Chapter 11: Sexual Reproduction and Meiosis

11.1 Overview of Heredity

• Heredity: The transmission of traits from one generation to the next.

• Variation: Differences in appearance among offspring compared to parents and siblings.

• Genetics: Scientific study of heredity and variation.

11.2 Genes and Chromosomes

• Genes: Units of heredity, made of DNA segments.

• Gametes: Reproductive cells (sperm and eggs) that carry genes to the next generation.

• Chromosomes: Humans have 46 chromosomes, packaged within somatic cells (non-reproductive).

• Locus: Specific position of a gene on a chromosome.

11.3 Modes of Reproduction

Asexual Reproduction

• Involves a single individual passing all genes, producing genetically identical clones.

• Examples: Hydra and Redwoods.

Sexual Reproduction

• Involves two parents resulting in offspring with unique gene combinations (variation).

11.4 Sexual Life Cycles

• Composed of meiosis and fertilization.

• Diploid Cells: Somatic cells with two sets of chromosomes (2n).

• Haploid Cells: Gametes with one set of chromosomes (n).

• Fertilization: Combines genetic material from two parents.

11.5 Meiosis Basics

• Meiosis: Reduces chromosome sets from diploid (2n) to haploid (1n).

• Involves two rounds of cell division: Meiosis I and Meiosis II.

• Results in four daughter cells, each with half the chromosomes of the parent cell.

11.6 Phases of Meiosis

Meiosis I

1. Prophase I: Chromosomes duplicate; homologous chromosomes undergo synapsis, forming bivalents.

2. Metaphase I: Bivalents align at the metaphase plate.

3. Anaphase I: Homologous chromosomes are pulled apart to opposite poles.

4. Telophase I: Nuclear envelopes may reform; cytokinesis may occur.

Meiosis II

• Similar to mitosis: sister chromatids are separated.

1. Prophase II: Nuclear envelopes dissolve; spindle apparatus forms.

2. Metaphase II: Chromosomes align at the metaphase plate.

3. Anaphase II: Sister chromatids are pulled to opposite poles.

4. Telophase II: Nuclear envelopes reform, resulting in four haploid cells.

11.7 Genetic Recombination and Variation

• Crossing Over: Exchange of chromosomal material between homologous chromosomes during prophase I.

• Chiasmata: Sites of crossover that maintain contact until anaphase I.

• Independent Assortment: Random orientation of chromosomes contributes to genetic variation.

11.8 Key Differences: Meiosis vs. Mitosis

1. Homologous Pairing: Only in meiosis, crossing over occurs.

2. Chromatid Cohesion: Sister chromatids segregate together in anaphase I.

3. Kinetochores Attachment: In meiosis I, kinetochores of sister chromatids attach to the same pole.

4. No DNA Replication: No replication between meiotic divisions.

11.9 Errors in Meiosis

• Nondisjunction: Failure of chromosomes to separate properly during division.

• Results in aneuploid gametes (missing or extra chromosomes), common in spontaneous abortions.

11.10 Meiosis in Human Life Cycle

• Germ-Line Cells: Located in testes (spermatogenesis) and ovaries (oogenesis), responsible for producing gametes through meiosis.

• Fertilization: Diploid zygote forms, undergoes mitotic divisions to develop into the adult organism.