Cell Division

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Chromosome Structure

  • Made up of DNA, protein and a small amount of RNA
  • The DNA is made up of small sections called genes

They are found in matching pairs - homologous chromosomes

Humans have 23 pairs of homologous chromosomes - one chromosome in each pair comes from mother, the other from father:

  • Total number of chromosomes in somatic boday cell - diploid - 2n
  • Gametes have half diploid number - haploid - n
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Mitosis

Produces two genetically identical daughter cells 

1) Interphase

  • Newly formed cell increases in size
  • DNA replicates
  • New organelles producded
  • ATP synthesis occurs

2) Prophase (longest part of cycle)

  • Chromosomes condense to form chromatids
  • Centrioles divide - move to opposite poles of cell
  • Spindle develops
  • Nuclear envelope disintergrates
  • Nucleolus disapears
  • Chromosomes move towards equator of cell
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Mitosis

3) Metaphase

  • Chromosomes arrange themselves at equator of cell
  • Chromosomes attach to spindle fibres at centromere
  • Fibres contract - draws chromatids slightly apart

4) Anaphase (very rapid stage)

  • Centromere splits
  • Spindle fibres contract and shorten
  • Chromatids pulled to poles, centromere first

5) Telophase

  • Chromosomes uncoil and lengthen
  • Spindle breaks down
  • Nucleus and nuclear membrane reform

Cytokinesis occurs by constriction of the centre of parent cell

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Mitosis

(http://cyberbridge.mcb.harvard.edu/images/mitosis5_1.png)

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Significance of Mitosis

Mitosis allows production of genetically identical cells - genetic stability

Growth of organism and repair of damaged tissues

  • In plants: root tips and growing points of shoots
  • In animals: human skin - dead surface cells replaced by identical cells

Asexual reproduction

  • No genetic variation
  • Large numbers of identical offspring produced
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Meiosis

Produces 4 genetically different haploid daughter cells / gametes

  • Meiosis I - chromosome number is halved
  • Meiosis II - new haploid cells divide again producing 4 genetically diffeent daughter cells

Meoisis II is identical to mitosis

(http://genetics.thetech.org/sites/default/files/NormalMeiosis.gif)

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Meiosis

Sources of Genetic Variation

1) Crossing Over

  • During prophase I
  • Chromosomes condese
  • Homologous pairs become bivalent - synapsis
  • Chromatids in bivalent join at chiasmata
  • Parts of chromosome are exchanged at chiasmata

2) Independent Assortment

  • During metaphase I
  • Homologous pairs arrange themselves radomly at equator of spindle

3) Random fusion of gametes

  • Diploid chromosome number restored when nuclei of haploid gametes fuse 
  • Forms zygote
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Differences

Mitosis

  • 1 division
  • 2 daughter cells
  • No homologous pairs
  • Diploid
  • Crossing over doesnt occur
  • Genetically identical daughter cells

Meiosis

  • 2 divisions
  • 4 daughter cells
  • Homologous pairs
  • Haploid
  • Crossing over occurs
  • Genetically different daughter cells
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