Cell Division

• 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

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

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

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

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

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

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