Meiosis and genetic variation

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  • Meiosis and Genetic Variation
    • Gametes
      • 1) Gametes are sperm cells and egg cells. Join at fertilisation to form zygote which divides and develops into new organism
      • 2) Normal body cells have diploid number (2n) of chromosomes- two of each chromosome, one from mum and one from dad
      • 3) Gametes have haploid number (n) of chromosomes- only one copy of each
      • 4) Fertilisation- haploid sperm fuses with haploid egg- making cell with normal diploid number of chromosomes
      • 5) Fertilisation is random- produces zygotes with different combinations of chromosomes to both parents. Mixing of genetic material increases genetic diversity within a species
    • Meiosis
      • Takes part in reproductive organs. Cells diploid to start, forms haploid cells
      • 1) DNA unravels and replicates so there are two copies of each chromosome (chromatids)
      • 2) DNA condenses, forms double armed chromosomes, made from two sister chromatids. Joined in middle by centromere
      • 3) Meiosis 1- Chromosomes arrange themselves into homologous pairs
      • 4) Homologous pairs separated, halving chromosome number
      • 5) Meiosis 2- Pairs of sister chromatids are separated, centromere divided
      • 6) Four haploid cells (gametes) that are genetically different from each other produced
    • Crossing over
      • Homologous pairs of chromosomes pair up. Chromatids twist around each other and some of chromatid swaps over. Contain same genes but different alleles
      • In meiosis 1- crossing over means each of four daughter cells contain chromatids with different alleles
      • Increases genetic variation
    • Independent segregation of chromosomes
      • 1) Each homologous pair made up of one maternal chromosome and one paternal chromosome
      • 2) When separated in meiosis 1, completely random which chromosome from each pair ends up in daughter cell
      • 3) Four daughter cells have completely different combination of maternal/paternal chromosomes
      • 4) Independent segregation
      • 5) 'Shuffling' of chromosomes leads to genetic variation in potential offspring
    • Chromosome mutations
      • 1) Correct human meiosis produces four daughter cells with 23 chromosomes in each
      • 2) Incorrect meiosis can produce variations in numbers of whole/ parts of chromosomes
      • 3) E.g. two cells may have 23 whole chromosomes. Other two may have muddled i.e one having two 6 chromosomes and the other having no 6 chromosomes
      • 4) Chromosome mutation caused by errors during meiosis
      • 5) Lead to inherited conditions because errors present in gametes
      • Non- disjunction- failure of chromosomes to separate
      • Downs syndrome caused by extra copy of chromosome 21
      • Non disjunction means chromosome 21 fails to separate properly during meiosis, one cell gets extra copy of chromosome 21 and another gets none
      • When gamete with extra copy fuses during fertilisation, resulting zygote has three copies of chromosome 21


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