The G₁ phase is a period in the cell cycle

Cell cycle

The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...

 during interphase

Interphase

Interphase is the phase of the cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for cell division. In preparation for cell division, it increases its size and makes a copy of its DNA...

, before the S phase

S phase

S-phase is the part of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Precise and accurate DNA replication is necessary to prevent genetic abnormalities which often lead to cell death or disease. Due to the importance, the regulatory pathways that govern this...

. 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 enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s, resulting in great amount of protein synthesis

Protein biosynthesis

Protein biosynthesis is the process in which cells build or manufacture proteins. The term is sometimes used to refer only to protein translation but more often it refers to a multi-step process, beginning with amino acid synthesis and transcription of nuclear DNA into messenger RNA, which is then...

 and a high metabolic rate in the cell. G₁ consists of four subphases:

1. Competence
   Competence (biology)
   In microbiology, genetics, cell biology and molecular biology, competence is the ability of a cell to take up extracellular DNA from its environment...
   
    (g1a)
2. Entry (g1b)
3. Progression (g1c)
4. Assembly (g1d)

These subphases may be affected by limiting growth factor

Growth factor

A growth factor is a naturally occurring substance capable of stimulating cellular growth, proliferation and cellular differentiation. Usually it is a protein or a steroid hormone. Growth factors are important for regulating a variety of cellular processes....

s, 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

S phase

S-phase is the part of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Precise and accurate DNA replication is necessary to prevent genetic abnormalities which often lead to cell death or disease. Due to the importance, the regulatory pathways that govern this...

 and enter a state of dormancy called the G₀ phase

G0 phase

The G0 phase is a period in the cell cycle in which cells exist in a quiescent state. G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or a distinct quiescent stage that occurs outside of the cell cycle...

. Most mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

lian cells do this. In order to divide, the cell re-enters the cycle in S phase

S phase

S-phase is the part of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Precise and accurate DNA replication is necessary to prevent genetic abnormalities which often lead to cell death or disease. Due to the importance, the regulatory pathways that govern this...

.

Status of the genome

The DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 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

Chromatin

Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...

 and if it were coiled into chromosomes, there would be no sister chromatids. Haploid organisms such as some yeast

Yeast

Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

 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 kinase

Cyclin-dependent kinase

thumb|350px|Schematic of the cell cycle. outer ring: I=[[Interphase]], M=[[Mitosis]]; inner ring: M=Mitosis; G1=[[G1 phase|Gap phase 1]]; S=[[S phase|Synthesis]]; G2=[[G2 phase|Gap phase 2]]...

s (CDK)———— S-phase promoting factor(SPF). The G₁ CDK proteins activate the transcription factor

Transcription factor

In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...

s for a variety of genes.These include genes which are responsible for DNA synthesis proteins and S phase CDK proteins.