- G1 phase
The G1 phase is a period in the
cell cycleduring interphase, after cytokinesisand before the S phase. For many cells, this phase is the major period of cell growth during its lifespan. During this stage new organellesare being synthesized, so the cell requires both structural proteins and enzymes, resulting in great amount of protein synthesis. The metabolic rate of the cell will be high. G1 consists of four subphases:
These subphases may be affected by limiting
growth factors, nutrient supply, and additional inhibiting factors. A rapidly dividing human cell which divides every 24 hours spends 9 hours in G1 phase.cite book | author=Lodish et al. | title=Molecular Cell Biology | publisher= W.H. Freeman and Co. | edition=4th | year=2000 ]
A cell may pause in the G1 phase before entering the
S phaseand enter a state of dormancy called the G0 phase. Most mammallian cells do this. In order to divide, the cell re-enters the cycle in S phase.
Status of the genome
DNAin a G1 diploid eukaryoticcell is 2n, meaning there are two sets of chromosomespresent in the cell. Haploidorganisms such as some yeastwill be 1n and thus have only one copy of each chromosome present.
There is a "restriction point" present at the end of G1 phase. This point is a series of safeguards to ensure the DNA is intact and that the cell is functioning normally. Functionally, the safeguards exist as proteins known as
cyclin-dependent kinases (CDK). The G1 CDK proteins activate the transcription factors for a variety of genes. These include genes which are responsible for DNA synthesis proteins and S phase CDK proteins.
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