The G
1 phase, gap 1 phase, or growth 1 phase, is the first of four phases of the
cell cycle
The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and sub ...
that takes place in
eukaryotic cell
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...
division. In this part of
interphase
Interphase is the portion of the cell cycle that is not accompanied by visible changes under the microscope, and includes the G1, S and G2 phases. During interphase, the cell grows (G1), replicates its DNA (S) and prepares for mitosis (G2). A c ...
, the cell synthesizes
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein.
mRNA is created during the ...
and
proteins in preparation for subsequent steps leading to mitosis. G
1 phase ends when the cell moves into the
S phase of interphase. Around 30 to 40 percent of cell cycle time is spent in the G
1 phase.
Overview
G
1 phase together with the
S phase and
G2 phase comprise the long growth period of the
cell cycle
The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and sub ...
cell division called
interphase
Interphase is the portion of the cell cycle that is not accompanied by visible changes under the microscope, and includes the G1, S and G2 phases. During interphase, the cell grows (G1), replicates its DNA (S) and prepares for mitosis (G2). A c ...
that takes place before
cell division in
mitosis (M phase).
During G
1 phase, the cell grows in size and synthesizes
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein.
mRNA is created during the ...
and
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
that are required for
DNA synthesis. Once the required proteins and growth are complete, the cell enters the next phase of the cell cycle, S phase. The duration of each phase, including the G
1 phase, is different in many different types of cells. In human somatic cells, the cell cycle lasts about 10 hours, and the G
1 However, in
Xenopus embryos,
sea urchin embryos, and
Drosophila embryos, the G
1 phase is barely existent and is defined as the gap, if one exists, between the end of mitosis and the S phase.
G
1 phase and the other subphases of the cell cycle may be affected by limiting
growth factors
A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for reg ...
such as nutrient supply, temperature, and room for growth. Sufficient
nucleotides
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules with ...
and
amino acids must be present in order to synthesize mRNA and proteins. Physiological temperatures are optimal for cell growth. In humans, the normal physiological temperature is around
37 °C (98.6 °F).
G
1 phase is particularly important in the cell cycle because it determines whether a cell commits to division or to leaving the cell cycle.
If a cell is signaled to remain undivided, instead of moving onto the S phase, it will leave the G
1 phase and move into a state of dormancy called the
G0 phase. Most nonproliferating vertebrate cells will enter the G
0 phase.
Regulation
Within the cell cycle, there is a stringent set of regulations known as the
cell cycle control system that controls the timing and coordination of the phases to ensure a correct order of events. Biochemical triggers known as
cyclin-dependent kinases
Cyclin-dependent kinases (CDKs) are the families of protein kinases first discovered for their role in regulating the cell cycle. They are also involved in regulating transcription, mRNA processing, and the differentiation of nerve cells. They ...
(Cdks) switch on cell cycles events at the corrected time and in the correct order to prevent any mistakes.
There are three checkpoints in the cell cycle: the G
1/S Checkpoint or the Start checkpoint in yeast; the
G2/M checkpoint; and the
spindle checkpoint
The spindle checkpoint, also known as the metaphase-to-anaphase transition, the spindle assembly checkpoint (SAC), the metaphase checkpoint, or the mitotic checkpoint, is a cell cycle checkpoint during mitosis or meiosis that prevents the separa ...
.
Biochemical regulators
During G
1 phase, the G
1/S cyclin activity rises significantly near the end of the G
1 phase.
Complexes of cyclin that are active during other phases of the cell cycle are kept inactivated to prevent any cell-cycle events from occurring out of order. Three methods of preventing Cdk activity are found in G
1 phase:
pRB binding to
E2F family transcription factors downregulate
expression
Expression may refer to:
Linguistics
* Expression (linguistics), a word, phrase, or sentence
* Fixed expression, a form of words with a specific meaning
* Idiom, a type of fixed expression
* Metaphorical expression, a particular word, phrase, o ...
of S phase cyclin genes;
anaphase-promoting complex
Anaphase-promoting complex (also called the cyclosome or APC/C) is an E3 ubiquitin ligase that marks target cell cycle proteins for degradation by the 26S proteasome. The APC/C is a large complex of 11–13 subunit proteins, including a cull ...
(APC) is activated, which targets and degrades S and M cyclins (but not G
1/S cyclins); and a high concentration of Cdk inhibitors is found during G
1 phase.
Restriction point
The
restriction point (R) in the G
1 phase is different from a checkpoint because it does not determine whether cell conditions are ideal to move on to the next phase, but it changes the course of the cell. After a vertebrate cell has been in the G
1 phase for about three hours, the cell enters a restriction point in which it is decided whether the cell will move forward with the G
1 phase or move into the dormant G
0 phase.
This point also separates two halves of the G
1 phase; the post-mitotic and pre-mitotic phases. Between the beginning of the G
1 phase (which is also after mitosis has occurred) and R, the cell is known as being in the G
1-pm subphase, or the post-mitotic phase. After R and before S, the cell is known as being in G
1-ps, or the pre S phase interval of the G
1 phase.
In order for the cell to continue through the G
1-pm, there must be a high amount of growth factors and a steady rate of protein synthesis, otherwise the cell will move into G
0 phase.
Conflicting research
Some authors will say that the restriction point and the G
1/S checkpoint are one and the same,
but more recent studies have argued that there are two different points in the G
1 phase that check the progression of the cell. The first restriction point is growth-factor dependent and determines whether the cell moves into the G
0 phase, while the second checkpoint is nutritionally-dependent and determines whether the cell moves into the S phase.
The G1/S checkpoint
The G
1/S checkpoint is the point between G
1 phase and the S phase in which the cell is cleared for progression into the S phase. Reasons the cell would not move into the S phase include insufficient cell growth, damaged DNA, or other preparations have not been completed.
At the G
1/S checkpoint, formation of the G
1/S cyclin with Cdk to form a complex commits the cell to a new division cycle.
These complexes then activate S-Cdk complexes that move forward with
DNA replication in the S phase. Concurrently,
anaphase-promoting complex
Anaphase-promoting complex (also called the cyclosome or APC/C) is an E3 ubiquitin ligase that marks target cell cycle proteins for degradation by the 26S proteasome. The APC/C is a large complex of 11–13 subunit proteins, including a cull ...
(APC) activity decreases significantly, allowing S and M cyclins to become activated.
If a cell does not clear to pass through to the S phase, it enters the dormant G
0 phase in which there is no cellular growth or division.
In cancer
Many sources have linked irregularities in the G
1 phase or the G
1/S checkpoint to uncontrolled growth of
tumors. In these cases where the G
1 phase is affected, it is generally because
gene regulatory proteins of the
E2F family have become unrestrained and increase G
1/S cyclin gene expression, leading to uncontrolled cell-cycle entry.
However, the cure for some forms of cancer also lies in the G
1 phase of the cell cycle. Many cancers including
breast
The breast is one of two prominences located on the upper ventral region of a primate's torso. Both females and males develop breasts from the same embryological tissues.
In females, it serves as the mammary gland, which produces and sec ...
and
skin cancers
have been prevented from proliferating by causing the tumor cells to enter G
1 cell cycle arrest, preventing the cells from dividing and spreading.
See also
*
G1/S transition
References
{{DEFAULTSORT:G1 Phase
Cell cycle