Meiosis and genetic variation (aqa new Unit 2)

bullet point summary of important points! Writern with references to CGP AQA Biology revision guide.

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Meiosis and genetic variation
Gametes: The form in which DNA is passed from one generation to another
Sperm fertilises egg to form Zygote (which developed into new organism)
Gametes have haploid number of chromosomes (23 in humans)
Fertilised egg has diploid number (46 in humans)
Gamete Formation: formed by meiosis.
1. DNA unravels and replicates two form chromatids (copies of chromosome)
2. DNA condenses to form 2 armed chromosomes called sister chromatids.
3. First division (Meiosis 1) = chromosomes arrange themselves in homologous pairs.
4. Pairs separate
5. Second division (Meiosis 2) = sister chromatids separate.
6. Haploid cells (4) genetically different.
Cross Over: In meiosis 1
Homologous pairs
Chromatids twist and bits cross over.
Contain same genes (diff. Combination of alleles.)
Meiosis: produces genetically different cells,
1. Crossing over
2. Independent segregation (where different combinations of maternal and paternal
chromosomes go into gametes.)


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