Meiosis

Form of cell division in which gametes are formed.

Key terms:

• Homologous chromosomes – pair or set of chromosomes
• Tetrads – A set of identical sister chromatids
• Chromatid – one half of a replicated chromosome
• Chromatin – Combination of DNA and Proteins in nucleus

Steps in Meiosis

1. Interphase: DNA replicated
2. Prophase I
   • Chromatin condenses into chromosomes
   • nuclear membrane breaks down
   • centrioles (in animals) migrates to poles of the cell and form spindle fibers which attach to each homologous chromosome pair
3. Metaphase I
   • tetrads line up at metaphase plate
4. Anaphase I
   • homologous chromosomes separate and move to opposite ends of the cell
5. Telophase I / cytokinesis
   • nuclear membrane reforms
   • cytokinesis: contractile ring at center pinches cells apart into 2 haploid daughter cells
6. Prophase II
   • nuclear membrane breaks down
   • cromatin condenses into chromosomes
   • spindle fibers from centrioles form and attach to chromatids
7. Metaphase II
   • chromosomes line up at cell center (metaphase plate)
8. Anaphase II
   • Sister chromatids separate and move towards opposite ends of the cell
9. Telophase II / cytokinesis
   • reform nuclear membranes
   • chromatids reform into chromatin
   • spindle breaks down

A difference between male and female gametes.

• Male → 4 gametes
• Female → 1 viable gamete, 3 polar bodies

In cytokinesis, the majority of the cytoplasm is saved for one egg. This increases its fitness to be nourished and grow.