G1 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. G1 phase is the first gap (G) between the activity of cell division in mitosis and DNA duplication in S phase. During this time the cell begins to grow in size to prepare for two copies of its DNA. Proteins that are needed to copy the DNA in S phase are also synthesized and assembled. In almost all cells this phase is the longest phase. However, the G1 phase is often dramatically shortened, almost to the point of elimination, in cells that are growing quickly, such as those in a developing embryo.

At a certain point in G1 phase the cell will analyze its internal and external environments to make sure conditions are favorable for S phase and DNA replication. This includes analyzing the size of the cell and determining if there are enough extracellular nutrients and growth factors to continue. If everything checks out then the cell continues on to S phase and replicates its DNA. If there are any problems then the cell will pause and remain in G1 phase until the problems have been resolved. For example, if the DNA is damaged then the protein p53 is activated and delays entry into S phase until the DNA is repaired. If the problems are more severe, such as there are not enough growth factors or nutrients available, then the cell will likely enter a special phase called G0 phase. In this phase the cell is dormant and will wait until enough nutrients are available to continue through the cell cycle.

Several proteins are involved in the transition through G1 phase and into S phase. During G1 phase a certain cyclin dependent kinase (cdk) that interacts with a protein called cyclin D become active. This kinase phosphorylates a protein called retinoblastoma (Rb) which in turn frees transcription factors bound by Rb. These transcription factors are needed in S phase for DNA replication. Another cdk that interacts with cyclin E promotes the transition from G1 phase to S phase.

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 G1 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 G1 phase by treating them with certain drugs. Two examples are lovastatin, a HMG-CoA reductase inhibitor, and mimosine, an inhibitor with an unknown target. This arrest can be reversed so the cells can continue through the cycle simply by removing the drugs.

Current Protocols in Cell Biology, Volume 1, Section 8
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