Cell cycle phases

The cell cycle is the period of the existence of the cell. It can last from the moment of its appearance (which occurs after the end of the division of the parent cell) until the moment of completion of its division. Or the cell cycle lasts from the moment of appearance of the cell to its death. More often the cell cycle is considered as the first case, i.e., the period from the moment of appearance of the cell to the end of its division.

The following consecutive phases of the cell cycle are distinguished:

1. The presynthetic period (G₁)
2. Synthetic period (S)
3. The post-synthetic period (G₂)
4. Cell division: mitosis (M)

The diagram below shows the phases of the cell cycle and their duration.

The first three phases of the cell cycle together are the interphase. It is clear that instead of mitosis there may be meiosis.

The duration of the cell cycle in different cells is not the same: there are rapidly dividing, others divide through relatively large intervals of time. There are differentiated cells that do not divide at all. They actually do not have the listed phases/periods. In the phase G₁ they enter the so-called phase G₀ (rest phase).

The processes occurring in the phases of the cell cycle are similar in all dividing eukaryotic cells.

The presynthetic phase is the longest one. During this period the cell grows, it synthesizes RNA, proteins, ATP, the formation of organelles, etc.

Replication (doubling) of DNA occurs during the synthetic period. Since the chromosomes consist not only of DNA molecules, but also of proteins (histones), their synthesis also occurs. By the end of the synthetic phase of the cell cycle, each chromosome consists of two identical chromatids.

In the post-synthetic period, the cell prepares for division. The formation of the structures necessary for this process is taking place. Energy is also accumulated in a large amount (ATP is synthesized).

Mitosis is a complex process, but it proceeds quickly (about 10% of the time) with respect to the duration of the entire cell cycle. Phases of mitosis are prophase, metaphase, anaphase, telophase.

The cell cycle is regulated by the internal mechanisms of the cell. Normally, if an error occurs at any phase, the next phase of the cell cycle does not occur (generally or until the defect is corrected).