Interphase is the period of the cell’s life cycle, concluded between the end of the previous division and the beginning of the next. From a reproductive point of view, this time can be called the preparatory stage, and from the biofunctional - vegetative. During the interphase period, the cell grows, completes the structures lost during division, and then metabolically rebuilds to switch to mitosis or meiosis, if any reasons (for example, tissue differentiation) do not remove it from the life cycle.
Since interphase is an intermediate state between two meiotic or mitotic divisions, it is otherwise called interkinesis. However, the second version of the term can only be used in relation to cells that have not lost their ability to divide.
general characteristics
Interphase is the longest part of the cell cycle. The exception is the greatly shortened interkinesis between the first and second divisions of meiosis. A remarkable feature of this stage is also that there is no duplication of chromosomes, as in the interphase of mitosis. This feature is associated with the need to reduce the diploid set of chromosomes to haploid. In some cases, intermeiotic interkinesis may be completely absent.
Interphase stages
Interphase is a generic name for three consecutive periods:
- presynthetic (G1);
- synthetic (S);
- postsynthetic (G2).
In cells that do not fall out of the cycle, the G2 stage directly passes into mitosis and is therefore otherwise called premitotic.
G1 is the interphase phase that occurs immediately after division. Therefore, the cell has half the size, as well as a reduced by about 2 times the content of RNA and proteins. Throughout the entire presynthetic period, all components are restored to normal.
Due to the accumulation of protein, the cell gradually grows. There is a completion of the necessary organelles and an increase in the volume of the cytoplasm. At the same time, the percentage of various RNAs increases and DNA precursors (nucleotide triphosphate kinases, etc.) are synthesized. For this reason, blocking the production of messenger RNAs and proteins characteristic of G1 precludes the cell from transitioning to the S period.
At stage G1, there is a sharp increase in the enzymes involved in energy metabolism. The period is also characterized by high biochemical activity of the cell, and the accumulation of structural and functional components is supplemented by the storage of a large number of ATP molecules, which will serve as an energy reserve for the subsequent restructuring of the chromosomal apparatus.
Synthetic stage
In the S-period of interphase, a key moment necessary for division occurs - DNA replication. At the same time, not only genetic molecules are doubled, but also the number of chromosomes. Depending on the time of inspection of the cell (at the beginning, in the middle or at the end of the synthetic period), one can detect the amount of DNA from 2 to 4 s.
The S-stage is a key transition point that “decides” whether division will occur. The only exception to this rule is the interphase between meioses I and II.
In cells that are constantly in a state of interphase, the S-period does not occur. Thus, cells that will not divide again stop at the stage with a special name - G0.
Postsynthetic stage
Period G2 is the final stage of preparation for division. At this stage, the synthesis of information RNA molecules necessary for the passage of mitosis is carried out. One of the key proteins that are produced at this time are tubulins, which serve as a building material for the formation of the division spindle.
At the border between the postsynthetic stage and mitosis (or meiosis), RNA synthesis is sharply reduced.
What are G0 cells
For some cells, interphase is a permanent condition. It is characteristic of some components of specialized tissues.
The state of inability to divide is conditionally indicated by stage G0, since the G1 period is also considered a phase of preparation for mitosis, although it does not include morphological changes associated with it. Thus, G0 cells are considered to have fallen from the cytological cycle. In this state of rest can be both permanent and temporary.
Most often, cells that complete differentiation and specialize in specific functions go into the G0 phase. However, in some cases, this condition is reversible. So, for example, liver cells with organ damage can restore the ability to divide and move from the G0 state to the G1 period. This mechanism underlies the regeneration of organisms. In the normal state, most of the liver cells are in the G0 phase.
In some cases, the G0 state is irreversible and persists until cytological death. This is characteristic, for example, of keratinizing cells of the epidermis or cardiomyocytes.
Sometimes, on the contrary, the transition to the G0 period does not at all mean a loss of the ability to divide, but only provides for a systematic suspension. This group includes cambial cells (for example, stem cells).