Cell Cycle

Life cycle of a cell

• The cell cycle consists of:

• G1 = growth and preparation of the chromosomes for replication

• S = synthesis of DNA (and centrioles)

• G2 = preparation for mitosis

• M = mitosis

Gap O• Gap 0 (G0): There are times when a cell

will leave the cycle and quit dividing. This may be a temporary resting period or more permanent. An example of the latter is a cell that has reached an end stage of development and will no longer divide (e.g. neuron).

 

GAP 1

• Gap 1 (G1): Cells increase in size in Gap 1, produce RNA and synthesize protein. An important cell cycle control mechanism activated during this period (G1 Checkpoint) ensures that everything is ready for DNA synthesis.

• S Phase: To produce two similar daughter cells, the complete DNA instructions in the cell must be duplicated. DNA replication occurs during this S (synthesis) phase.

• Gap 2 (G2): During the gap between DNA synthesis and mitosis, the cell will continue to grow and produce new proteins. At the end of this gap is another control checkpoint (G2 Checkpoint) to determine if the cell can now proceed to enter M (mitosis) and divide.

Related to these events are two factors that appear to control the

entry into the M-phase

• Internal control: Substances inside the cell regulate the timing of specific phases of the cell cycle. Example DNA Replication

• Environmental Factors: External factors that may start mitosis or prevent mitosis from occurring.

• Example contact inhibition, causes cells to stop dividing when cells reach a certain population density (once a cut is healed)

Two genes that also control cell division

• Protoonco genes: Encourage cell growth by producing a certain protein

• Tumor Suppressor genes: Encourage cells to stop growing by producing a certain protein.

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Growth = Mitosis

• In order for organisms to grow cells must increase in number.

• In order for organisms to replace old cells cell division must occur.

• This type of cell division is called mitosis and it starts with the same number of chromosomes it ends with and it produces two identical cells.

Stages of mitosisInterphase: G1, S, G2,(G0)

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ProphaseChromosomes become visible

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MetaphaseChromosomes align at the

center of the cell

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AnaphaseChromosomes get pulled apart

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TelophaseTwo separate cells begin to

form•

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Cancer

• Cancer occurs when cells cells loose their ability to divide and do not stop dividing. Contact inhibition is lost

• A mutation occurs on a protoonco gene or a tumor suppressor gene turning it into an onco gene that has uncontrolled division

Tumors

• Two main kinds of tumors:

• Benign – stay in the same spot – more treatable of the two – can become malignant.

• Malignant – release enzymes that can spread and invade other tissue – much more serious of the two.

• Metastasis – the process of malignant tumor spreading

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