Chapter 11 Meiosis and Sexual Reproduction

Chapter 11: Meiosis & Sexual Reproduction

Part 1: Meiosis

• Understanding Chromosomes:

  • Chromosomes consist of homologous pairs, with one chromosome from each parent.

  • Types of Chromosomes:

  • Autosomes: Non-sex chromosomes (humans have pairs 1-22)

  • Sex Chromosomes: Determined as X & Y

    • XX = Female

    • XY = Male

• Ploidy Measurement:

  • Diploid (2n):

  • Humans have 23 pairs (46 total chromosomes)

  • Haploid (n):

  • Single set in gametes (n = 23)

  • Important in avoiding chromosome count doubling each generation.

• Meiosis Definition:

  • A reduction division process necessary in sexually reproducing organisms to halve chromosome sets.

  • Results in 4 haploid cells (either sperm OR egg).

• Meiosis Process:

  • Involves two rounds of division:

  • Meiosis I: Homologous chromosomes separate (2n -> n).

  • Meiosis II: Sister chromatids separate to produce four haploid daughter cells.

• Stages of Meiosis I:

  • Prophase I: Homologous chromosomes pair (synapsis) and crossing over occurs, forming tetrads.

  • Metaphase I: Tetrads align at the metaphase plate with random orientation (independent assortment).

  • Anaphase I: Homologous chromosomes move to opposite poles.

  • Telophase I: Reform nuclear envelope around separated chromosomes.

• Differences Between Mitosis and Meiosis:

  • Crossing Over: Only during Meiosis I, causing genetic variation.

  • Separation of Chromosomes:

  • Meiosis: Homologous pairs separate; no sister chromatid separation in Anaphase I.

  • Mitosis: Sister chromatids separate.

Part 2: Reproduction

• Types of Reproduction:

  • Asexual Reproduction: Identical offspring (clones) such as budding, binary fission, etc.

  • Advantages: Rapid population increase, no mate required.

  • Disadvantage: No genetic variation.

  • Sexual Reproduction:

  • Mixing genes to create variable offspring, promoting diversity.

  • Gametes produced through meiosis.

• Diploid-Dominant Life Cycle:

  • Observed in humans and most animals.

  • Diploid (2n) cells primarily in the life cycle; only haploid (n) cells are gametes (sperm and egg).

• Methods Contributing to Genetic Variation:

  1. Crossing Over: Genetic recombination during prophase I.

  2. Independent Assortment: Different combinations during meiosis I; total variations = 2ᶮ (n = number of chromosome pairs).

  3. Random Fertilization: Contribution from genetically distinct sperm and egg; resulting in many possible combinations.

Gamete Formation

• Spermatogenesis:

  • Formation of sperm cells from spermatogonia (2n) within testes.

  • Meiosis leads to 4 haploid spermatozoa (n).

• Oogenesis:

  • Formation of egg cells from primary oocytes in ovaries.

  • Produces large secondary oocyte and smaller polar bodies, ultimately forming one ovum and potentially 2-3 polar bodies.

Review: Mitosis vs. Meiosis

• Mitosis:

  • Produces genetically identical diploid cells (2n).

  • One division, no crossing over, outcomes = two identical cells.

• Meiosis:

  • Produces non-identical, haploid cells (n).

  • Two divisions, involves crossing over, outcomes = four cells with genetic variation.