Meiosis I

Meiosis I

• Sperm and egg cells have half the number of chromosomes because when the sperm and egg fuse they will then produce a diploid fertilized egg.
• Meiosis is a two-step cell division process: meiosis I and meiosis II.
• In each round of cell division, the cell goes through interphase and the same 5 phases as in mitosis: prophase, prometaphase, metaphase, anaphase, and telophase.
• Interphase occurs before entering meiosis I:
• The cell grows during the G1 phase of interphase;
• The DNA is replicated in the S phase;
• The cell prepares for division in the G2 phase.

Meiosis I

1. Prophase I

• As in mitosis, chromosomes being to condense.
• Each chromosome lines up with its homologous pair.
• An overlap forms – called the chiasmata.
• Crossing over occurs – the exchange of DNA between the two homologous chromosomes.
• Genetic variability leads to offspring being more likely to thrive!

1. Metaphase I

• After crossing over, chromosomes begin to move to metaphase plate.
• Homologous pairs (rather than individual chromosomes like in mitosis) line up in a random orientation $\to$ independent assortment.

1. Anaphase I

• Homologous chromosomes are pulled apart by the microtubules.
• Sister chromatids remain attached, unlike in mitosis.

1. Telophase I

• Chromosomes are separated.
• Cytokinesis occurs.
• End result is two haploid daughter cells.