Meiosis Lecture Notes

Overview of Meiosis

• Meiosis, also known as reduction division, is a type of cell division that produces gametes (sex cells: sperm in males, eggs in females).
• Consists of two stages of cell division: Meiosis I and Meiosis II.

Phases of Meiosis I

Prophase I

• Starts with a diploid cell: 2 sets of uncoiled chromosomes (one from each parent).
• DNA Replication: Chromatin condenses into X-shaped chromosomes (sister chromatids).
• Synapsis: Pairing and binding of homologous chromosomes forming a tetrad (4 chromatids).
  • Genes on chromosomes may have different versions called alleles.
• Crossing Over (Recombination):
  • Exchange of alleles between chromatids of homologous chromosomes.
  • Leads to genetic variety in offspring.
• Nuclear Membrane Disappears: Centrioles move to opposite cell ends, spindle fibers form.

Metaphase I

• Homologous chromosomes line up at the equator.
• Attach to spindle fibers from opposite poles.

Anaphase I

• Spindle fibers separate homologous chromosomes, pulling them to opposite poles.

Telophase I

• Chromosomes at separate poles (still sister chromatids).
• Nuclear membrane reforms, spindle fibers disappear.
• Cytokinesis: Results in two genetically different haploid daughter cells.

Meiosis II

• No DNA replication before starting.

Prophase II

• Nuclear membrane disappears again.
• Spindle fibers form from paired centrioles.

Metaphase II

• Chromosomes line up at the equator, attach to spindle fibers.

Anaphase II

• Sister chromatids separate and move to opposite poles.
  • Once separated, they are called chromosomes.

Telophase II

• Spindle fibers disappear, nuclear membranes reform.
• Cytokinesis: Produces four genetically different haploid daughter cells.

Key Points to Remember

• Meiosis starts with a diploid cell.
• Produces genetically different haploid gametes.
• Two stages: Meiosis I (homologous chromosomes separate) and Meiosis II (sister chromatids separate).
• Prophase I is crucial for genetic diversity due to synapsis and crossing over.
• All gametes produced are haploid.

Specific to Prophase I

• Synapsis: Homologous chromosomes pair (tetrad formation).
• Crossing Over: Exchange of alleles leading to genetic differences.
• Genetic Variation: Result of crossing over and allele exchange.