DNA Part III:

The Cell Cycle“The Life of a Cell”

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Cells must reproduce else they die. The "life of a cell" is termed the cell cycle. The cell cycle has distinct phases, which are called G1, S, G2, and M.

Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G0 phase.

Cells & Cell Reproduction

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• During this time organelles are reproducing, protein synthesis is occurring for growth and differentiation.

• Because, transcription is occurring, the DNA is uncoiled.

• This phase is the most variable, ranging from almost nothing to years.

The G1 Phase of the Cell Cycle

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The G1 Phase of the Cell Cycle

The average time for G1 is around 10 hours if the cell cycle lasts 24 hours.

Most cells that differentiate will do so during this phase. Cells arrested in G1 may no longer have the capability of reproducing and are said to be in G0.

Certain cells in G0, however, when given some external or internal cues may revert back to G1 and enter the cell cycle again.

Nerve and muscle cells are usually arrested in G0.

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The S or synthesis phase is the second phase of the cell cycle.

• DNA uncoils

• DNA replication occurs

• Additional organelle replication occurs

• This phase ensures that each emerging daughter cell will have the same genetic content as the mother cell.

S Phase of the Cell Cycle

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The G2 or Gap 2 phase occupies the time from the end of S until the onset of mitosis.

•During this time, the cell prepares for mitosis by making and organizing necessary proteins such as the tubulin needed to construct microtubules which used to make spindle fibers.

G2 Phase of the Cell Cycle

• During mitosis the nucleus is replicated and the cytoplasm divides to produce two genetically identical daughter cells.

• The phases are triggered by the accumulation of cell signals.

M Phase or Mitosis

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This graph represents the amount of DNA found in the cell during the cell cycle.

The Amount of DNA Varies During the Cell Cycle

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• The control of the cell cycle is dependent on an accumulation of “signal molecules”.

• Quite often these signal molecules must be phosphorylated in order to be functional. This are simple illustrations.

Internal Controls of the Cell Cycle

• Cyclins are a family of proteins that control the progression of cells through the cell cycle by activating cyclin-dependent kinase (Cdk) enzymes.

• A kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation.

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Cyclins vs. Kinases

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Cyclins vs. Kinases

• Certain cyclins are made at certain times during the cell cycle, and their concentration will rise and fall. Cyclins are also destroyed after they are no longer needed by the cell.

• CDKs are not destroyed as they are only activated or deactivated.

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•Certain kinases may have two forms (active and inactive).•Kinases are enzymes (proteins) that phosphorylate certain molecules or other enzymes. •Most cell cycle signals are phosphorylated by kinases.

Kinases Phosphorylate Cell Signal Molecules

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Cyclins Activate Kinases

Most cell cycle kinases are activated by molecules called cyclins. A kinase that requires a cyclin for activation is called a cyclin-dependent kinase or CDK.

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Cyclins Activate Kinases

Once the CDK phosphory-lates certain signals, the cyclin is destroyed.

In the cell, the concentration of cyclins will rise and fall depending on the phase of the cell cycle.

When the cyclin is destroyed the CDK returns to an inactive form.

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Cyclins Activate Kinases

Above is an example of how the M cyclin concentration affects MPF or M/CDK activity.

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Cyclic Nature of Cyclins in the Cell Cycle

This graph displays the cyclic nature of various cyclins in a given cell cycle

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Different Types of Cyclins

These are some known cyclin/CDK complexes and their role in the cell cycle.

Cyclin/CDK Complex

Cyclin Function of Cyclin/CDK Complex

G1-CDK Cyclin D Drives the transition G1 S transition

G1/S-CDK Cyclin E Cyclins bind to CDK at the end of G1 and commits the cell to DNA replication.

S-CDK Cyclin A Cyclins bind the CDK during S and are necessary for the initiation of DNA replication

M-CDK Cyclin B Cyclins promote the events of Mitosis

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Cyclins/CDKs Control the Cell Cycle

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External Factors Also Influence the Cell Cycle

Cell reproduction and the healing of a wound.

I. Hemostasis

II. Inflammation

III. Proliferation

IV. Remodeling