Chapter 6- Cell division

• How does cell division produce genetic variation?

  • Independent assortment of homologous chromosomes in metaphase 1- produces large number of allele combinations
  • Crossing over at chiasmata in prophase 1- chromatids will have new combinations of alleles- variation depends on distance between crossover points
  • Mutation- changes base sequence
  • DNA checks during duplication may not recognise damage
  • One more or less chromosome present
  • Gametes not being genetically identical produces large number of allele combinations
• what processes happen during interphase?

  • G$_1$
  • S phase
  • G$_2$
• state and describe what happens in each phase during interphase

  • G$_1$ - first growth phase. Proteins are synthesised and organelles replicate. Cell increases in size
  • S phase- synthesis phase- this is when DNA replication happens
  • G$_2$ - second growth phase- cell continues to grow in size, energy stores are increased and the DNA is checked for errors/damage
• what two stages does cell division involve?

  • mitosis
  • cytokinesis
• what happens if the cell doesn't satisfy the requirements of cell division?

  enters G$_0$ - this is when cells leave the cycle
• what generally happens during interphase?

  • DNA replicated and checked for errors
  • protein synthesis occurs in the cytoplasm
  • normal metabolic processes within the cell continues (for example, respiration continues throughout cell division)
  • organelles grow and divide
• what divides during mitosis and what divides during cytokinesis?

  • Mitosis- nucleus
  • cytokinesis- cytoplasm
• state the reasons for why a cell might leave the cell cycle and enter

  G$_0$


  • differentiation- a differentiated cell is specialised and can no longer divide by mitosis, carries out its function indefinitely
  • DNA may be damaged and no longer viable, damaged cell cannot divide and enters permanent arrest
  • some cells that enter G$_0$ can be stimulated to go back into the cell cycle and start dividing again, for example lymphocytes in an immune response
• state and describe what happens in each stage during interphase

  • G$_1$ - first growth phase. Proteins are synthesised and organelles replicate. Cell increases in size
  • S phase- synthesis phase- this is when DNA replication happens
  • G$_2$ - second growth phase- cell continues to grow in size, energy stores are increased and the DNA is checked for errors/damage

  
• what are checkpoints in the cell cycle?

  control mechanisms- verify whether or not the processes at each stage of the cell cycle have been accurately completed before the cell can progress into the next phase
• state and describe each of the three checkpoints at interphase

  • G$_1$ checkpoint- Cell size and DNA damage checked for. END of G$_1$ phase, before entry into the S phase. if the cell satisfies requirements, goes to the S phase, if not, enters G$_0$
  • G$_2$ checkpoint- at the end of the G$_2$ phase, before the mitotic phase. Checks for cell size, DNA replication and DNA damage. Checks that DNA has been replicated without error, enters G$_0$ if not
  • spindle assembly/metaphase checkpoint- all chromosomes should be attached to spindle fibres and correctly aligned. Mitosis cannot proceed until this checkpoint is passed

  
• why can't prokaryotic organisms undergo mitosis?

  Have no nucleus to divide, divide instead by binary fission
• what is a chromatid?

  A DNA molecule, replicates during mitosis to form two sister chromatids attached by the centromere

  
• True or false? By the end of prophase, spindle fibres are attached to the centromeres of chromsosomes

  TRUE
• state how cytokinesis occurs in animal cells

  • cleavage furrow forms around the middle of the cell
  • cell membrane pulled inwards by the cytoskeleton until it's close enough to fuse around the middle, forming two cells

  
• how does cytokinesis occur in plant cells?

  • plant cells have cell walls so not possible for cleavage furrow to be formed
  • vesicles from the golgi apparatus assemble in the same place as where the metaphase plate was formed
  • vesicles fuse with each other and the cell surface membrane, dividing the cell into two
  • new sections of cell wall form along the new sections of cell membrane
• how does the number of chromosomes vary during meiosis?

  2n -> 4n -> 2n -> n
• what independent assortment mean?

  orientation of homologous chromosomes is random and maternal or paternal chromosomes can end up facing either pole
• difference between metaphase and metaphase I?

  homologous chromosomes line along metaphase plate
• how is anaphase I different from anaphase?

  Anaphase I: Homologous chromosomes separate and are pulled to opposite poles Anaphase: Sister chromatids separate and are pulled to opposite poles
• when does crossing over take place?

  during anaphase I
• how is anaphase different from anaphase I

  crossing over occurs
• what are the chromatid formed as a result of crossing over?

  recombinant chromatids
• when does independent assortment occur?

  during metaphase I and II
• how is anaphase I different to anaphase II

  in anaphase II, individual chromatids are pulled apart, homologous chromosomes for I
• true or false? By the end of anaphase I, sister chromatids are different genetically

  TRUE- as crossing over has occurred