9.2 - Meiosis & Genetic Variation

• why is meiosis important?

  allows for number of chromosomes in a cell to be halved forming gametes so when they fuse during fertilisation the number of chromosomes is right

  produces genetic variation among offspring through independent segregation & crossing over of homologous pairs which gives rise to new alleles so new features so better suitability for survival
• stages of meiosis

  interphase: DNA replicates

  meiosis I: homologous pairs of chromosomes are separated

  meiosis II: pairs of sister chromatids are separated
• stages in meiosis I

  prophase I: chromosomes condense and appear as two sister chromatids - crossing over may occur

  metaphase l: homologous pairs line up in the middle of the cell independently of each other

  anaphase l: homologous pairs separate, pulled apart by spindle fibres

  telophase l: nuclei forms around chromosomes forming two intermediate diploid cells
• stages in meiosis II

  prophase ll: spindle forms & chromosomes migrate to the middle of the cell

  metaphase Il: chromosomes line up on spindle fibres in the centre of the cell

  anaphase ll: sister chromatids pulled apart at the untromere

  telophase ll: nucleus forms around chromosomes

  cytokinesis Il: cytoplasm divides forming 4 haploid daughter cells
• what is independent segregation?

  when homologous pairs line up in meiosis I at radom so combination of maternal & paternal chromosomes that go into each daughter cell is completely random
• how does independent segregation cause variation

  each homologous pair has the same gene but different alleles so independent segregation produces new genetic combinations so each gamete is different
• what is crossing over?

  in meiosis I chromatids of each homologous twist around each other creating tension so parts of the chromatids break off and rejoin with chromatids of homologous partner so new combination of maternd and paternal

  bivalent
• recombination in crossing over

  broken off portions of chromatid recombine with another chromatid
• how does crossing over cause variation?

  produces 4 genetically different haploid daughter cells
• possible combinations of chromosomes for each daughter cell

  2^n

  n=number of pairs of homologous chromosomes
• possible combinations during random fertilisation

  (2^n)²

  n= number of pairs of homologous chromosomes

  ²=gametes - 1 paternal, 1 maternal
• meiosis vs mitosis

  mitosis produces diploid daughter cells, meiosis produces haploid

  in mitosis daughter cells are genetically identical, in meiosis they are genetically different

  mitosis produces 2 daughter cells, meiosis produces 4

  in mitosis there's 1 division, in meiosis there's 2.

  in meiosis there's crossing over and independent segregation where as in mitosis there isn't