2.1.6 Division, Diversity and Organisation

• The cell cycle is a series of events during which the cell duplicates its contents and splits into two
• Cell cycle = Interphase + Mitosis
• p53 is the tumor surpressor gene. it triggers checkpoints in the cell cycle
• checkpoints...

  • prevent uncontrolled cell division
  • detect and repair damaged DNA
  • ensure cycles isn't reversed and DNA is only replicated once
• the cell is not dividing during interphase
• Interphase consists of three phases:

  1. G1 phase -> growth
  2. S phase -> synthesis
  3. G2 phase -> growth
• Mitosis is used for:

  • Growth of the organism
  • Repair of tissues
  • Replacement of old cells
  • Asexual reproduction
• Mitosis produces two genetically identical daughter cells
• Stages of Mitosis:

  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase
• Prophase:

  • chromatin condenses into chromosomes
  • centrioles move to opposite poles
  • tubulin threads form spindle fibres
  • nuclear envelope breaks down into vesicles
• Metaphase:

  • chromosomes line up at equator
  • spindle fibres attach to centromere
• Anaphase:

  • motor proteins pull sister chromatids along spindle fibres to opposite poles
  • centromere divides
• Telophase:

  • chromosomes uncoil
  • spindle fibres disintegrate
  • nuclear envelope reforms
• Cytokinesis occurs after mitosis
• Cytokinesis:

  • cell membrane and cytoplasm divide
• Cytokinesis in animals:

  • cell membrane and cytoplasm divide
  • membrane folds in forming cleavage furrow
• Cytokinesis in plants:

  • cell membrane and cytoplasm divide
  • end plate forms and new plasma membrane and cell wall formed
• Homologous chromosomes are identical chromosomes that contain the same genes but different alleles
• Hayflick constant is the number of times a cell can divide before it dies. Cells should only undergo 50 cycles
• Meiosis produces four non-genetically identical haploid daughter cells.
• Cells produced by meiosis are gametes, used for sexual reproduction
• Before cell division, DNA replicates so that each chromosome consists of two identical copies called sister chromatids
• Prophase 1:

  • chromosomes condense
  • nuclear envelope breaks down
  • spindle fibres form
  • homologous chromosomes pair to form bivalents containing four chromatids

  Crossing over
• crossing over occurs in prophase 1
• crossing over: Non-sister chromatids wrap around each other and exchange sections of DNA. They swap alleles.
• The chiasmata are the crossing points of the chromosomes during crossing over
• Metaphase 1:

  • spindle fibres attach to centromeres
  • Homologous pairs (bivalents) move to equator

  Independent assortment
• Independent assortment occurs in metaphase 1 and metaphase 2
• The orientation of each bivalent is random, this is independent assortment
• Anaphase 1:

  • Homologous chromosomes pulled to opposite poles
  • Centromeres not divided, chromosome consists of two chromatids
• Telophase 1:

  • Nuclear membrane reforms around each set of chromosomes

  Cytokinesis occurs
• Prophase 2:

  • nuclear envelope breaks down
  • Centrioles replicate and migrate to opposite poles
  • Spindle fibres formed
• Metaphase 2:

  • Spindle fibres attach to centromeres
  • Chromosomes align randomly along equator

  Independent assortment
• Anaphase 2:

  • chromatids pulled to opposite poles
  • centromeres divide
• Telophase 2:

  • nuclear envelope reform

  Cytokinesis occurs
• Genetic variation in meiosis:

  1. Crossing over - prophase 1
  2. Independent assortment of homologous chromosomes- metaphase 1
  3. Independent assortment of sister chromatids - metaphase 2
• In Growth 1:

  • organelles replicated
  • cell increases in size
  • protein synthesis
  • respiration
  • p53 gene - > triggers G1/S checkpoint
• In S phase:

  • DNA replicated
  • This is rapid to prevent mutations
• In Growth 2:

  • cell grows in size
  • protein synthesis
  • respiration
• G1 checkpoint checks cell size and for DNA damage. If needed, the cell will go into G0 (rest) phase, else will continue with the cycle