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G1 phase

G1 phase

Overview
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 take 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 number of organelles, and makes a copy of its DNA...

, after cytokinesis
Cytokinesis
Cytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...

 and before the S phase
S phase
The S phase, short for synthesis phase, is a period in the cell cycle during interphase, between G1 phase and the G2 phase. Following G1, the cell enters the S stage, when DNA synthesis or replication occurs. At the beginning of the S stage, each chromosome is...

. 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. The metabolic rate of the cell will be high. 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, and additional inhibiting factors.
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Encyclopedia
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 take 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 number of organelles, and makes a copy of its DNA...

, after cytokinesis
Cytokinesis
Cytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...

 and before the S phase
S phase
The S phase, short for synthesis phase, is a period in the cell cycle during interphase, between G1 phase and the G2 phase. Following G1, the cell enters the S stage, when DNA synthesis or replication occurs. At the beginning of the S stage, each chromosome is...

. 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. The metabolic rate of the cell will be high. 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, 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
The S phase, short for synthesis phase, is a period in the cell cycle during interphase, between G1 phase and the G2 phase. Following G1, the cell enters the S stage, when DNA synthesis or replication occurs. At the beginning of the S stage, each chromosome is...

 and enter a state of dormancy called the G0 phase
G0 phase
The G0 phase or G zero is a period in the cell cycle where 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 and or as a distinct quiescent stage which occurs outside of...

. Most mammal
Mammal
Mammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...

lian cells do this. In order to divide, the cell re-enters the cycle in S phase
S phase
The S phase, short for synthesis phase, is a period in the cell cycle during interphase, between G1 phase and the G2 phase. Following G1, the cell enters the S stage, when DNA synthesis or replication occurs. At the beginning of the S stage, each chromosome is...

.

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 and some viruses. The main role of DNA molecules is the long-term storage of information...

 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 complex combination of DNA and protein that makes up chromosomes. It is found inside the nuclei of eukaryotic cells. It is divided between heterochromatin and euchromatin forms....

 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 about 1,500 species currently described; they dominate fungal diversity in the oceans. Most reproduce asexually by budding, although a few do so by binary fission...

 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|Growth phase 1]]; S=[[S phase|Synthesis]]; G2=[[G2 phase|Growth phase 2]]...

s (CDK)———— S-phase promoting factor(SPF). The G1 CDK proteins activate the transcription factor
Transcription factor
In the field of molecular biology, a transcription factor is a protein that binds to specific DNA sequences and thereby controls the transfer 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.