Module 2.1.6- Cell division, cell diversity and cellular organisation

• Outline some events that occur during interphase


  DNA is replicated and checked for errors in the nucleus

  protein synthesis occurs in the cytoplasm

  mitochondria grow and divide, increasing in number in the cytoplasm

  chloroplasts grow and divide

  normal metabolic processes of cells occur
• 3 stages of interphase

  G1, S, G2
• G1 phase


  proteins from which organelles are synthesised and produced and organelles replicate. The cell increases in size.
• G1 checkpoint


  checks for cell size, nutrients, growth damage, DNA damage and if it is not satisfied, the cell enters G0.
• Synthesis phase


  DNA is replicated in the nucleus
• G2 phase


  cell continues to increase in size, energy stores are increased and the duplicated DNA is checked for errors
• G2 checkpoint


  checks for cell size, DNA replication, DNA damage.
• G0 phase


  A nondividing state occupied by cells that have left the cell cycle, sometimes reversibly.
• Reasons why G0 phase occurs


  Differentiation- cell is specialised and can no longer divide

  DNA may be damaged

  Ageing cells
• Spindle assembly checkpoint (metaphase checkpoint)


  check for chromosome attachment to spindle and have aligned before anaphase
• Prokaryotic organisms reproduce by...

  binary fission
• Chromatids


  Bodies of tightly coiled chromatin; visible during cell division. Two identical chromatids bound at a centromere make a chromosome.
• Centromere


  the region of the chromosome that holds the two sister chromatids together during mitosis
• How to view stages of mitosis under a light microscope in plants


  From growing root tips in plants

  root tips treated with a chemical to allow cells to be separated

  they can then be squashed to form a single layer of cells on a microscopic slide

  stains that bind to DNA make chromosomes visible

  
• Prophase- mitosis


  Chromatin coils and condenses the nucleolus disappears and nuclear envelope dissolves spindle forms

  2 centrioles migrate to opposite poles of the cell spindle fibres attach to centromeres

  
• Metaphase- mitosis


  chromosomes moved by spindle fibres on the metaphase plate and held in position

  
• Anaphase- mitosis


  Phase of mitosis in which the chromosomes separate and move to opposite ends of the cell by the shortening of spindle fibres.

  
• Telophase- mitosis


  chromatids have reached the poles and are now chromosomes

  two new sets of chromosomes assemble at each pole and nuclear envelope reforms around them

  chromosomes uncoil and nucleolus is formed

  
• Cytokinesis in animals

  cleavage furrow forms

  cell-surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle forming two cells

  
• Cytokinesis in plants


  vesicles from the Golgi apparatus begin to 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

  
• Meiosis


  a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores.

  
• Homologous chromosomes


  Pair of chromosomes that are the same size, same appearance and same genes.
• Alleles


  Different forms of a gene
• Meiosis 1


  Homologous chromosomes separate to form 2 haploid cells
• Meiosis 2


  Splits the sister chromatids and results in 4 haploid cells
• Prophase 1- meiosis 1


  Chromosomes become visible; nuclear envelope breaks down; crossing-over occurs.

  homologous chromosomes pairing up are called bivalents

  
• How can genetic variation occur in mitosis


  Crossing over

  Independent assortment

  Random fertilisation

  Mutation
• Crossing over


  Process in which homologous chromosomes exchange portions of their chromatids during meiosis.

  
• Metaphase 1- meiosis 1


  Paired homologous chromosomes (bivalents) line up across the centre of the cell. Orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair so can result in many different combinations of alleles facing the poles. (independent assortment resulting in genetic variation)

  
• Independent assortment


  Maternal or paternal chromosomes can end up facing either pole

  result in many different combinations of alleles facing the poles
• Anaphase 1- meiosis 1


  The fibres pull the homologous chromosomes toward opposite ends of the cell. sister chromatids cross over to form recombinant chromatids

  
• Chiasmata


  X-shaped regions where crossing over occurred.

  
• Telophase 1- meiosis 1

  chromosomes assemble at each pole and nuclear envelope reforms

  2 haploid daughter cells are formed

  each daughter cell contains only one chromosome of the homologous pair.
• Prophase 2- meiosis 2


  Chromosomes (consisting of 2 chromatids) condense nuclear envelope breaks downspindles form in each new cell spindle fibres attach to chromosomes.

  
• Metaphase 2- meiosis 2


  individual chromosomes assemble on metaphase plate due to crossing over, the chromatids are no longer identical so there is independent assortment again and more genetic variation

  
• Anaphase 2- meiosis 2


  sister chromatids separate and move to opposite poles

  
• Telophase 2- meiosis 2


  chromatids assemble at poles

  chromosomes uncoil and form chromatin again

  nuclear envelope reforms and nucleolus becomes visible

  4 cells haploid due to reduction division

  
• Reduction division


  another name for meiosis I, the division where homologous pairs separate.
• Meiosis 1 vs Meiosis 2

  1: homologous chromosomes separate

  2: sister chromatids separate
• Significance of mitosis

  Asexual reproduction

  Growth

  Tissue repair