AP Biology : 5.1 Meiosis – Study Notes

D. Meiosis
➢ Production of gametes
➢ Limited to sex cells in gonads

  •  gonads=sex organs
  •  Testes in males and ovaries in females
  •  Made up of germ cells

➢ Produces haploid cells which then combine to restore the diploid (2n) number during fertilization
➢ 2 rounds of cell division: meiosis I and meiosis II
➢ Just like in mitosis, double-stranded chromosomes are formed during S phase of interphase
Meiosis I

Prophase I
■ Nuclear membrane disappears
■ Chromosomes becomes visible
■ Centrioles move towards opposite ends of cell
● Chromosomes line up side-by-side with their homologs (counterparts)
● 2 sets of chromosomes come together to form a tetrad (aka bivalent) consisting of 4 chromatids
Crossing over
● Exchange of segments between homologous chromosomes
● Genetic variation
● Begins in Prophase I as homologous chromosomes line up gene by gene
● Produces recombinant chromosomes (DNA combined from each parent)
● Homologous portions of two nonsister chromatids trade placed
● Chromatids that are farther apart are more likely to cross over

  • Metaphase I
    ■ Tetrads line up along metaphase plate
    ■ Random alignment–more genetic variation
    ● Offspring will be a combination of all 4 grandparents
  •  ANaphase I
    ■ Each pair of chromatids within a tetrad separates and moves to opposite poles
    ■ Chromatids DO NOT separate at centromere
  • Telophase I
    ■ Nuclear membrane forms around each set of chromosomes
    ■ 2 daughter cells
    ■ Nucleus contains haploid number of chromosomes, but each chromosome is a duplicated chromosome consisting of 2 chromatids

➢ Meiosis II

  •  Purpose is just to separate sister chromatids
  •  Prophase II is the same
  •  Metaphase II: chromosomes move toward metaphase plate lining up in a single file, not in pairs
  •  Anaphase II:chromatids split at the centromere and each chromatid is pulled to opposite ends of cell
  •  Telophase II: nuclear membrane forms around each set of chromosomes and a total of 4 haploid cells are produced
  • Meiosis I separates homologous chromosomes; Meiosis II separates sister chromatids

➢ Gametogenesis

  •  Spermatogenesis if sperm cells are produced
  •  Oogenesis if egg cell/ovum is produced
    ■ Produces one ovum instead of 4
    ■ Other 3 cells, called polar bodies get only a tiny amount of cytoplasm and eventually degenerate
    ■ Allows female to conserve as much cytoplasm as possible for the surviving ovum

➢ Meiotic Errors

  •  Nondisjunction: chromosomes fail to separate properly

■ Produces wrong number of chromones
■ Usually results in miscarriage or significant genetic defects
■ Ex. Down syndrome is a result of 3 copies of the 21st chromosome

    •  Translocation
      ■ One or more segments of a chromosome break and are either lost or reattach to
      another chromosome
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