3.24c Mitosis 3

1. Looking down a microscope, at a cell, we can see the nucleus, a spherical structure. However, the chromosomes would not be seen.  
2. It is during the interphase where the DNA replication takes place. 
3. The first sign of mitosis, we see when we look down a microscope, is when the nuclear membrane breaks down and the chromatids (pairs of chromosomes) become visible. This is a phase known as the prophase. 
4. Now that the nucleus is gone, inside the cell there is a network of protein molecules known as the spindle, each made out of a fibre. These extend from one pole of the cell to the other. During late prophase, the pair of chromatids will move towards the spindle and will join onto one of the spindle/fibre at the centromere. 
5. The next stage is known as the metaphase: 
• The pair of chromatids is attached to the spindle firbe by the centromere.
• The characteristicof this stage is that the chromosomes are in the middle, arranged across the equator of the cell.
6. Anaphase: 
• The spindle/fibre shortens, pulling one chromatid upwards and the other downwards. 
• The pair of chromatids are moving apart to the poles. the chromatids are being separated. 
7. Telophase: 
• The chromosomes are at opposite ends of the cell. 
• The nucleus begins to reform around the chromosomes at either end of a cell. 
• These will be the new nuclei of the new cells 
• In the telophase we see the formation of two nuclei 
8. Cytokinesis: 
• In this process, the cell splits into two. 
• This is not a part of mitosis. 
• We see the nucleus that had reformed in the anaphase. 
• The cell begins to divide its cytoplasm in half 
• The membrane of the cell fuses across the equator to form the two cells
• The two new cells will contain 1 chromosome just like the parental cells do. 
9. In human cells we won't see one pair of chromosomes separating, but 23 separating at the same time.