3.24 Mitosis

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

- Mitosis is a form of cell division which results in growth which occurs by an increase in the number of cells.

- A normal cell has a nucleus. The number of chromosomes in a nucleus is known as a diploid number which can be abbreviated to the formula 2N. 2N for humans is 46.

- In the process of mitosis this normal cell will divide to form two new cells both with a nucleus. If we take a closer look at the nucleus from each of these cells, we find they both have a diploid nucleus. Meaning they both still have 46 chromosomes meaning they are identical. Sometimes described as daughter cells.

- They are identical because they have the same number of chromosomes and because they have the same set of chromosomes. This means that if we find one chromosome in one of the cells, we will find an identical version of that chromosome in the other cell.

- From this information a number of questions come to mind.
      1) How are the copy of chromosomes made?
      2) How do they separate into the two cells?

These questions are answered in 3.24b



- From 3.24a we have established that a normal cell will copy its chromosomes and divide into two identical cells during mitosis.

- This copy is a process called DNA Replication. In this process each chromosome undergoes a copying process to form an identical copy of itself with all the same genes. These two copies are held together by a central structure called a centromere. These two copies held together by the centromere are called pairs of chromatids.

- This process of DNA Replication takes place in the nucleus while the nucleus is still intact. This means we can't see this process. This is known as the interphase of the cell cycle.

- The process in which the copies of the chromosomes are seperated is dealt with in 3.24c



- Looking at the stages of mitosis:

1) It is during the 'Interphase' of a cell that DNA replication occurs

2) The first sign that a cell is entering mitosis and cell division, is when we see the breakdown of the nuclear membrane. Basically, the nucleus breaks down. This is a phase known as the 'Prophase'. At this point the chromosomes become visible as a pair of chromatids.

3) At this stage the nucleus is gone, and inside the cell a network of protein molecules are present which are known as the spindle, each of which are fibres. They extend from one end (pole) of the cell to the other. During late prophase the chromosome pair will move towards the centre of the spindle and join on to one of the spindle fibres at the centromere.

4) What happens during late phase can be seen in the next stage of mitosis which is known as 'Metaphase.' In the diagram only one pair of chromotids are illustrated. The chromosomes can be seen in the middle attached at the centromere.

5) The next stage is called the 'Anaphase.' Here the fibre (spindle fibre) shortens and pulls one chromotid in one direction towards on of the poles, and one chromatid in the other direction towards the opposite pole. They are moving a part and being separated.

6) The next stage which is the end of mitosis is called 'Telophase.' At thus stage the nucelus begins to reform around the chromosomes at each end of the cell. This will be the new nucleus for the new cell.

7) And finally now the stage called 'Cyto Kinesis' is when the cell splits into two new cells. THIS IS NOT PART OF MITOSIS. The cytoplasm of the original cell splits, and the membrane fuses across the equator to form the new cells. It is important to understand that each of the new cells contain one chromosome.