Meiosis Notes

Ploidy

• Ploidy: number of chromosome sets in a cell; each set = 'n'.

• Haploid (n): one set of chromosomes (gametes).

• Diploid (2n): two sets of chromosomes (somatic and germ cells).

  • One set from each parent.

Karyotype and Chromosomes

• Karyotype: describes the number and appearance of autosomes and sex chromosomes.

• Homologous chromosomes: matching pair of chromosomes (one from each parent).

• Autosomes: non-sex chromosomes.

• Sex chromosome: determines individual's sex.

Meiosis Overview

• Sexual reproduction involves two parents, forming a zygote from two gametes (egg and sperm).

• Meiosis: one diploid germ cell produces four haploid cells.

• Germ cell: diploid cell undergoing meiosis.

• Gamete: haploid sex cell for sexual reproduction.

• Meiosis involves two rounds of division:

  • Meiosis I: Homologous chromosomes separate.

  • Meiosis II: Sister chromatids separate.

Meiosis I

• Prophase I:

  • Synapsis occurs: Homologous chromosomes pair up.

  • Crossing over occurs within tetrads.

  • This results in more genetic diversity

• Metaphase I:

  • Tetrads line up along metaphase plate.

  • Independent assortment occurs.

• Anaphase I: Homologous chromosomes separate.

• Telophase I: Nuclear envelope reforms, cytokinesis begins.

Meiosis II

• Prophase II: Nuclear envelope breaks down, spindle forms.

• Metaphase II: Chromosomes align at metaphase plate, spindle fibers attach to kinetochores.

• Anaphase II: Sister chromatids separate.

• Telophase II: Nuclear envelope reforms, cytokinesis begins.

Genetic Diversity in Meiosis

• Crossing Over:

  • Parts of non-sister chromatids exchange at chiasma, resulting in recombinant chromatids (maternal and paternal DNA).

  • Chiasma (location where synapasis occurs) can occur at multiple locations.

• Independent Assortment:

  • Random alignment of homologous chromosomes during metaphase I, leading to different combinations of chromosomes in daughter cells.

Errors in Meiosis

• Nondisjunction: Chromosomes not separated properly during Meiosis I or II.

  • Results in daughter cells with incorrect chromosome numbers.

• Consequences: Zygotes formed from affected gametes have incorrect chromosome numbers, leading to developmental abnormalities.

• Down Syndrome (Trisomy 21): three copies of chromosome 21.

Mitosis vs. Meiosis

• Mitosis:

  • Two genetically identical diploid cells.

  • One round of cell division.

  • No genetic recombination.

  • Function: growth/development and tissue repair.

• Meiosis:

  • Four genetically different haploid cells.

  • Two rounds of cell division.

  • Genetic recombination (crossing over and independent assortment).

  • Tetrads form.

  • Function: produce gametes for sexual reproduction.

• Similarities:

  • Both start with one diploid cell.

  • Both involve interphase.