2.6 - Cell division, cell diversity & cellular organisation

• List all the stages of the cell cycle in order

  Interphase:

  • G1 - first growth phase
  • S - synthesis = DNA replication
  • G2 - second growth phase = growth & mitosis preparation

  Mitosis & Cytokinesis

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• What happens in G1?

  Cells grow

  Organelles duplicate

  Protein synthesis takes place

  G1 checkpoint
• What happens in the G1 checkpoint?

  Cell checks for growth damage

  Checks to see if all chemicals needed for replication are present

  Checks for damage in the DNA before S-phase

  If its damaged it enters G0
• What is G0 / Growth zero?

  Apoptosis = programmed cell death

  If DNA is damaged, Apoptosis destroys the cell
• What happens in the Synthesis (S) stage?

  DNA replication. To be ready to divide by mitosis
• What happens in G2?

  Cell keeps growing and making proteins needed for cell division

  Energy store increases

  centrioles replicate

  G2 checkpoint
• What happens in the G2 checkpoint?

  Cell checks that all DNA has been replicated without damaged so it can enter mitosis
• What are the 4 stages of Mitosis?

  • Prophase
  • Anaphase
  • Metaphase
  • Telophase

  PMAT
• What are checkpoints used for?

  TO prevent uncontrolled division that would lead to tumours

  And to detect and repair damage to DNA
• What happens during prophase?

  • Nuclear envelope breaks down
  • Chromosomes condense and start to become more visible
  • Centrioles appear and move to opposite ends
  • Spindle fibres form
• What happens during Metaphase?

  Chromosomes attach to spindle fibres by their centromere

  They line up along the equator
• What happens during Anaphase?

  Centromere of chromatids split, separating each pair of sister chromatids

  Spindles contract, pulling chromatids to opposite sides, centromere first
• What happens during Telophase?

  Chromosomes decondense becoming long and thin again.

  A nuclear envelope forms around each group of chromosomes. Two nuclei form
• What happens during Cytokinesis

  The cytoplasm divides forming two new daughter cells that are genetically identical to the original cell and each other.
• When does cytokinesis usually start and end?

  It usually starts it Anaphase and ends in Telophase
• What phase of Mitosis is this image?

  Anaphase

  
• What phase of mitosis is this image? - the one in the middle?

  Prophase

  
• What stage of mitosis is this image?

  Metaphase

  
• What stage of mitosis is in this image?

  telophase

  
• What does meiosis produce?

  Gametes for Sexual Reproduction
• How many gametes does meiosis produce?

  4
• What is a haploid?


  A haploid is a cell or organism that has only one set of chromosomes.

  They have half the normal number of chromosomes
• Describe what the cells formed by meiosis are like genetically?

  The cells formed y meiosis are genetically different
• Meiosis involves two divisions: meiosis I and meiosis II
• Meiosis 1 is the reduction division, it halves the chromosome number
• What happens in Interphase during meiosis?

  DNA unravels and replicates to produce double-armed chromosomes called sister chromatids
• What happens in Meiosis 1 - Prophase 1?

  • Chromosomes condense getting shorter and fatter
  • They arrange themselves into homologous pairs. (same gene pairs up, one from mum one from dad)
  • Crossing over occurs
  • Centrioles move to opposite ends of the cell forming spindle fibres
  • Nuclear envelope breaks down
• What happens in Meiosis 1 - Metaphase 1?

  Homologous pairs line up across the centre of the cell and attach to the spindle fibres by their centromeres
• What happens in Meiosis 1 - Anaphase 1?

  The spindles contract, separating the homologous pairs- one chromosome goes to each end of the cell
• What happens in Meiosis 1 - Telophase 1?

  A nuclear envelope forms around each group of chromosomes.

  Each new nucleus contains half the original number of chromosomes but each chromosome consists of two chromatids
• What happens in Meiosis 1 - Cytokinesis?

  Cytoplasm divides and two haploid daughter cells are produced
• What is crossing over, that occurs in prophase 1 of meiosis 1?

  When homologous pairs of chromosomes come together and pair up, the chromatids twist around each other and bits of chromatids swap over. The chromatids contain the same genes but now have different combination of alleles
• What happens in Meiosis 2 - Prophase 2?

  • DNA doesnt replicate
  • Spindles form
• What happens in Meiosis 2 - Metaphase 2?

  Similar to metaphase 1:

  • chromosomes attach to the equator of the spindle by their centromere
  • They get randomly arranged in the middle
• What happens in Meiosis 2 - Anaphase 2?

  • spindles contract
  • Sister chromatids separate and are pulled towards poles
• What happens in Meiosis 2 - Telophase 2?

  • Sister chromatids decondense
  • nuclear envelope appears
  • 4 genetically different haploid gametes are formed
• What two things during meiosis lead to genetic variation in cells?

  Crossing over and independent assortment of chromosomes
• How does crossing over of chromatids causes genetic variation?

  Crossing over in meiosis 1 prophase 1 means each of the 4 daughter cells formed from meiosis contains chromatids with different alleles
• How does independent assortment of chromosomes lead to genetic variation?

  Independent assortment when chromosomes line up in Metaphase 1 and are separated in Anaphase 1 leads to random distribution of maternal and paternal chromosomes in each pair.

  Its completely random which chromosomes ends up in which daughter cell.

  'shuffling'
• What are stem cells?


  Unspecialised cells that can develop into different types of cell

  Stem cells are a renewing source of undifferentiated cells.