Meiosis Lecture Notes

Overview of Meiosis

• Meiosis is essential for sexual reproduction.

• It involves the fusion of two gametes during fertilization, creating a new organism with the same chromosome number as its parents.

• Gametes possess half the chromosome number of their respective diploid parents, allowing for this match during fertilization.

Gamete Production

• Gametes are produced through meiotic cell division.

• They are classified as haploid (n), which means they contain half the number of chromosomes of diploid (2n) parent cells.

  • For example, human gametes contain n = 23 chromosomes, while parent cells have 2n = 46 chromosomes.

Ploidy Levels

• Ploidy defines the number of complete chromosome sets in a cell.

• A haploid cell (n) has one complete set of chromosomes, represented in humans by 23 chromosomes (e.g., sperm or egg cells).

• A diploid cell (2n) contains two complete sets, one from each parent, totaling 23 pairs of chromosomes in humans (e.g., somatic cells).

Transformation of Cells Preceding Meiosis

• During the S-phase (DNA synthesis phase), DNA replication occurs, doubling the amount of DNA but not the number of chromosomes; thus, it still remains a diploid cell.

  • For example, after DNA replication, a diploid somatic cell contains 4 complete sets of chromosomes (D, d, D, d), but only two unique chromosomes (weighing more in DNA).

Phases of Cell Division

• Interphase includes the following stages:

  • G1 phase (Gap 1)

  • S phase (DNA synthesis)

  • G2 phase (Gap 2)

• After interphase, cells reach a critical decision point where they can undergo either mitosis or meiosis.

Comparing Mitosis and Meiosis

• Differences between mitosis and meiosis include:

  1. Mitosis results in two genetically identical diploid daughter cells; meiosis produces four genetically unique haploid daughter cells.

  2. Each daughter cell in meiosis contains half the chromosome count of the parent cell, while mitosis maintains it.

  3. Meiotic cell division results in unique genetic combinations, while mitosis does not.

Structure of Meiosis

• Meiosis consists of two successive divisions: Meiosis I and Meiosis II.

  • Meiosis I reduces the chromosome number by separating homologous chromosomes.

  • Meiosis II separates sister chromatids, akin to mitosis.

Meiosis I Detailed Overview

• Steps of Meiosis I include:

  • Prophase I:

  • Homologous chromosomes pair (synapsis) forming bivalents, allowing for genetic exchange (crossing over).

  • Crossing Over:

  • Non-sister chromatids exchange genetic material, enhancing genetic variability.

  • Prometaphase I:

  • Meiotic spindles attach to kinetochores on chromosomes.

  • Metaphase I:

  • Bivalents align at the equatorial plane, with random orientation relative to each other.

  • Anaphase I:

  • Homologous chromosomes separate; sister chromatids remain together.

  • Telophase I and Cytokinesis:

  • Results in two daughter cells, each with one set of chromosomes (n), reducing ploidy from 2n to n.

Meiosis II Overview

• Meiosis II involves the separation of sister chromatids without prior DNA replication.

  • Each stage in meiosis II resembles mitosis:

  1. Prophase II: Preparatory phase for separation.

  2. Prometaphase II: Meiotic spindles attach to both sister chromatids.

  3. Metaphase II: All chromosomes align along the equatorial plane.

  4. Anaphase II: Separation of sister chromatids occurs, transitioning to individual gametes.

  5. Telophase II and Cytokinesis: Yield four distinct haploid cells from two sex cells.

Differences Between Mitosis and Meiosis II

• While meiosis II is similar to mitosis, key distinctions include:

  • In meiosis II, only half the chromosome sets are present (1 of the homologues).

  • Only recombined chromosomes are utilized in meiosis II, contributing to genetic diversity.

  • Cytoplasmic division differs significantly between sexes:

  • In females, one oocyte and three polar bodies result.

  • In males, cytoplasmic division is equal, leading to four viable sperm cells.

Summary Table of Mitosis and Meiosis