The Cell Cycle

Final checkpoint during mitosis

Animations

Animation 1

Animation 2

The Cell Cycle – G1

• The first phase is a growth phase (G1)• The new cell starts growing and replicating

its organelles• At the end of G1, the checkpoint is to

gauge if it is a good time for cell to keep growing or dividing or if a delay is necessary.

• If the cell is under any sort of stress, it will pause at this stage.

The Cell Cycle - S

• During the synthesis (S) phase, DNA replication occurs

• It is at this point that the diploid cell (2n) doubles to 4n, so that when it divides there will be two complete copies of the DNA

The Cell Cycle – G2

• During the next growth phase (G2), the cell continues to grow in preparation for division

• The checkpoint during this phase is extremely important.

• If any errors have occurred during transcription, the cell must undergo apoptosis (programmed cell death)

The Cell Cycle - M

• During Mitosis (M), the single parent cell gives rise to two identical daughter cells.

• Once again a checkpoint assesses whether any errors have occurred during division.

• A negative result at the checkpoint here will result in apoptosis.

The Cell Cycle - C

• Cytokynesis (C) is actually the last part of Mitosis

• At this point the cell, which momentarily has twin nuclei, will divide in to two new cells (2N) and the process will begin again.

What happens if the checkpoints don’t work?

• If cell cycle checkpoints are operated by enzyme proteins.

• These enzymes can be prevented from carrying out normal function by inhibitors.

• These enzymes could also be stimulated to maintain an active state permanently.

• Either way can lead to uncontrolled and unmonitored cell division, this is commonly known as ….

CANCER

• Cells multiply uncontrollably and regardless of accumulating errors will never apoptose.

• Does not form useful cells as cells spend far more time in division rather than growth (a reversal of the normal situation).

• How does chemotherapy help?

Chemotherapy

• During interphase, DNA is somewhat protected from radiation due to the double membrane of the nucleus.

• During mitosis there is no nuclear envelope to protect the DNA

• Chemotherapy destroys DNA, thus kills far more cancer cells than budy cells as they spend far more time dividing

• The collateral damage of killing healthy body cells is what makes people sick when undergoing chemotherapy (and makes their hair fall out)