Meiosis & Sexual Reproduction

Cell division / Asexual reproduction

mitosis:

• produce cells with same information

  • identical daughter cells

  • exact copies (clones)

• same amount of DNA

  • same number of chromosomes

  • same genetic information

Asexual reproduction:

• single-celled eukaryotes

  • yeast (fungi)

  • protists

    • paramecium

    • amoeba

• simple multicellular eukaryotes

  • hydra

• disadvantages? no natural variation

Homologous Chromosomes:

• paired chromosomes

  • both chromosomes of a pair carry “matching” genes

    • control same inherited characters

    • homologous = same information

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how do we make sperm and eggs?

• must reduce 46 chromosomes → 23

  • must reduce the number of chromosomes by half

Meiosis: production of gametes

• alternating stages

  • chromosome number must be reduced

    • diploid → haploid

    • 2n → n

      • humans: 46 → 23

    • meiosis reduces chromosome number

    • makes gametes

  • fertilization restores chromosome number

    • haploid → diploid

    • n → 2n

Meiosis

• reduction division

  • special cell division for sexual reproduction

  • reduce 2n → 1n

  • diploid → haploid

    • “two” → “half”

  • makes gametes

    • sperm, eggs

Preparing for meiosis

• 1st step of meiosis

  • duplication of DNA

  • why bother?

    • meiosis evolved after mitosis

    • convenient to use. “machinery” of mitosis

    • DNA replicated in S phase of interphase of meiosis (just like in mitosis)

Steps of meiosis

• meiosis 1

  • interphase

  • prophase 1

  • metaphase 1

  • anaphase 1

  • telophase 1

    • 1st division of meiosis separates homologous pairs ( 2n → 1n ) “reduction division”

• meiosis 2

  • prophase 2

  • metaphase 2

  • anaphase 2

  • telophase 2

    • 2nd division of meiosis separates sister chromatids (1n → 1n) “just like mitosis”

Meiosis 1

• 1st division of meiosis

• seperates homologous pairs

Trading pieces of DNA

• crossing over

  • during prophase 1, sister chromatids interwine

    • homologous pairs swap pieces of chromosome

      • DNA breaks & re-attaches

Crossing over

• 3 steps

1. cross over

2. breakage of DNA

3. re-fusing of DNA

• new combination of traits

Meiosis 2

• 2nd division of meiosis

• seperates sister chromatids

mitosis vs meiosis

The value of sexual reproduction

• sexual reproduction introduces gentic variation

  • genetic recombination

    • independent assortment of chromosomes

    • random alignment of homologous chromosomes in Metaphase 1

  • crossing over

    • mixing of alleles across homologus chromosomes

  • random fertilization

    • which sperm fertilizes which egg?

• dividing evolution

  • providing variation for natrual selection

Variation for genetic recombination

• independent assortment for chromosomes

  • meiosis introduces genetic variation

  • gametes of offspring do not have same combination of genes as gametes from parents

    • randdom assortment in humans produces 8,388,608 different combination in agmetes

Variation from crossing over

• crossiing over creates completely new combination of traits on ech chromosome

  • creates an infinate variety in gametes

Variation from random fertilization

• sperm + egg = ?

  • any 2 parents will produce a zygote with over 70 trillion possible diploid combination

Sexual reproduction creates variability

• sexual reproduction allows us to maintain both genetic similarity and diffrences

Sperm production

• spermatogenesis

• continuois & prolific process

• each ejaculation = 100-600 million sperm

Egg Production

• Oogensis

  • eggs in ovaries halted before Anaphase 1

  • Meiosis 1 completed during maturation

  • Meiosis 2 completed after fertilization

  • 1 egg + 2 polar bodies

Diffrences across kingdoms

• not all organisms use haploid & diploid stages in same way

  • which on is dominant (2n or n) differs

  • but still alternate between haploid & diploid

    • must for sexual reproduction