🧬 Meiosis and Sexual Reproduction

Overview

Meiosis: Specialized cell division producing gametes (sex cells)

Key features:

• Two divisions: Meiosis I and Meiosis II
• Produces 4 haploid cells from 1 diploid cell
• Daughter cells are genetically different

Chromosome numbers:

• Diploid (2n): two sets of chromosomes (one from each parent)
• Haploid (n): one set of chromosomes
• Humans: 2n = 46, n = 23

Meiosis I (Reductional Division)

Interphase: DNA replicates (same as mitosis)

Prophase I (LONGEST phase)

• Chromatin condenses
• Synapsis: Homologous chromosomes pair up
• Tetrad (bivalent): 4 chromatids (2 homologous chromosomes)
• Crossing over: exchange of DNA between homologs
  • Occurs at chiasmata
  • Creates genetic variation
• Nuclear envelope breaks down
• Spindle forms

Metaphase I

• Tetrads align at metaphase plate
• Random orientation of homologs
• Independent assortment occurs

Anaphase I

• Homologous chromosomes separate
• Sister chromatids stay together
• Different from mitosis!

Telophase I and Cytokinesis

• Two haploid cells form
• Each has one chromosome from each homologous pair
• Sister chromatids still joined

Meiosis II (Equational Division)

Similar to mitosis but with haploid cells

Prophase II

• Chromosomes condense
• Spindle forms
• No crossing over

Metaphase II

• Chromosomes align at metaphase plate
• Single file (not pairs)

Anaphase II

• Sister chromatids separate
• Move to opposite poles

Telophase II and Cytokinesis

• Nuclear envelopes reform
• 4 haploid cells produced
• Each genetically unique

Sources of Genetic Variation

1. Crossing Over (Prophase I)

• Exchange of DNA between homologous chromosomes
• Creates new allele combinations
• Recombinant chromosomes

2. Independent Assortment (Metaphase I)

• Random orientation of homologous pairs
• 2ⁿ possible combinations (n = # of chromosome pairs)
• Humans: 2²³ = ~8 million combinations

3. Random Fertilization

• Any sperm can fertilize any egg
• (2²³)² = ~70 trillion combinations

Mitosis vs. Meiosis

| Feature | Mitosis | Meiosis | |---------|---------|---------| | Purpose | Growth, repair | Gamete production | | Divisions | One | Two | | Daughter cells | 2 diploid | 4 haploid | | Genetic identity | Identical to parent | Different from parent | | Crossing over | No | Yes (Prophase I) | | Homolog pairing | No | Yes (synapsis) |

Errors in Meiosis

Nondisjunction: Homologs or sister chromatids fail to separate

Results:

• Aneuploidy: abnormal chromosome number
• Monosomy: 2n - 1 (one chromosome missing)
• Trisomy: 2n + 1 (one extra chromosome)

Examples:

• Down syndrome: Trisomy 21 (3 copies of chromosome 21)
• Turner syndrome: Monosomy X (45, X)
• Klinefelter syndrome: XXY (47, XXY)

Key Concepts

1. Meiosis has two divisions producing 4 haploid cells
2. Meiosis I: homologous chromosomes separate
3. Meiosis II: sister chromatids separate
4. Crossing over (Prophase I) exchanges DNA
5. Independent assortment randomizes chromosome distribution
6. Three sources of variation: crossing over, independent assortment, random fertilization
7. Nondisjunction causes aneuploidy