Cell Cycle

The cell cycle describes the stages of cell division. It is often depicted as a circle separated into segments with each segment representing a phases of the cycle.

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There are 4 phases, plus one special state for resting cells.

Gap Phase One (G1)

This phase begins after the completion of cell division.

Cells increase in size and begin to synthesize RNA and protein in preparation for DNA replication.

Synthesis Phase (S)

The period* during which DNA is replicated.

Gap Phase 2 (G2)

Begins when DNA synthesis is complete.

Cell continues to grow until it has doubled in size and has enough cellular material for two cells.

Mitosis (M)

The cell divides creating two daughter cells.

The diagram sometimes has an additional phase:

Resting Phase (G0)

A cell enters G0 by failing to pass a checkpoint in G1 that determines if a set of conditions needed to proceed with the cell cycle are met.

Cells enter resting phase for a number of reasons and transition into this phase can be either reversible or irreversible. Cells in G0 may function normal*ly, but are not actively synthesizing the material needed to proceed through the cell cycle.

There are also checkpoints at different places along the cycle where the cellular machinery makes sure that certain conditions have been met before proceeding to subsequent phases of the cycle.

By showing growth during the cell cycle as a chart, we can see where different types of growth occur.

RNA and protein concentration*s increase throughout the G1, S, and G2 phases.

As a result, the amount of RNA and protein doubles from the beginning of G1 to the end of G2.

These amounts then drop back down as the cell divides during M Phase as mitosis and cytokinesis occur.

Contrast this with DNA which also doubles over the cycle.



But in the case of DNA, all synthesis occurs during the S Phase.

In a normally dividing cell, the cycle begins at G1 right after it is formed by the division of its parent cell.

At the cell enters G1, the nucleus contains a single copy of its genome.




The chromosome*s have uncoiled to facilitate DNA, RNA, and protein synthesis.

The cell increases in volume* as RNA, protein, and organelles are synthesized.

The proteins and enzymes required for DNA synthesis are made in anticipation of entering S Phase.

The primary checkpoint for the whole cell cycle occurs towards the end of G1.

Active growth during G1 signals that conditions are good for DNA replication.

Initiation of DNA replication marks the transition to S Phase.

Once the cell transitions from G1 to S, it is committed to cell division.

In S Phase all other cell functions continue - RNA concentration, protein concentration, and cell volume increases as the amount of DNA doubles.

Transition to G2 occurs at the completion of DNA synthesis.

In G2 the cell finishes doubling in size and synthesizes enough organelles and other cellular material to support two cells.

Cellular components begin to move to the proper side of the cell in anticipation of cell division. This includes the centriols which will pull sister chromatid*s to opposite sides of the cell during mitosis.

The transition from G2 to M occurs as DNA re-condense to form characteristic x-shaped chromosomes.

As the cell proceeds with mitosis, spindle fibers form between the centrioles and each chromosome, the nuclear membrane breaks down, and the chromosomes line in the middle of the cell along the mitotic plate.

The cell begins to divide, and sister chromatids are pulled to opposite ends of the cell.

At the end of mitosis, nuclear membranes form around each set of chromatids and cytokinesis occurs resulting in two cells each with half the cellular material of the parent cell.

Each cell begins the cycle again with Gap Phase 1.

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