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Meiosis: Structure, Mechanisms, Variation, and Consequences

Meiosis – Life Sciences Grade 12: Comprehensive Study Notes

Structure of Chromosomes

  • Chromatin condenses into visible chromosomes before cell division.

  • A chromosome has 2 chromatids joined at the centromere.

  • Each chromosome consists of genes (DNA portions).

  • Key terms:

    • Chromosome: DNA molecule with associated proteins carrying genes.

    • Chromatid: One of two identical halves of a replicated chromosome.

    • Centromere: Region joining chromatids; attachment site for spindle fibers.

Chromosomes in Somatic and Sex Cells

  • Somatic cells: Diploid (2n), make up most body tissues.

  • Sex cells (gametes): Haploid (n), egg and sperm.

  • Humans: 2n = 46 chromosomes in somatic cells; n = 23 chromosomes in gametes.

Terminology and Basic Numbers

  • Gametes: Haploid (n).

  • Somatic cells: Diploid (2n).

  • Karyotype: Chromosome number and appearance; used to detect genetic diseases.

  • Haploid (n): One complete set of chromosomes.

  • Diploid (2n): Two complete sets of chromosomes.

  • Autosomes: Non-sex chromosomes (human pairs 1–22).

  • Gonosomes: Sex chromosomes (X and Y in humans).

  • Homologous pair: Pair of chromosomes with same genes, one from each parent.

  • Human Karyotype: 23 pairs (22 autosome pairs + 1 pair of gonosomes).

    • Females: XX (23rd pair); Males: XY (23rd pair).

    • Somatic cells: 2n = 46 (44 autosomes + 2 gonosomes).

    • Gametes: n = 23.

Meiosis: Overview and Life Cycle Relevance

  • Meiosis produces 4 haploid cells from 1 diploid parent cell; cells are genetically diverse.

  • Occurs in plants and animals to produce gametes (animals) or spores (plants).

  • Purpose: Halve chromosome number for fertilization to restore diploidy.

  • Halving effect: Meiosis reduces chromosome number by half.

  • Doubling effect: Fertilization restores diploid chromosome number.

Meiosis Phases: I and II

  • Meiosis includes 2 divisions: Meiosis I and Meiosis II.

  • Interphase (before Meiosis I only): DNA replication occurs, chromosomes become double-stranded.

  • End result: 4 haploid daughter cells, genetically distinct.

Meiosis I

  • Prophase I:

    • Nuclear membrane dissolves; chromatin condenses.

    • Crossing over occurs: exchange of genetic material between non-sister chromatids of homologous chromosomes; increases genetic variation.

  • Metaphase I:

    • Homologous pairs (bivalents) align at the equator randomly (independent assortment).

  • Anaphase I:

    • Homologous chromosomes separate and move to opposite poles; sister chromatids remain attached.

  • Telophase I:

    • Chromosomes gather at poles; cytoplasm divides (cytokinesis) into two haploid cells.

    • Each chromosome still has two sister chromatids.

Meiosis II

  • No interphase between Meiosis I and II.

  • Prophase II:

    • Chromosomes condense; nuclear envelope breaks down.

  • Metaphase II:

    • Single chromosomes (each with two chromatids) align at the equator.

  • Anaphase II:

    • Sister chromatids separate and are pulled to opposite poles; each chromatid is now a single chromosome.

  • Telophase II:

    • Nuclei form; cytoplasm divides, creating four haploid daughter cells.

Crossing Over and Genetic Variation

  • Crossing over (Prophase I): Exchanges genetic material, producing recombinant chromosomes.

  • Independent assortment (Metaphase I): Random alignment of homologous pairs.

  • These mechanisms create genetic diversity.

  • Number of possible gamete combinations in humans from independent assortment: 2^{n} where n=23, so approximately 2^{23} \approx 8.39 \times 10^{6}.

Differences Between Meiosis I and Meiosis II

  • Crossing over: Meiosis I (Prophase I) only.

  • Alignment at equator: Meiosis I aligns homologous pairs; Meiosis II aligns single chromosomes (sister chromatids).

  • Separation: Meiosis I separates homologous chromosomes; Meiosis II separates sister chromatids.

  • Chromosome number: Meiosis I halves it (2n \rightarrow n); Meiosis II maintains the haploid number for its division.

  • Chromosome condition: Meiosis I chromosomes are double-stranded; Meiosis II chromosomes become single-stranded after Anaphase II.

  • Products: Meiosis I yields two haploid cells; Meiosis II yields four haploid cells.

Similarities Between Meiosis and Mitosis

  • DNA replication occurs.

  • Nuclear and cytoplasmic division.

  • Both produce new cells.

Consequences of Abnormal Meiosis

  • Non-disjunction: Failure of homologous chromosomes or sister chromatids to separate.

    • Occurs in Anaphase I or Anaphase II.

    • Leads to gametes with abnormal chromosome numbers.

  • Aneuploidy: Organism has one or few chromosomes above or below normal.

  • Trisomy: Extra copy of a chromosome (e.g., Down’s syndrome is trisomy 21).

  • Monosomy: Missing copy of a chromosome.

  • Polyploidy: More than two complete sets of chromosomes (e.g., 3n, 4n).

  • Down’s syndrome (Trisomy 21): Extra chromosome 21 (47 total). Detected by amniocentesis and karyotyping.

  • Polyploidy in agriculture: Common in plants; induced to create seedless fruits (e.g., watermelons), larger organs, or increased disease resistance. Colchicine is used to induce it.

Summary: Core Takeaways

  • Meiosis produces four genetically distinct haploid gametes, halving chromosome number and generating variation.

  • Genetic variation comes from crossing over and independent assortment.

  • Abnormal meiosis (non-disjunction) can cause chromosomal disorders like Down’s syndrome.

  • Polyploidy has significant agricultural applications.