Chapter 6

• Significance of mitosis

  • necessary when all daughter cells have to be identical - this is the case during growth, replacement & repair of tissues in multicellular organisms
  • necessary for asexual reproduction - genetically identical offspring
  • necessary in eukaryotic single-celled organisms
• Why do prokaryotic organisms not need mitosis?

  they don't have a nucleus so they reproduce asexually by binary fission
• 4 stages of mitosis

  1. prophase
  2. metaphase
  3. anaphase
  4. telophase
• Phrophase

  • chromatin fibres coil & condense into visible chromosomes
  • Nucleolus disappears and nuclear membrane begins disintegrate
  • spindle microtubules form between cell poles
  • 2 centrioles migrate to opposite poles of the cell
  • towards end spindle fibres begin to attach to centromeres & start to move chromosomes to centre of cell
  • by end of prophase nuclear envelope has disappeared

  
• Metaphase

  • (at beginning) spindle fibres attach to centromeres
  • chromosomes line up along the centre (equator) of the cell

  
• Anaphase

  • centromeres holding pairs of chromatids together divide
  • chromatids pulled to opposite poles of the cell by spindle fibres
  • v shape of chromatids is result of them being dragged by their centromeres through liquid cytosol

  
• Telophase

  • chromatids that have reached poles now called chromosomes
  • 2 new sets of chromosomes assemble at each pole & nuclear envelope reforms around them
  • chromosomes start to uncoil & nucleolus is formed
  • cell division/ cytokinesis begins

  
• How is plant tissue used to show cell cycle & stages of mitosis?

  • root tips are treated w a chemical to allow cells to separate
  • then they can be squashed to form a single layer of cells on a microscope slide
  • stains that bind DNA are used to make chromosomes visible
• 2 main phases of the cell cycle

  • interphase
  • miotic (division) phase
• What happens during interphase ?

  • DNA is replicated & checked for errors
  • protein synthesis occurs
  • mitochondria grow & divide - inc in number
  • chloroplasts grow & divide - inc in number
  • normal metabolic processes occur
• 3 stages of interphase

  • G1 - first growth phase
  • S - synthesis phase
  • G2 - second growth phase
• G1 - first growth phase

  proteins from which organelles are synthesised are produced & organelles replicate. Cell increases in size
• S - synthesis phase

  DNA is replicated in the nucleus
• G2 - second growth phase

  • cell continues to increase in size
  • energy stores are increased
  • DNA is checked for errors
  • cell prepared for division
• 2 stages of the miotic phase

  • mitois - nucleus divides
  • cytokinesis - cytoplasm divides & 2 cells are produced
• What is the G0 phase?

  phase when cell leaves the cycle, either temporarily or permanently
• Reasons for G0 phase

  • differentiation
  • DNA damaged
  • age
• What is used to regulate the cell cycle?

  • checkpoints
  • they monitor & verify whether processes at each phase have been completed accurately before cell can progress to next phase
• What does G1 checkpoint check for?

  • cell size
  • nutrients
  • growth factor
  • DNA damage
• What does G2 checkpoint check for?

  • cell size
  • DNA replication
  • DNA damage
• What does spindle assembly checkpoint check for?

  chromosome attachment to spindle - mitosis can't proceed to anaphase until checkpoint passed
• Label cell cycle

  A) G1

  B) S

  C) G2

  D) spindle assembly checkpoint

  E) G1 checkpoint

  F) G2 checkpoint

  
• Differences in mitosis in plants & animals

  • animals have centrioles; plants don't
  • in animals cells become rounded before division; in plants there is no shape change
  • in animals spindle disappears before cytokinesis; in plants some of the spindle remains during cytokinesis
• Cytokinesis


  • division of a cell
  • results in 2 genetically identical daughter cells
  • each cell receives half of the organelles & cytoplasm from the original cell
• Method of cytokinesis in animals

  • a cleavage furrow forms (i.e. cell surface membranes are pulled together by the cytoskeleton)
• Method of cytokinesis in plants

  • cell wall prevents cleavage furrows
  • 2 daughter cells separated by new cell wall production down the centre of the original cell
• Define meiosis

  A form of nuclear division in which the chromosome number is halved
• What are homologous chromosomes?

  • pairs of matching chromosomes which have the same gene loci (positions)
  • have genes in the same positions and are the same length & size
• What are alleles?


  different versions of the same gene
• How many divisions does meiosis involve?

  • 2
  • Meiosis I - seperates homologous chromosomes to produce 2 haploid cells
  • Meiosis II - chromatids separated to form total of 4 haploid daughter cells
• Stages of meiosis

  • prophase 1
  • metaphase 1
  • anaphase 1
  • telophase 1
  • prophase 2
  • metaphase 2
  • anaphase 2
  • telophase 2
• Prophase 1

  • chromosomes condense, nuclear envelope disintegrates, nucleolus disappears, spindles form
  • homologous chromosomes pair up, forming bivalents
  • moving chromosomes through cytoplasm results in chromatids entangling - called crossing over

  
• Metaphase 1

  • homologous pairs (instead of individual chromosomes) line up along metaphase plate (equator)
  • independent assortment - orientation of each homologous pair is random & independent of any other pair - results in genetic variation

  
• Anaphase 1

  • homologous chromosome pairs are pulled to opposite poles (random segregation)
  • sections of DNA on 'sister' chromatids which became entangled during crossing over break off and rejoin on points called chiasmata - may result exchange of DNA between chromatids
  • if DNA is exchanged, new combinations of alleles from the same gene form
  • chromatids have diff allele combination than the original ones - sister chromatids no longer genetically identical = genetic variation
• Telophase 1

  • chromosomes assemble at each pole & nuclear membrane reforms
  • cell undergoes cytokinesis and divides into 2 cells - chromosome number goes from diploid to haploid

  
• Prophase 2

  • chromosomes (which still consist of two chromatids) condense & become visible again
  • nuclear envelope breaks down & spindles form

  
• Metaphase 2

  • individual chromosomes assemble on metaphase plate
  • due to crossiving over, chromatids no longer identical so there is independent assortment again & more genetic variation than in metaphase 1

  
• Anaphase 2 


  • chromatids pulled to opposite poles after division of centromeres
  • (same as anaphase of mitosis)
  • ensures that each daughter cell receives one sister chromatid from each chromosome

  
• Telophase 2 


  • chromatids assemble at poles
  • chromosomes uncoil
  • nuclear envelope reforms & nucleolus becomes visible
  • cytokinesis results in division of cells forming 4 haploid daughter cells formed - all genetically different from each other due to crossing over & independent assortment
• Diploid cells

  
  

  • contain 2 complete sets of chromosomes - one from each parent
  • 23 chromosome pairs (46 total)
  • formed by mitosis
  • e.g. zygote