Meiosis

Modes of Reproduction

• Asexual Reproduction

  • involves only 1 parent

  • produces (nearly) identical offspring

  • pros: rapid, efficient

• Sexual Reproduction

  • usually involves 2 parents

  • produces unique offspring

  • pros: genetic diversity

Sets of Chromosomes

• Homologous Chromosomes

  • AKA homologs

  • pairs of chromosomes

    • same size

    • same genes

  • includes

    • 1 maternal

    • 1 parental

  • sex chromosomes

    • determine sex of individual

    • humans- 1 pair (x,y)

  • autosomes

    • all of the other chromosomes

    • humans have 22 pairs

• Diploid Cells (2n)

  • contain both homologs from each pair

  • somatic body cells

• Haploid Cells (n)

  • only contain one homolog from each pair

  • gametes (sex cells)

Sexual Life Cycles

• begins w/ fertilization

  • n+n= 2n

  • haploid sperm and haploid egg= diploid zygote

    • zygote: first diploid cell of an embryo

• embryo undergoes mitosis to make more cells

• once mature, cells in reproductive organs start meiosis, sperm or egg

Meiosis

• only sexual reproduction

• 2 rounds of division

  • reduces the # of chromosomes in each cell (reduction division)

  • 1 diploid cell= 4 haploid cells

• 1. Interphase

  • same as before in mitosis

    • growth+DNA replication

    • DNA= chromatin

• 2. Prophase I

  • mostly same as mitosis

  • additional:

    • synapsis occurs

      • homologous pair up in tetrads

      • held together by synaptonemal complex (proteins)

  • crossing over occurs

    • exchange of DNA b/t non-sister chromatids of homologs

    • occurs at chiasmata

    • “mixes up” paternal and maternal DNA

• 3. Metaphase I

  • homologous pairs line up in the middle

• 4. Anaphase I

  • homologous pairs are separated

• 5. Telophase I and Cytokinesis

  • same in mitosis

  • cyto. divides into 2 haploid cells

    • b/t have ½ the original number of chromosomes

    • but still have 2 copies of each gene bc they are replicated chromosomes

• 6. Interkenesis

  • not for all cells

  • a suspended rest period

  • no additional DNA replication

• 7. Prophase II

  • pretty much the same except with two cells instead of one

  • no synapsis or crossing over

    • synapsis: fusing of chromosomes

    • crossing: exchange of genetic information between two chromosomes

• 8. Metaphase II

  • single chromosomes line up in the middle in both cells

• 9. Anaphase II

  • sister chromatids are separated in both cells

• 10. Telophase II and Cytokinesis

  • telo=same

  • cyto. divides into 4 genetically unique haploid daughter cells

• G1 of Interphase (prereplication)

  • 4 unreplicated chromosomes

  • 2 copies of each gene

• G2 of Interphase (postreplication)

  • 4 replicated chromosomes, 8 chromatids

  • 4 copies of each gene

• End of Meiosis I

  • 2 replicated chromosomes

    • 4 chromatids

  • 2 copies of each gene

• End of Meiosis II

  • 2 unreplicated chromosomes

  • 1 copy of each gene

Genetic Diversity

• 1. Crossing Over

  • exchanged DNA creates recombinant chromosomes

    • carry 2 genes derived from 2 parents

  • non recombinant, entirely maternal or paternal DNA

• 2. Independent Assortment

  • chromosomes line up and separate randomly in meta/anaphase

  • leads to diff. combos of chrome. in sex cells

• 3. Random Fertilization

  • nothing really influences which sperm arrives first

  • endless possibilities combos for the zygote

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