The Cell Cycle

Biology 1-2

The Cell Cycle All cells come from

pre-existing cells. The cell cycle is a

series of events from the time a cell divides into two daughter cells until the daughter cells themselves divide.

The Cell Cycle cont’d

Cells make copies of themselves: to add new cells during growth and to replace dead or damaged cells. The copies are identical to each other.

The Cell Cycle cont’d

For a cell to divide properly, it must receive multiple signals. The signals allow the cell to pass through 3

checkpoints. Without the signals, the cell will not divide.

The Cell Cycle cont’d

G1 checkpoint• The cell receives a signal and makes a copy of its

DNA.• Cells not actively dividing have not received a

signal to pass through this checkpoint.

G2 checkpoint• If the cell receives this signal, the cell splits in half;

each half receiving a complete, identical set of DNA (mitosis).

The Cell Cycle cont’d M checkpoint

• If the cell receives the signal, mitosis stops and the two daughter cells grow up.

• Daughter cells wait for the signal to pass through the G1 checkpoint and divide again.

Cancer

Cancer cells are cells that do not “listen” to the normal controls of the cell cycle. Tumor-a mass of abnormally dividing cells.

• Benign tumor-stays in its original spot.• Malignant tumor-the tumor spreads.

• When a malignant tumor is present, the person is said to have cancer.

Cancer cont’d

Cancer begins when normal cells change into cancer cells (transformation). Could be genetic or environmentally caused.

• Environmentally caused cancers form when cancer-causing agents damage the genes that control the checkpoints or signals.

Cancer that is “in the family” (genetically caused) results from faulty genes that create the checkpoints or signals.

Cancer cont’d

Transformation begins when either: the genes that make the checkpoints are

broken.• Broken checkpoints are stuck in the “on” position,

even without a signal present. the genes that make the signals are broken.

• Signals are hyperactive and/or immortal; they continually tell the cell to pass through the checkpoints.

Cancer cont’d

Transformation is aided by a failure of the immune system. Tumor suppressor genes are broken and

cannot kill the cancer cell. The cancer cell “ignores” the suicide signal.

Cancer cont’d

The cell begins to grow and divide uncontrollably. No longer does what it is supposed to do. Leads to organ failure/death if it spreads

enough.• Too many cells, no one doing their jobs.

Some cancer cells may break off and spread (metastasis).