meiosis

• Gametes are the only cells which do not contain the same genetic info. 
• Normal cells have the diploid number of chromosomes - each cell contains two of each chromosome, one from each parent. 
• Matching pairs of chromosomes are called homologous pairs, humans have 23 homologous pairs, so 46 chromosomes in total - diploid number is 46. 
• Gametes are found in sperm and egg cells, they have a haploid number of chromosomes with only one copy of each chromosome. The haploid number for humans is 23. 
• When a haploid sperm fuses with a haploid egg, a cell with the diploid number of chromosomes is formed.
• In sexual reproduction fertilisation is random and produces zygotes with different combinations of chromosomes from each parent. This increases genetic diversity within a species. 
• Meiosis takes place in the reproductive organs of eukaryotic organisms. Cells that divide by meiosis are diploid and the zygotes formed are haploid. 
• 1. Before meiosis starts, the DNA unravels and replicates, so there are two copies of each chromosome called chromatids. 

  
• 2. The DNA condenses to form double-armed chromosomes, each made from two sister chromatids. The sister chromatids are joined by a centromere. 

  
• 3. Meiosis I is when the chromosomes arrange themselves into homologous pairs. 

  
• 4. These homologous pairs are separated, halving the chromosome number

  
• 5. Meiosis II is when the pairs of sister chromatids that make up each chromosome are separated. 

  
• 1. 6. Four haploid cells that are genetically different from each other are produced.

  
• mitosis involves one division, meiosis invites two.
• in meiosis the number of chromosomes is halved, in mitosis the number of chromosomes remains the same.
• homologous chromosomes do not pair up in mitosis, they do pair up in meiosis.
• after meiosis the daughter cells are genetically different, after mitosis the daughter cells are genetically identical.
• four daughter cells are formed from meiosis, only two are formed from mitosis.