G2 phase is a phase of the cell cycle, which consists of G1 phase, S phase, G2 phase, and finally mitosis where the cell divides to make two daughter cells. G2 phase is the first gap (G) between the DNA duplication that occurs in S phase and the cell division that occurs in mitosis. During this time the cell continues to grow in size so it can divide into two full size daughter cells in mitosis. The cell also checks the DNA that was duplicated during S phase to make sure everything was copied and that there were no errors. This is critical, since any unchecked, serious problems with the DNA will be passed on to the daughter cells, resulting in their death or worse, abnormal growth that may be the start of cancer. Proteins that are needed for mitosis, such as those involved in forming the mitotic spindle, are synthesized and assembled during G2 phase. This phase is the third longest in the cell cycle behind G1 phase and S phase. However, the G2 phase is often dramatically shortened, almost to the point of elimination, in cells that are growing quickly, such as those in a developing embryo.

Certain signaling pathways and checkpoints are in place to move the cell through G2 phase and into mitosis. Special proteins called cyclins and kinases called cyclin dependent kinases (cdk) are involved in this progress. Cyclin A and a certain cdk help to move the cell through G2 phase. Cyclin B binds to a different cdk to form a complex that helps move the cell into mitosis. Additionally, there are checkpoints throughout G2 phase that monitor the DNA, cell size, and extracellular environment to make sure everything is ready for mitosis. If there are any problems then the progress through G2 phase into mitosis is delayed. For example, if there are errors in the DNA then various proteins involved with DNA repair such as p53, ATM, and ATR will activate a signaling pathway to halt the progression of G2 phase until the DNA is repaired.

At any given time, a certain population of cells growing in tissue culture will be in various stages of the cell cycle. Researchers will often want to determine what percentage of the cells are in G2 phase. The main way of determining this is by using flow cytometry. The cells are treated with a DNA dye, such as Hoechst, and analyzed by flow to determine the amount of DNA and therefore the position in the cell cycle. (See the bottom of the Hoechst dye w/u for more detail on how this works.)

Cells in tissue culture can be arrested specifically in G2 phase by treating them with certain drugs. Two examples are nocodazole and taxol, drugs that inhibit microtubule formation and prevent entry into mitosis. This arrest can be reversed so the cells can continue through the cycle simply by removing the drugs.

Albert’s Molecular Biology of the Cell, Third Edition
Current Protocols in Cell Biology, Volume 1, Section 8

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