G1 phase

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The G₁ phase is a period in the cell cycle during interphase, after cytokinesis and before the S phase. For many cells, this phase is the major period of cell growth during its lifespan. During this stage new organelles are being synthesized, so the cell requires both structural proteins and enzymes, resulting in great amount of protein synthesis and a high metabolic rate in the cell. G₁ consists of four subphases:

1. Competence (g1a)
2. Entry (g1b)
3. Progression (g1c)
4. Assembly (g1d)

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.^[1]

A cell may pause in the G₁ phase before entering the S phase and 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.^[1]

Status of the genome

The DNA in a G₁ diploid eukaryotic cell is 2n, meaning there are two sets of chromosomes present in the cell. The genetic material exists as chromatin and if it were coiled into chromosomes, there would be no sister chromatids. Haploid organisms such as some yeast will be 1n and thus have only one copy of each chromosome present.

Restriction Point

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)———— S-phase promoting factor(SPF). 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.^[1]

References

1. ^ ^{a b c} Lodish et al. (2000). Molecular Cell Biology (4th ed.). W.H. Freeman and Co.. 

v • d • e Cell cycle Interphase G1 phase · S phase · G2 phase M phase Mitosis (Preprophase · Prophase · Prometaphase · Metaphase · Anaphase · Telophase) · Cytokinesis Cell cycle checkpoints Restriction point · Spindle checkpoint · Postreplication checkpoint Other cellular phases G0 phase · Apoptosis

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