coiled structures.

  Spindle fibres form, extending from opposite poles of the cell.

  Metaphase:

  The sister chromatids (identical copies of a chromosome) line up along the metaphase plate

  Anaphase:

  Spindle fibres contract and pull the sister chromatids apart toward opposite poles of the cell.

  Telophase:

  A nuclear envelope reforms around each set of separated chromatids at the poles.

  Cytokinesis (division of the cytoplasm) occurs resulting in two daughter cells.
• What features do homologous chromosomes have

  one is from the mother and the other from the father

  same loci of genes

  carry different or similar alleles

  pair up in meiosis

  similar length to each other