Meiosis I
Homologous chromosomes separate in meiosis I
Prophase I
- DNA has already replicated and condenses and becomes visible as chromosomes
- Each chromosome consists of two sister chromatids joined together by a centromere
- The chromosomes are arranged side-by-side in homologous pairs
- A pair of homologous chromosomes is called a bivalent
- As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)
- In this stage centrioles migrate to opposite poles and the spindle is formed
- The nuclear envelope breaks down and the nucleolus disintegrates
Metaphase I
- The bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres
- The bivalents line up by independent assortment (random orientation)
Anaphase I
- The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle
- The centromeres do not split
Telophase I
- The chromosomes arrive at opposite poles
- Spindle fibres start to break down
- Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
- Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I
Meiosis I is reduction division
- Meiosis I is referred to as reduction division because homologous chromosomes separate and move to opposite poles of the cell.
- Therefore, the number of chromosomes per cell is reduced by a factor of 2
The different stages of meiosis I in an animal cell