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Meiosis and Sexual Life Cycles

Asexual vs Sexual Reproduction

  • Asexual

    • Produces clones (genetically identical)

    • Single parent

    • Little variation in population - only through mutations

    • Fast and energy efficient

    • Eg. budding, binary fission

  • Sexual

    • Meiosis produces gametes (sex cells)

    • 2 parents: male/female

    • Lots of variation/diversity

    • Slower and energy consumptive

    • Eg. humans, trees

Chromosomes

  • Somatic (body) cell: 2n = 46 chromosomes

  • Each pair of homologous chromosomes includes 1 chromosome from each parent

  • Autosomes: 22 pairs of chromosomes that do not determine sex

  • Sex chromosomes: X and Y

    • Females: XX

    • Males: XY

  • Gametes (n=23): 22 autosomes + 1 sex chromosome

    • Egg: 22 + X

    • Sperm: 22 + X  **or** 22 + Y

Life Cycle

  • Life cycle: reproductive history of organism, from conception → production of own offspring

  • Fertilization and meiosis alternate in sexual life cycles

  • Meiosis: cell division that reduces # of chromosomes (2n → n), creates gametes

  • Fertilization: combine gametes (sperm + egg)

    • Fertilized egg = zygote (2n)

  • Zygote divides by mitosis to make multicellular diploid organism

Alternation of Generations

  • Plant and some algae

  • Sporophyte (2n): makes haploid spores by meiosis

    • Spore → gametophyte by mitosis

  • Gametophyte (n): makes haploid gametes by mitosis

Meiosis I

  • Interphase: chromosomes replicated

  • Prophase I:

    • Synapsis: homologous chromosomes pair up

    • Tetrad: 4 sister chromatids

    • Crossing over at the chiasmata

  • Metaphase I: Tetrads line up

  • Anaphase I:

    • Pairs of homologous chromosomes separate

    • (Sister chromatids still attached by centromere)

  • Telophase I & Cytokinesis:

    • Haploid set of chromosomes in each cell

    • Each chromosome = 2 sister chromatids

    • Some species: chromatin & nucleus reforms

Meiosis II

  • Prophase II:

    • No interphase

    • No crossing over

    • Spindle forms

  • Metaphase II:

    • Chromosomes line up

  • Anaphase II:

    • Sister chromatids separate

  • Telophase II:

    • 4 haploid cells

    • Nuclei reappear

    • Each daughter cell genetically unique

Events Unique to Meiosis I

  • Prophase I: Synapsis and crossing over

  • Metaphase I: pairs of homologous chromosomes line up on metaphase plate

  • Anaphase I: homologous pairs separate → sister chromatids still attached at centromere

Sources of Genetic Variation

  • Crossing Over: exchange genetic material, resulting in recombinant chromosomes

  • Independent Assortment of Chromosomes: random orientation of homologous pairs in Metaphase I

  • Random Fertilization: Any sperm + Any egg

Meiosis and Sexual Life Cycles

Asexual vs Sexual Reproduction

  • Asexual

    • Produces clones (genetically identical)

    • Single parent

    • Little variation in population - only through mutations

    • Fast and energy efficient

    • Eg. budding, binary fission

  • Sexual

    • Meiosis produces gametes (sex cells)

    • 2 parents: male/female

    • Lots of variation/diversity

    • Slower and energy consumptive

    • Eg. humans, trees

Chromosomes

  • Somatic (body) cell: 2n = 46 chromosomes

  • Each pair of homologous chromosomes includes 1 chromosome from each parent

  • Autosomes: 22 pairs of chromosomes that do not determine sex

  • Sex chromosomes: X and Y

    • Females: XX

    • Males: XY

  • Gametes (n=23): 22 autosomes + 1 sex chromosome

    • Egg: 22 + X

    • Sperm: 22 + X  **or** 22 + Y

Life Cycle

  • Life cycle: reproductive history of organism, from conception → production of own offspring

  • Fertilization and meiosis alternate in sexual life cycles

  • Meiosis: cell division that reduces # of chromosomes (2n → n), creates gametes

  • Fertilization: combine gametes (sperm + egg)

    • Fertilized egg = zygote (2n)

  • Zygote divides by mitosis to make multicellular diploid organism

Alternation of Generations

  • Plant and some algae

  • Sporophyte (2n): makes haploid spores by meiosis

    • Spore → gametophyte by mitosis

  • Gametophyte (n): makes haploid gametes by mitosis

Meiosis I

  • Interphase: chromosomes replicated

  • Prophase I:

    • Synapsis: homologous chromosomes pair up

    • Tetrad: 4 sister chromatids

    • Crossing over at the chiasmata

  • Metaphase I: Tetrads line up

  • Anaphase I:

    • Pairs of homologous chromosomes separate

    • (Sister chromatids still attached by centromere)

  • Telophase I & Cytokinesis:

    • Haploid set of chromosomes in each cell

    • Each chromosome = 2 sister chromatids

    • Some species: chromatin & nucleus reforms

Meiosis II

  • Prophase II:

    • No interphase

    • No crossing over

    • Spindle forms

  • Metaphase II:

    • Chromosomes line up

  • Anaphase II:

    • Sister chromatids separate

  • Telophase II:

    • 4 haploid cells

    • Nuclei reappear

    • Each daughter cell genetically unique

Events Unique to Meiosis I

  • Prophase I: Synapsis and crossing over

  • Metaphase I: pairs of homologous chromosomes line up on metaphase plate

  • Anaphase I: homologous pairs separate → sister chromatids still attached at centromere

Sources of Genetic Variation

  • Crossing Over: exchange genetic material, resulting in recombinant chromosomes

  • Independent Assortment of Chromosomes: random orientation of homologous pairs in Metaphase I

  • Random Fertilization: Any sperm + Any egg

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