Meiosis and Sexual Life Cycles Notes

Overview of Meiosis and Sexual Life Cycles

• Chapter 10 - Focus on the process of meiosis and the distinctions between somatic cells, gametes, and the cycle of sexual reproduction.

Somatic Cells vs. Gametes

• Somatic Cells:

  • All body cells except reproductive cells.
  • Contain two of each chromosome type (Diploid, $2n$ = 2 complete sets).
  • Formed by mitosis, producing identical daughter cells.

• Gametes:

  • Sperm and eggs, with one copy of each chromosome (Haploid, $n$ = 1 complete set).
  • Formed by meiosis, producing unique cells.

Asexual vs. Sexual Reproduction

• Asexual Reproduction:

  • Uses mitosis; results in identical daughter cells.
  • Common in unicellular organisms.

• Sexual Reproduction:

  • Involves meiosis and fertilization.
  • Produces genetic diversity.
  • Male and female gametes unite to form a diploid zygote.

Human Life Cycle

• Ovaries and testes produce haploid gametes through meiosis.
• Gametes restore diploid condition upon fertilization.
• Diploid zygote develops into an adult.

Chromosomes and Genes

• Chromosomes:

  • Somatic cells have 46 chromosomes in pairs (22 autosomes + sex chromosomes).
  • Each pair contains homologous chromosomes carrying the same genes but may have different versions (alleles).

• Genes:

  • Units of heredity; one set of chromosomes is inherited from each parent.

Homologous Chromosomes

• Appearance:
  • Look alike, same length, centromere in the same location.
  • Contain different versions of genes (non-sister chromatids).

Meiosis Phases

Meiosis Overview

• Only occurs in gonads for gamete production.
• One diploid cell divides to form four haploid gametes.
• DNA replicates once, but cell divides twice.

Meiosis I and Meiosis II

• Meiosis I:
  • Homologous chromosomes separate, resulting in two haploid cells.
• Meiosis II:
  • Sister chromatids separate, resulting in four haploid gametes.

Stages of Meiosis

Meiosis I:

• Prophase I:

  • Spindle formation, nuclear membrane disappears, homologous chromosomes form tetrads, crossing over occurs.

• Metaphase I:

  • Homologous chromosomes line up along the metaphase plate randomly (independent assortment).

• Anaphase I:

  • Maternal and paternal chromosomes separate to opposite poles (disjunction).

• Telophase I and Cytokinesis:

  • Each haploid cell forms, with chromosomes still consisting of sister chromatids.

Meiosis II:

• Prophase II:

  • Chromosomes condense, spindle apparatus forms (no replication).

• Metaphase II:

  • Chromosomes line up at metaphase plate with sister chromatids attached.

• Anaphase II:

  • Sister chromatids separate and move to opposite poles.

• Telophase II and Cytokinesis:

  • Nuclear membranes reform; results in four haploid gametes.

Comparison: Mitosis vs. Meiosis

• Mitosis:

  • One division, produces two identical diploid cells, role in growth and repair.

• Meiosis:

  • Two divisions, produces four unique haploid gametes, role in sexual reproduction.

Genetic Variation

• Mechanisms of Genetic Variation:
  • Crossing Over:
  • Occurs in Prophase I, leads to recombination of genes between non-sister chromatids.
  • Independent Assortment:
  • Maternal and paternal chromosomes assort randomly into gametes during Metaphase I.
  • Random Fertilization:
  • Any sperm can combine with any egg, leading to unique diploid combinations in zygotes.

Evolutionary Significance

• Genetic variation through sexual reproduction is vital for adaptation and evolution.
• Natural selection acts upon this variation, driving evolutionary change.