3.24

3.24 understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes

Mitosis is cell division that causes growth, an increase in number of cells. A normal cell has diploid number of chromosomes (2n). For humans, this would be 2n = 46. Through mitosis, a cell will divide into two. Each will contain a diploid nucleus. These cells are identical daughter cells. We know that they have the same number of chromosomes and same set of chromosome (i.e. if you choose a random chromosome, you will find an identical version within the cell).

Phases

1)     Interphase – this is when DNA replication takes place. During this process, each chromosome undergoes a copying process to form an identical copy of itself. The pair of chromosomes (exact copies) are held together by centromere. We refer to the copied pair as a pair of chromatids. The replication process takes place inside the nucleus while the nucleus is still intact.

2)     Prophase – we can see the breakdown of the nuclear membrane. Chromosomes are now a pair of visible chromatids.

3)     Late prophase – network of protein molecules begin to form (spindle). These spindle fibres extend from one pole to the other. The chromosome pair moves towards the spindle and join at the centromere.

4)     Metaphase – the pair reaches the middle of the spindle fibre. The characteristics of this phase are that the chromosome is in the middle and the spindle fibre is arranged across the equator of the cell.

5)     Anaphase – the spindle fibre shortens. This pulls one chromatid in one direction and the second one in the other direction. The characteristics of this phase are that the pair move apart, move towards the poles of the cell and they separate.

6)     Telophase – the nucleus begins to reform around the chromosomes at either ends of the cell. Here two nuclei are formed and two sets of chromosomes remain at opposite ends of the cell. 

7)     Cytokinesis – cells split into two. This is not part of mitosis! The reformed nucleus form into two cells when the cytoplasm divides in half. The membrane fuses across the equator forming two new cells, each containing ONE chromosome. In humans, 23 pairs separate all at the same time.