Sexual Reproduction and Meiosis

Sexual Reproduction and Meiosis

Overview of Sexual Reproduction

  • Definition: A process by which a new organism is produced through the fusion of two sex cells, also known as gametes.
  • Types of Sex Cells:
    • Egg cell (female gamete)
    • Sperm cell (male gamete)

Fertilization

  • Process: The joining of an egg and a sperm cell.
  • Outcome: A zygote is formed, which develops into a new organism with a unique genetic identity due to the combination of genetic material from two different organisms of the same species.

Types of Cells

  • Diploid vs. Haploid Cells:
    • Diploid Cells: Body cells (e.g. brain, skin, bones) composed of 46 chromosomes in humans (2n); these are non-gamete cells.
    • Haploid Cells: Sex cells (gametes) that contain half the number of chromosomes (n), resulting in 23 chromosomes in humans.

Chromosome Numbers

  • Diploid Number: Total number of chromosomes (2n) in non-gamete cells of a species.
    • Example:
    • Humans: 46 chromosomes
    • Nematodes: 4 chromosomes

Cell Division

Mitosis and Meiosis

  • Types of Cell Division:
    • Mitosis: Produces two clones of the original cell, each maintaining the same number of chromosomes.
    • Meiosis: Results in four gametes that are not identical to the original cell, containing half the number of chromosomes.

Chromosomes and Chromatin

  • Chromosomes: Long strands of DNA with histone proteins; these are visible during cell division.
  • Chromatin: A tangled mass of DNA visible only when a cell is not dividing.
  • Structure of Chromosomes:
    • Contains a centromere, histones, and DNA wrapped around nucleosomes.

Sister Chromatids and Homologous Chromosomes

  • Sister Chromatids: Identical copies of a chromosome created during DNA replication.
  • Homologous Chromosomes: A pair of chromosomes containing the same genes but potentially different alleles.

Meiosis

Stages of Meiosis
  1. Interphase (Preparation for Meiosis):
    • G1 Phase: Cell growth, synthesis of proteins and organelles, and normal cellular functioning.
    • S Phase: DNA replication occurs, synthesizing a complete copy of the DNA, producing identical sister chromatids for each chromosome.
    • G2 Phase: Further growth, synthesis of proteins and enzymes necessary for cell division, and final preparation for meiosis.
  2. Meiosis I (Reduction Division):
    • Prophase I: Chromosomes condense, homologous chromosomes pair up (synapsis) to form bivalents, and crossing over occurs at points called chiasmata. The nuclear envelope breaks down, and the spindle apparatus forms.
    • Metaphase I: Homologous chromosome pairs (bivalents) align randomly at the cell's equator (metaphase plate), demonstrating independent assortment.
    • Anaphase I: Homologous chromosomes separate and are pulled to opposite poles by spindle fibers. Sister chromatids remain attached at their centromeres.
    • Telophase I: Chromosomes decondense at the poles, the nuclear envelope may reform around each set of haploid chromosomes, and cytokinesis typically follows, resulting in two haploid daughter cells.
  3. Meiosis II (Division of Haploid Cells):
    • Prophase II: Chromosomes re-condense in the two haploid cells, a new spindle apparatus forms, and the nuclear envelope (if reformed) breaks down. Notably, DNA replication does not occur again.
    • Metaphase II: Individual chromosomes (each composed of two sister chromatids) align at the equator of each haploid cell.
    • Anaphase II: Sister chromatids are separated and move as individual chromosomes to opposite poles.
    • Telophase II: The nuclear envelope reforms around the decondensing chromosomes at each pole, followed by cytokinesis, resulting in four haploid daughter cells.
Detailed Process of Meiosis I
  • Prophase I:
    • Chromosome Condensation: Chromatin condenses into visible chromosomes.
    • Homologous Pairing: Homologous chromosomes align and form pairs (synapsis), forming a structure called a bivalent or tetrad.
    • Crossing Over: Genetic material is exchanged between non-sister chromatids of homologous chromosomes. This exchange occurs at specific points known as chiasmata, leading to genetic recombination.
    • Formation of Spindle Apparatus and breakdown of the nuclear envelope.
    • Chromosomes Attach to Spindle Fibers by their centromeres.
  • Metaphase I:
    • Homologous pairs align at the metaphase plate of the cell, with each pair orienting independently, contributing to independent assortment.
  • Anaphase I:
    • Homologous chromosomes are separated and pulled to opposite poles, effectively halving the chromosome number. Sister chromatids remain joined.
  • Telophase I and Cytokinesis:
    • Nuclear envelopes may reform around the haploid sets of chromosomes, indicating the end of the first meiotic division, leading to two haploid daughter cells.
Detailed Process of Meiosis II
  • Prophase II:
    • Chromosomes condense once more, and spindle apparatus forms in each daughter cell. The nuclear envelope, if present, disintegrates.
  • Metaphase II:
    • Chromosomes align individually at the metaphase plate in each haploid daughter cell.
  • Anaphase II:
    • Sister chromatids are separated and moved to opposite poles as individual chromosomes.
  • Telophase II & Cytokinesis:
    • Nuclear envelopes reform around the chromosomes, which decondense. Cytokinesis completes, resulting in a total of four non-identical haploid gametes.

Final Result of Meiosis

  • The end product of meiosis is four haploid daughter cells, each with unique genetic material and half the chromosome number (n) of the original parent cell.

Comparison Between Meiosis I and Meiosis II

  • Meiosis I: Separation of homologous chromosomes producing two haploid daughter cells; this is where genetic recombination (crossing over and independent assortment) occurs.
  • Meiosis II: Separation of sister chromatids producing four haploid daughter cells; no further genetic recombination occurs in this phase.

Purpose of Meiosis

  • Maintaining Chromosome Number: Gametes formed by meiosis must have half the chromosomes (23 or n) to restore the diploid number (46 or 2n) upon fertilization, preventing genetic abnormalities like Down Syndrome.
  • Creating Genetic Diversity: Crossing over and independent assortment generate variations essential for evolution and adaptation.
  • Repairing Genetic Defects: Recombination can replace defective genes from parents, contributing to healthier offspring.
  • Consequences of Improper Chromosome Number: If chromosome number is not halved, polyploid cells can occur, leading to fatal outcomes in most animals, although some plants can survive this condition.