1. Homologous chromosomes (matching chromosomes from mother and father) pair up.
2. Crossing over occurs in this stage.
3. Spindle fibres form
4. Nuclear Membrane breaks down
|Metaphase I||1. Homologous pairs line up along the equator of the cell|
1. Homologous chromosomes separate and are pulled to opposite poles by the spindle fibres.
|1. One chromosome from each homologous pair is at each pole of the cell.|
1. There is one chromosome of the homologous pair in each cell.
1. The X-shaped chromosomes form a single line across the middle of the cell.
1. Sister chromatids move to opposite pols of the cell. Once they separate, each sister chromatid is considered to be a chromosome.
1. Spindle fibres begin to disappear, and a nuclear membrane forms around each set of chromosomes.
Characteristics unique to meiosis:
- Pairing of homologous chromosomes – Meiosis I differs from mitosis because in meiosis I a pair of matching chromosomes, one from each parent, lines up at the equator. This happens in metaphase In mitosis chromosomes line up alone.
- Crossing Over – Occurs during meiosis I. Non-sister chromatids “cross over” and exchange pieces of DNA with each other. This creates more diversity in chromosomes since they have changed pieces with another.
- Independent Assortment – Chromosomes sort themselves independently of other pairs. That means each chromosome pair has 2 possible arrangements (Chromosome from father on right, Mother on left OR chromosome from father on left, mother on right). Times this by 23 chromosomes and we get over 8 million possibilities!