1. Why Do Cells Divide?
Reproduction
Growth
Replacement of
Tissues
Repair

2. THE CELL CYCLE I. Cell Cycle – is a repeating sequence of cellular
growth and division
Five Phases of the Cell Cycle:
1. First Growth (G1) Phase – the cell grows & carries out its normal routine functions
2. Synthesis (S) Phase – DNA is copied, resulting in each chromosome having 2 chromatids attached at the centromere
3. Second Growth (G2) Phase – the cell prepares to divide by assembling microtubules, making extra cytoplasm and organelles
centromere
chromatid
Interphase
Interphase

3. **Cells spend 90% of their time in the first 3
phases, which are called interphase.
4. Mitosis – the process during cell division
in which the nucleus of a cell is divided
into 2 nuclei
5. Cytokinesis – the process during cell
division in which the cytoplasm divides
**Mitosis and cytokinesis produce new cells that are identical to the original cells and allow organisms to grow, replace damaged tissues, and in some organisms reproduce asexually.

4. Cell Cycle Diagram First Growth Phase Synthesis Phase
Second Growth Phase

5. II. Control of the Cell Cycle
The cell cycle is controlled proteins. It is regulated by feedback signals from the cell which has key checkpoints at which signals the cell to go on to the next phase of the cell cycle or it may delay the next phase.

6. Three Principal Checkpoints:
     1. Cell Growth (G1) Checkpoint – this checkpoint decides whether the cell will divide
     2. DNA Synthesis (G2) Checkpoint – DNA replication is checked at this point by DNA repair enzymes; if this checkpoint is passed, proteins help to trigger mitosis
3. Mitosis Checkpoint – this checkpoint triggers the exit from mitosis and it signals the beginning of the G1 phase

7. III. Cancer cells; it is a disorder of cell division
    Cancer – is the uncontrollable growth of abnormal
cells; it is a disorder of cell division
- cancer cells do not respond normally to
the body’s control mechanisms and
continue to divide

8. Some Mutations Cause Cancer by:
1. Over producing growth-promoting
molecules, thus speeding up the cell
cycle.
2. Inactivating the control proteins that
normally act to slow or stop the cell