meiosis and genetic variation

• what is a gamete

  reproductive cells (sperm and egg)
• what is a diploid cell

  cell that has a diploid (2n) set of chromosomes - 2 homologous chromosomes ( one from mum one from dad)
• what is an example of a diploid cell

  body cells
• what is a haploid cell


  a cell that contains one set of chromosomes (n)
• what is an example of a haploid cell

  gamete
• what happens at fertilisation in terms of chromosomes

  haploid sperm fuses with haploid egg to form zygote with diploid number of chromosomes
• what is random fertilisation

  during sexual reproduction any sperm can fertilise any egg so produces zygotes with different combinations of each chromosomes from each parent
• random fertilisation in wildlife may cause reproduction between different species/ cross pollination, how is this prevented

  courting rituals
• describe meiosis

  1. in a diploid cell DNA unravels and replicates so there are 2 copies of each chromosome (chromatids)

  2. DNA condenses to form double armed chromosomes from 2 sister chromosomes connected by a centromere

  3. meiosis I (first division) - chromosomes arrange themselves into homologous pairs

  4. homologous pairs are separated - cell splits into 2 each with diploid number of chromosomes

  5. Meiosis II (second division)- the pairs of sister chromatids are separated (the centromere is divided)

  6. 4 genetically different haploid cells are produced
• what are the two main ways genetic diversity is created during meiosis

  crossing over and independent segregation
• what is crossing over

  recombination of maternal and paternal ALLELES during meiosis

  
• describe the process of crossing over

  -during prophase of meiosis I, homologous pairs of chromosomes form bivalents (pair up closely)

  - chromatids twist around each other -point where they join is called chiasma

  - fragments of non sister chromatids swap over so alleles are exchanged
• what is independent segregation

  -when the homologous pairs are separated in meiosis I it is completely random which chromosome from each pair ends up in the daughter cell

  -could recombine maternal and paternal chromosomes

  
• compare the outcomes of meiosis with the outcomes of mitosis

  meiosis- produces 4 daughter cells, produces haploid cells, cells have half number of chromosomes than parent cell, daughter cells are genetically different

  mitosis- produces 2 daughter cells, produces diploid cells, daughter cells have same number of chromosomes as parent cells, daughter cells are genetically identical
• why are the outcomes of mitosis different to meiosis

  mitosis only has one vision, meiosis has 2

  no pairing or separating of homologous chromosomes in mitosis so no crossing over or independent segregation (so daughter cells are genetically identical)
• what is chromosome mutation

  a variation in the number or parts of chromosomes that arise due to errors in meiosis
• why can chromosome mutate lead to inherited disease


  the chromosome variation is within the gametes
• what is non-disjunction

  the failure for chromosomes to separate completely during meiosis so one cell gets 2 copies of the same chromosome and the other gets none
• how does non- disjunction lead to downs syndrome

  -the failure of chromosome 21 to separate during meiosis (can be during meiosis I or meiosis II)

  - this means one cell will have 2 copies of chromosome 21 after meiosis and another have none

  - when a gamete with an extra copy of this chromosome fuses with another gamete at fertilisation there would be 3 copies of chromosome 21 which causes downs syndrome.