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