Topic 4B

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Diploid body cells and Gametes

  • Normal body cells have the diploid number (2n) of chromosomes, this means that each cell has 2 of each chromosome, 1 from mum and 1 from dad
  • The chromosoms are the same size and contain the same genes, although they could have different versions of the gene called alleles.
  • These pairs of matching chromosomes are called homologus pairs
  • Humans have 23 homologus pairs which menas they have 46 chromosomes in total. This means the diploid number in humans is 46
  • Gametes are the sperm cells in men and egg cells in women.
  • Gametes have a haploid (n) number and in humans this is 23 and they only conain one copy of each chromosome in a homolgous pairs.
  • During fertilisation the 2 gametes join together to form a zygote, which divides and develops.
  • Random fertilisation produces zygotes with different combinations of chromosomes to both parents.
  • This mixing of genetic material in sexual reproduction increases genetic diversity
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Meiosis

  • Meiosis is a type of cell division and takes place in the reproductive organs.
  • Before meiosis starts, the DNA unravels and replicates so there are 2 copies of each chromosome called chromatids
  • The DNA condenses to form double-armed chromosomes, each made from 2 sister chromatids. The chromatids are joined in the middle by a centromere
  • Meiosis I (first division)- the chromosomes arrange themselves into homologus pairs
  • These homologus pairs are them seperated, halving the chromosome number
  • Meiosis II (second division)- the pairs of sister chromatids that make up each chromosome are separated (the centromere is divided)
  • 4 haploid  cells that are genetically different from each other are produced 

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Creating genetic variation in gametes

  • During meiosis I, homologus chromosomes come together and pair up. The chromatids twist around each other and bits of chromatids swap over, this means that the chromatids still contain the same genes but now have a different combination of alleles. Also it meas that each of the 4 daughter cells formed from meiosis II contain chromatids with different alleles.
  • Each homologus pair of chromosome is made from 1 chromosome from your mother and father. So when the homologus pairs are separated in meiosis 1 it is completely random which chromosome from each pair ends up in the daughter cell. So the 4 daughter cells produced by meiosis have completely different combinations of those from the mum and dad. This is called independent segregation of the chromosomes and this "shuffling" leads to genetic diversity
  • (http://www.bristol.k12.ct.us/uploaded/faculty/solomona/Images_for_websites/Genetics/sf9x5b.jpg)
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Life Cycles

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