Meiosis

Meiosis

• Overview of Meiosis

  • Process of cell division that reduces the chromosome number by half to form gametes (sperm and egg).

  • Consists of two main divisions: Meiosis 1 and Meiosis 2.

  • Essential for sexual reproduction, allowing genetic variation.

Key Definitions

• Interphase: The phase before meiosis where the DNA is replicated.

• Homologous Chromosomes: Paired chromosomes carrying genes for the same traits, one from each parent (diploid, 2n).

• Gametes: Male and female reproductive cells (sperm and egg), which are haploid (1n).

Asexual Reproduction

• Involves one parent and does not require gametes.

• Offspring are genetically identical to the parent.

• Common in bacteria, plants, and some animals.

• Types include binary fission in bacteria and budding in hydras.

Cell Division

• Mitosis:

  • Produces two identical daughter cells, maintaining the chromosome number (2n).

  • Involves a single division (no genetic variation).

• Comparison:

  • Mitosis results in clones, while meiosis introduces variation through crossing over and independent assortment.

Sexual Reproduction

• Involves two parents and gametes (sperm and egg).

• Results in offspring with mixed DNA, providing genetic diversity.

• Fusion of sperm and egg during fertilization restores the diploid number.

Meiosis Process

Meiosis 1

• First Division: Separates homologous chromosomes.

• Goes through stages:

  • Interphase: DNA replication occurs.

  • Prophase 1: Homologous chromosomes pair (synapsis) and exchange genetic material (crossing over).

  • Metaphase 1: Homologous pairs align at the metaphase plate.

  • Anaphase 1: Homologous chromosomes are pulled apart.

  • Telophase 1: Cell divides into two haploid cells.

Meiosis 2

• Second Division: Similar to mitosis, but results in four genetically unique haploid cells.

• Stages include:

  • Prophase 2: Chromosomes condense without DNA replication.

  • Metaphase 2: Chromosomes align individually.

  • Anaphase 2: Sister chromatids are separated.

  • Telophase 2: Cells divide, resulting in four distinct haploid cells.

Genetic Variation

• Crossing Over (Genetic Recombination):

  • Occurs during Prophase 1 and involves the exchange of DNA between homologous chromosomes.

  • Introduces new combinations of alleles.

• Independent Assortment:

  • The random distribution of homologous chromosome pairs during metaphase 1.

  • Leads to diverse genetic combinations in gametes.

• Random Fertilization:

  • Any sperm can fertilize any egg, resulting in over 70 trillion potential combinations in offspring.

Significance of Meiosis

• Necessary for the production of gametes (gametogenesis) for sexual reproduction.

• Maintains chromosome numbers across generations.

• Introduces genetic diversity, essential for adaptation and evolution.

Mitosis vs. Meiosis

Summary

• Mitosis produces clones for growth while meiosis ensures genetic diversity through a complex process involving two divisions, pairing of homologous chromosomes, and crossing over.