Cell Division

Specification:

• Understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes
• Understand that mitosis occurs during growth, repair, cloning and asexual reproduction
• Understand that division of a cell by meiosis produces four cells, each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes

Mitosis:

• A cell nucleus divides into two identical 'daughter' nuclei.
  
• The daughter cells produced by mitosis are genetically identical to the parent cell.
• Mitosis is important for the growth of an organism, for repair of worn-out parts of the body and for asexual reproduction in plants.
  
  Stages in mitosis:

1. A pair of chromosomes in the nucleus.
2. Each chromosome makes an identical copy of itself.
3. Cell starts to divide into two. each daughter cell has a complete set of chromosomes.
4. Two new daughter cells, each identical to the parent cell they divided from.
   
   Meiosis:

• 2 cell divisions, producing 4 sex cells, daughter nuclei are haploid (contains half the number of chromosomes as the parent nucleus).
  
• Meiosis produces haploid gametes.
• Meiosis results in variations in the gametes produced.
• Variations occur due to crossing over and also due to independent assortment of chromosomes.
• Independent assortment of chromosomes means one chromosome from each pair can combine with either chromosome of the other pair. This results in four different gametes being produced from two pairs of chromosomes. --since fertilisation is random (any sperm fuses with any egg), such variations in the gametes produce variations in the offspring. 

Mitosis	Meiosis
Daughter cells contain same number of chromosomes as parent cell	Daughter cells contain half the number of chromosomes as parent cell
Pairing of homologous chromosomes doesn't occur	Homologous chromosomes pair at prophase I
No crossing over	Crossing over may occur
Daughter cells are identical to parent cell	Variations occur in daughter cells
Two daughter cells are produced from one parent cell	Four daughter cells are produced from one parent cell
Involves only one nuclear division	Involves two nuclear divisions
Occurs in normal body cells (somatic cells) during growth or repair of body parts	Occurs in the gonads (Sex organs) during gamete formation