Why Do Cells Divide? Reproduction Growth Replacement of Tissues Repair

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

**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.

Cell Cycle Diagram First Growth Phase Synthesis Phase Second Growth Phase

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.

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

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

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