1. The nucleus controls the process of cell division

2. Cell Division Cell division is a means of increasing the number of cells in an organism. Every species has a definite and characteristic number of chromosomes (the chromosome complement) present in each cell. Humans have a chromosome complement of 46 chromosomes. The nucleus of the cell controls cell activities, including cell division. Division of the nucleus is called Mitosis, and this results in the formation of two genetically identical daughter nuclei. Mitosis maintains the chromosome complement.

3. Stages of Mitosis – In stage 1 of mitosis, the nucleus contains long, uncoiled chromosomes – a bit like fine threads scattered throughout the nucleus. chromosomes are only visible in the cell during mitosis. The nucleus, at this stage, is surrounded by a nuclear membrane. Structures called centrioles are present in the cytoplasm. Between stages 1 and 2 each chromosome replicates, each chromosome doubling to form two identical chromatids joined at a point called the centromere. At this point chromosomes are easily stained using a dye.

4. Stage I Chromosomes, look like fine threads Nuclear membrane Pair of centrioles necessary for cell division

5. Stage2 In stage 2 the chromosomes are shorter and thicker and they are made up of two chromatids joined at the centromere The nuclear membrane is still present

6. Stage 2 Chromosome made of 2 chromatids centromere Centrioles move to the poles of the cell

7. Stage 3 In stage 3,the nuclear membrane has disappeared Spindle fibres have formed. Chromosomes are attached by their centromere to structures called centrioles which are now present at the poles of the cell. Chromosomes, consisting of chromatids, line up at the equator of the cell

8. Stage 3. Spindle fibres Chromosomes are at the equator centrioles are at the poles Note, the nuclear membrane has disappeared

9. Stage 4 In stage 4,each centromere splits and the pair of identical chromatids become separated. one chromatid from each pair moves to one pole of the cell and the other to the opposite pole of the cell. During cell division, division of the nucleus is followed by division of the cytoplasm.

10. Stage 4. One chromatid goes to one pole The other goes to the opposite pole

11. Stage 5 The cytoplasm starts to divide, nuclear division has now been completed. In this stage, a nuclear membrane forms around each set of chromatids The chromatids are now called chromosomes again

12. Stage 5. Nuclear division is complete, the cytoplasm starts to divide and the nuclear membrane forms again around the chromosomes Cytoplasm starts to divide

13. Stage 6 In stage 6, the cytoplasm has now divided completely Two GENETICALLY Identical Daughter Cells have been formed. These daughter cells contain the same number of chromosomes as the original cell – the chromosome complement has been maintained. There is now cell growth prior to the process being repeated. Q. why is it important that new cells have the same number of chromosomes as the original? A. to ensure that no genetic information is lost.

14. Stage 6. Two new genetically identical daughter cells have been formed: no genetic information has been lost Chromosomes are thin threads