Chapter 9 part 1 Meiosis and the Genetic Basis of Sexual Reproduction

Sexual Reproduction

• Involves the production of haploid (n) cells by parents.
• Fusion of haploid cells (fertilization) produces a diploid (2n) cell.

Meiosis Overview

• Meiosis: Nuclear division in sexual reproduction, daughter cells receive a haploid number of chromosomes.
• Reduces chromosome number from 2n to n.
• Produces four genetically different daughter nuclei.

Basics of Meiosis

• Two major functions:
  • Reducing chromosome number.
  • Shuffling chromosomes to produce genetically different gametes (sperm and egg).
• Homologous chromosomes (homologues): Same size, shape, centromere location, and genes.
• Alleles: Alternate forms of a gene.
• Humans: 23 pairs of chromosomes (2n = 46), 22 pairs of autosomes, 1 pair of sex chromosomes (XX female, XY male).

Life Cycle

• Involves mitosis and meiosis.
• Meiosis reduces chromosome number from diploid to haploid.
• Gametes (egg and sperm) have one member of each homologous pair (n).
• Egg and sperm join to form a diploid zygote (2n).

Meiosis Process

• Two rounds of nuclear division: Meiosis I and Meiosis II.
• Meiosis I: Homologous pairs line up (synapsis), then separate.
• Meiosis II: Sister chromatids separate, resulting in 4 daughter cells.

Crossing-Over

• Occurs during prophase I, homologous chromosomes pair up forming a tetrad (synapsis).
• Chiasma: Point where exchange of genetic material occurs between non-sister chromatids.
• Increases genetic variability.

Importance of Meiosis

• Chromosome number stays constant across generations by producing haploid gametes.
• Generates genetic variation through crossing-over and random chromosome combinations.
• Fertilization produces new combinations: (2^{23})^2 or 70,368,744,000,000 possible zygotes.

Phases of Meiosis

• Meiosis I and II, each with prophase, metaphase, anaphase, and telophase.
• Before meiosis I, each chromosome duplicates into two sister chromatids.

Meiosis I

• Prophase I: Tetrads form, crossing-over occurs, nuclear envelope fragments.
• Metaphase I: Tetrads align at spindle equator.
• Anaphase I: Homologues separate, dyads move to poles.
• Telophase I: Daughter nuclei are haploid with duplicated chromosomes.

Meiosis II

• Prophase II: Chromosomes condense, nuclear envelope fragments.
• Metaphase II: Dyads align at spindle equator.
• Anaphase II: Sister chromatids separate, becoming daughter chromosomes.
• Telophase II: Four haploid daughter cells form, genetically different.

Gamete Formation

• Spermatogenesis: Produces sperm in testes.
• Oogenesis: Produces eggs in ovaries.

Fertilization

• Each parent contributes half of the chromosomes.

Meiosis vs. Mitosis

• Meiosis: Two divisions, produces 4 genetically different haploid cells, crossing-over occurs.
• Mitosis: One division, produces 2 genetically identical diploid cells, no crossing-over.

Key Differences

• Meiosis occurs in specific tissues (testes and ovaries), mitosis in somatic cells.
• Meiosis is for sexual reproduction, mitosis for growth and repair.
• Meiosis: Synapsis and crossing over in prophase I, chromosome number reduced.
• Mitosis: No synapsis or crossing over, chromosome number maintained.