The cell cycle is the sequence of events occurring in an ordered fashion which results in cell growth and cell division

[Medicoman]

The cell cycle is the sequence of events occurring in an ordered fashion which results in cell growth and cell division.

• The cell cycle is a continuous process that includes all significant events of the cell, ranging from duplication of DNA and cell organelles to subsequent partitioning of the cytoplasm.

PHASES OF CELL CYCLE:

1. G0 PHASE:

• Gap 0 phase or G0 phase of the cell cycle is a period of time where the cell is present in a quiescent stage or resting phase, as it neither divides nor grows.

• The G0 phase can be considered either an extended G1 phase or a separate phase-out of the cell cycle.

• Usually, cells enter the G0 phase when they reach maturity like in the case of muscle cells and nerve cells, but the cells continue to perform their function throughout their life.

• In some cases, however, cells might enter the G0 phase from the checkpoint in the G1 phase due to the lack of growth factors or nutrients.

• In the G0 phase, the cell cycle machinery of the cell is dismantled, and the cell continues to remain in the G0 phase until there is a reason for the cell to divide.

• There are some cells like the parenchymal cells of the liver and kidneys that enter the G0 phase semi-permanently and can be induced to divide.

• Even though the G0 phase is often associated as senescence, the G0 phase is a reversible stage where a cell can enter the cell cycle again to divide.

2. G1 PHASE:

• The G1 phase of the cell cycle is a part of the interphase where the cell begins to prepare for cell division.

• A cell enters the G1 phase after the M phase of the previous cycle, and thus, it is termed as the first gap phase of the first growth phase.

• In this phase, no DNA synthesis takes place, but RNA synthesis occurs in order to produce proteins required for proper cell growth.

• G1 phase is considered a time of resumption where the cell finally picks up normal cell metabolism that had slowed down during the M phase of the previous cycle.

• The process and steps of the G1 phase are highly variable, even within the cells of the same species.

• The most important event of the G1 phase, however, is the transcription of all three types of RNAs which then undergo translation to form proteins and enzymes necessary for other events in the cell cycle.

• The duration of the G1 phase is also highly variable among cells. In some cells, it occupies about 50% of the total cell cycle time, whereas, in rapidly dividing cells, the phase is entirely omitted.

3. SYNTHESIS PHASE:

• The S phase or synthesis phase of the cell cycle is a part of the interphase where important events like DNA replication and formation of histone proteins take place.

• The processes of the S phase are tightly regulated as the synthesis of proteins and replication of DNA require utmost precision.

• The production of histone proteins and other proteins are crucial in this phase as the newly replicated DNA molecules require histone proteins to form nucleosomes.

• The entry into the S phase is regulated by the G1/S checkpoint that only allows cells with enough nutrients and healthy DNA to enter the next phase.

• The phase is moderately long, occupying about 30% of the total cell cycle time.

• During this phase, the content of DNA doubles in the cell, but the number of chromosomes remain the same as the division of chromosome doesn’t take place just yet.

• The regulatory mechanism of the S phase also ensures that the process of DNA synthesis takes place before the M phase and with precision.

• In order to preserve the epigenetic information, different regions of the DNA are replicated at different times.

• Similarly, actively expressed genes tend to replicate during the first half of the S phase, whereas inactive genes and structural DNA tend to replicate during the latter half.

• Therefore, at the end of the S phase, each chromosome of the cell has double the amount of DNA with a double set of genes.

4. G2 PHASE:

• The G2 phase or Gap Phase 2 or Growth Phase 2 is a phase of the cell cycle where the cell collects nutrients and releases proteins in order to prepare the cell for the M phase.

• The G2 phase is also a part of the interphase when the cell is still in the resting phase while preparing for cell division.

• The G2 phase is also important as it checks for DNA damage (during replication) to ensure that the cell is in proper condition to undergo division.

• The phase might be skipped in some rapidly dividing cells that directly enter the mitotic phase after DNA replication.

• It is, however, an essential phase that checks for mutations and DNA damage to prevent excessive cell proliferation.

• Even though information on the regulation and working of the G2 phase has been studied, its role in cancer initiation and development is yet to be determined.

• DNA repair is a crucial step in the G2 phase as it repairs breaks that might be present in the DNA strand after replication.

• The entry of the cell from the G2 phase to the M phase is regulated by the G2 checkpoint, where different proteins and complexes are involved.

• In the case of DNA damage or insufficient nutrients, the cell remains in the G2 phase and is not passed for cell division.

5. MITOTIC PHASE:

• The M phase or Mitotic phase of the cell cycle is the most crucial and dramatic phase of the entire cycle where the cell divides to form identical daughter cells.

• The most important event of this phase is the karyokinesis (nuclear division) where the chromosomes separate into form two distinct cells.

• The process of mitosis might differ from one organism to another and even from one cell to another.

• Mitosis begins with the condensation of chromosomes which then separate and move towards opposite poles.

• A cell entering the M phase has a 4N concentration of genetic material and ends with two cells, each containing a 2N concentration of DNA.

• Mitotic cell division occurs via four distinct steps; prophase, metaphase, anaphase, and telophase.

• Prophase is the first stage of mitosis where the chromosome of the cell divides into two chromatids held together by a unique DNA region called the centromere. As the prophase progresses, the chromatids become shorter and thicker. Prophase also includes the division of centriole that move toward the two opposite ends of the cell.

• Metaphase is the second and the longest stage of cell division where the chromatids are lined up on the metaphase plate. The chromatids are shorter and thicker and are still held together by a centromere.

• Anaphase is the next stage of mitosis involving the splitting of each chromosome into sister chromatids to form daughter chromosomes. After splitting, the chromatids are moved towards the pole due to the shortening of the microtubules.

• Telophase is the final stage of mitosis which involves the reorganization of two nuclei and the entry of the cell into the next phase. During this phase, a nuclear envelope is formed around the chromosomes to form two distinct daughter nuclei.

• Telophase indicates the end of the M phase, which initiates the division of cell organelles and separation of cytoplasm into two cells (cytokinesis).