Meiosis

Meiosis

Meiosis in outline
1. one diploid nucleus --> 4 haploid nucleus (by meiosis)
2. meiosis I
  1. 1 nuclei --> 2 nuclei
  2. 4 chromatids --> 2 chromatids
3. meiosis II
  1. 2 nuclei --> 4 nuclei
  2. 2 chromatids --> 1 chromatid
4. known as reduction division
Meiosis and sexual life cycles
1. fusion of gametes
2. halving of chromosome numbers allows
3. asexual cycle
  1. same chromosomes as parent
  2. genetically identical
4. sexual cycle
  1. different chromsomes
  2. genetic diversity
5. involves fertilization
  1. union of sex cells/gametes
  2. doubles chromosome number
6. occurs during gamete creation
Replication of DNA before meiosis
1. all chromosomes consist of sister chromatids
2. during interphase
3. initially genetically identical chromatids
4. very accurate process
5. replication does not occur again
Bivalents formation and crossing over
1. pairing homologous chromosomes
  1. 4 DNA molecules
  2. process called synapsis
  3. homo chromosomes are bivalent
2. crossing over
  1. after synapsis
  2. one chromatid in HC breaks off and joins with other
    1. occurs at same position so mutual exchange of genes
      1. some alleles are different so new allele combinations are formed
  3. occurs at random positions
  4. at least one in each bivalent
Random orientation of bivalents
1. spindle microtubules grow from cell poles
  1. attach to chromosome centromeres after nuclear membrane breaks down
  2. not the same as mitosis
  3. each chromosome attached to one pole only
  4. two HCs in bivalent attached to diff poles
  5. pole depends on which way pair is facing
    1. called orientation
2. equal chance for chromosome to attach to each pole
3. orientation of one doesn't affect others
Halving the chromosome number
1. during separation in Meiosis I
  1. centromere does not divide
  2. whole chromosome moves
2. initially two chromosomes in each bivalent held by chiasmata
  1. slide to the end; chromosomes can separate
  2. entire process called disjunction
3. caused by separation of HC pairs to opposite poles
Meiosis and genetic variation
1. promoted by crossing over and random orientation
2. equal chance for both alleles of a gene
3. meiosis promotes random allele combinations
4. RO generates genetic variation in genes on different chromosomes
  1. in metaphase I
5. crossing over in prophase I increases the number of possible allele combinations
Fertilization and genetic variation
1. promoted by fusion of gametes from diff parents
2. allows alleles from parents to mix
3. combination of alleles created is unique
4. GV essential to evolution

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