G1 phase

The G1 phase is a period in the cell cycle during interphase, 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, temperature, and additional inhibiting factors. A rapidly dividing human cell which divides every 24 hours spends 9 hours in G1 phase.

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.

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.