MEIOSIS

žEach chromosome is made up of 2 chromatids and a centromere.

žThe 2 chromatids are held together by the centromere.

žEach chromosome contains small portions of DNA called the genes.

Chromosomes

  • Chromosomes consist of two chromatids joined by a centromere.

  • Somatic (body) cells → diploid (2n = 46) chromosomes.

  • Gametes (sperm/ova) → haploid (n = 23) chromosomes.

  • Chromosomes are arranged in pairs: 22 autosomes + 1 pair of sex chromosomes (XX in females, XY in males).

🌱 Meiosis Overview

  • Type of cell division producing 4 genetically different haploid cells.

  • Occurs in sex organs:

    • Spermatogenesis → sperm (males).

    • Oogenesis → ova (females).

  • Ensures chromosome number remains constant across generations.

🌀 Phases of Meiosis

Meiosis I

  • Interphase: DNA replication → double-stranded chromosomes.

  • Prophase I: Nuclear membrane disappears, crossing over occurs (exchange of genetic material).

  • Metaphase I: Homologous pairs align at equator.

  • Anaphase I: Whole chromosomes pulled to opposite poles.

  • Telophase I: Two haploid cells form.

Meiosis II

  • Similar to mitosis, but no interphase.

  • Prophase II: Chromosomes visible, nuclear membrane disappears.

  • Metaphase II: Single chromosomes align at equator.

  • Anaphase II: Chromatids separate.

  • Telophase II: Four haploid cells form, all genetically different.


  • Meiosis Diagram

🌍 Importance of Meiosis

  • Maintains chromosome number (halving and doubling effect).

  • Produces variation through:

    • Crossing over (Prophase I).

    • Random arrangement of chromosomes (Metaphase I & II).

    • Random fertilisation.

📊 Differences Between Meiosis I & II

  • Crossing over occurs only in Meiosis I.

  • Meiosis I halves chromosome number; Meiosis II keeps it constant.

  • Meiosis I separates homologous chromosomes; Meiosis II separates chromatids.

🔄 Meiosis vs Mitosis

  • Meiosis: 2 divisions, 4 haploid gametes, genetic variation.

  • Mitosis: 1 division, 2 diploid somatic cells, genetically identical.

⚠ Abnormal Meiosis

  • Non-disjunction → chromosomes fail to separate.

  • Aneuploidy: extra/missing chromosomes (e.g., Down Syndrome – Trisomy 21).

  • Polyploidy: extra sets of chromosomes (common in plants, rare in animals).

🌾 Advantages of Polyploidy in Agriculture

  • Produces seedless fruits (bananas, watermelons).

  • Larger flowers, fruits, and storage organs.

  • Infertile plants become fertile (e.g., wheat).

  • Greater disease resistance.