Bio 112 Genetics Unit

Goals of Meiosis

  • Specialized cell division

  • Reduces chromosome number

  • Produces genetically distinct gametes (sperm and eggs)

  • Generates 4 progeny cells from 1 diploid parent cell (2n=2)

Mechanisms of Genetic Variation

  • Critical for survival; aids adaptation

  • Achieved through:

    • Crossing-over: DNA segments exchange during Prophase I

    • Independent assortment: Random chromosome segregation in Metaphase I

Overview of Meiosis Steps

  • Meiosis consists of two sequential divisions:

    1. Meiosis I: Homologous chromosomes separate. Each of the two resulting cells is haploid (n=1) but contains sister chromatids still attached at their centromeres.

    2. Meiosis II: Similar to mitosis, wherein sister chromatids finally separate. Each meiosis cycle creates four haploid gametes from the original diploid cell.

Phases of Meiosis II

  • Anaphase II: Sister chromatids are pulled apart to opposite poles.

  • Telophase II and Cytokinesis: Result in four genetically varied haploid daughter cells, each having half the genetic content of the parent cell.

Interphase before Meiosis

  • Germline cell undergoes G1, S, G2 (DNA replication)

Details of Meiosis I

  • Early Prophase I: Chromosome condensation, nuclear membrane disassembles

    • Synapsis forms tetrads

  • Crossing-Over: Occurs at chiasmata, increasing genetic diversity

  • Metaphase I: Random alignment of homologous chromosomes

  • Anaphase I: Homologous pairs pulled apart

  • Telophase I: Formation of two haploid cells

Independent Assortment

Metaphase I:

Homologous chromosomes align at the metaphase plate, oriented randomly. The outcome is a further increase in genetic diversity due to the random distribution of maternal and paternal chromosomes to gametes.

Anaphase I and Telophase I:

During Anaphase I, homologous pairs are pulled apart to opposite poles. In Telophase I, cells may undergo cytokinesis to form two haploid cells, completing the first meiotic division.

Meiosis II Similarities to Mitosis:

Meiosis II bears resemblance to mitosis, wherein sister chromatids separate resulting in four haploid daughter cells. The genetic content is halved and the variety among each gamete is maintained.

Genetic Variation Through Random Fertilization:

  • Random combination of maternal/paternal chromosomes

  • Egg and sperm have ~8 million combinations; ~64 trillion offspring combinations