1. Cell Division

2. Cell Increase and Decrease
Cell division increases the number of somatic cells
Consists of mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm)
Apoptosis, programmed cell death, decreases the number of cells

3. The Cell Cycle
Cell cycle: orderly set of stages that take place between the time a cell divides and the time the resulting cells also divide.

4. The Stages of Interphase
Most of the cell cycle is spent in interphase
In between episodes of mitosis
Cell carries on its usual functions
Prepares to divide (grows larger, number of organelles doubles, amount of DNA doubles)
Lasts 20 hours in mammals

5. The Stages of Interphase
G1 occurs before DNA synthesis
S includes DNA synthesis
G2 occurs after DNA synthesis
G = growth
S = synthesis
M = mitosis

6. The Stages of Interphase
G1 stage
Cell doubles its organelles
Accumulates materials needed for DNA synthesis

7. The Stages of Interphase
S stage
DNA replication occurs
At the beginning of stage, each chromosome is composed of one DNA molecule (chromatid). At the end, each chromosome consists of two identical DNA molecules (sister chromatids)
DNA replication has resulted in
duplicated chromosomes

8. The Stages of Interphase
G2 stage
Cell synthesizes proteins needed for cell division
G0 stage
“Resting stage”
Cell is neither dividing nor
preparing to divide
Ex. nerve cells

9. The Mitotic Stage
Sister chromatids of each chromosome separate, becoming daughter chromosomes that are distributed to two daughter nuclei
After cytokinesis, two daughter cells are left
About 4 hours in mammal cells

10. Control of the Cell Cycle
Controlled by internal and external signals.
Growth factors: hormones that act as external signals that regulate mitosis

11. Control of the Cell Cycle
The stop signs represent checkpoints when the cell cycle can possibly stop

12. Control of the Cell Cycle
Cyclin: protein that control the progression of cells through the cell cycle
Must be present from G2 to M, and G1 to S

13. Control of the Cell Cycle
G1 checkpoint: cell cycle will stop if DNA is damaged
In mammals, protein p53 stops cell cycle at G1.
If p53 cannot repair DNA, will bring about apoptosis

14. Control of the Cell Cycle
G2 checkpoint: Cell cycle stops if DNA has not finished replicating
If DNA is damaged, stopping the cell cycle allows time for damage to be repaired

15. Control of the Cell Cycle
M checkpoint: Mitosis stops if chromosomes not properly aligned

16. Apoptosis Programmed cell death
Cell fragments engulfed by white blood cells or neighbouring cells

17. Apoptosis Cells harbour the enzymes that bring about apoptosis
Held in check by inhibitors
Unleashed by internal or external signals
“Initiators” receive signal to activate “executioners” which activate enzymes that dismantle cell
ex. executioners turn on enzymes that tear apart the cytoskeleton and enzymes that chop up DNA.