🧬 AP BIO MEIOSIS STUDY NOTES

🔄 Types of Reproduction

ASEXUAL REPRODUCTION

• Involves only one parent.

• Produces offspring that are genetically identical to the parent (clones).

• No gametes involved; cells divide by processes like binary fission, budding, or parthenogenesis.

• Examples:

  • Bacteria (binary fission)

  • Yeast and Hydra (budding)

  • Lizards, ants, bees (parthenogenesis)

Advantages:

• Faster reproduction.

• Requires less energy since no mate is needed.

Disadvantages:

• Little to no genetic variation. This makes the population more vulnerable to environmental changes and diseases.

SEXUAL REPRODUCTION

• Involves two parents.

• Offspring are genetically different from both parents.

• Genetic variation is created through:

  • Crossing over (Prophase I of meiosis)

  • Independent assortment (Metaphase I)

  • Random fertilization (fusion of unique gametes)

Advantages:

• Increases genetic diversity, which helps populations survive changes in the environment.

Disadvantages:

• Slower process.

• Requires more energy and time to find a mate.

• Greater exposure to predators and environmental risks during mating.

🔬 What is Meiosis?

• Meiosis is a type of cell division that reduces the number of chromosomes by half, producing haploid gametes (sperm or egg).

• It occurs in the gonads:

  • Testes in males (produces sperm)

  • Ovaries in females (produces eggs)

• Diploid (2n) parent cells → Haploid (n) gametes.

  • Humans: 46 → 23 chromosomes

  • Gorilla: 48 → 24

  • Dogs: 78 → 39

  • Elephants: 56 → 28

🧩 Chromosomes and Homologous Pairs

• Homologous chromosomes: A pair of chromosomes (one from each parent) that have the same genes in the same order, but may have different alleles.

• Humans have 23 pairs of homologous chromosomes (46 total):

  • Pairs 1–22: Autosomes (control traits unrelated to sex)

  • Pair 23: Sex chromosomes

    • XX = Female

    • XY = Male

📚 Phases of Meiosis

➤ Meiosis has two divisions: Meiosis I and Meiosis II

➤ One DNA replication (S phase), but two rounds of division

📍 Meiosis I — Reduction Division

Purpose: Separate homologous chromosomes, reduce chromosome number by half.

1. Prophase I:

   • Chromosomes condense and become visible.

   • Homologous chromosomes pair up in a structure called a tetrad.

   • Crossing over occurs — exchange of genetic material between non-sister chromatids (increases genetic variation).

2. Metaphase I:

   • Tetrads align at the metaphase plate.

   • Independent assortment: Each pair aligns randomly, leading to many possible chromosome combinations in gametes.

3. Anaphase I:

   • Homologous chromosomes separate and move to opposite poles.

   • Sister chromatids remain attached.

4. Telophase I and Cytokinesis:

   • Two haploid cells form.

   • Chromosome number is reduced by half.

🧘 Interkinesis

• A short resting phase between meiosis I and II.

• NO DNA replication happens here.

📍 Meiosis II — Like Mitosis

Purpose: Separate sister chromatids of each chromosome.

1. Prophase II: Chromosomes condense again; spindle fibers form.

2. Metaphase II: Chromosomes line up at the equator (single file).

3. Anaphase II: Sister chromatids separate and move to opposite poles.

4. Telophase II and Cytokinesis:

   • Four genetically unique haploid cells form.

   • In males: 4 sperm cells. In females: 1 egg + 3 polar bodies.

⚖ Comparison: Mitosis vs Meiosis

⚠ Errors in Meiosis – Nondisjunction

• Nondisjunction: Chromosomes fail to separate properly during Anaphase I or II.

• Leads to aneuploidy: abnormal number of chromosomes.

Where It Happens:

• Meiosis I error: All resulting cells are affected.

• Meiosis II error: Only 2 out of the 4 cells are abnormal.

✅ KEY CONCEPTS TO REMEMBER

• Meiosis = Reduction division → Genetic diversity.

• Crossing over = Exchange of genes.

• Independent assortment = Random chromosome alignment.

• 1 diploid → 4 haploid gametes.

• Nondisjunction = Chromosome separation failure → genetic disorders.