Cell Cycle - Definition, Phases of Cell Cycle

The cell cycle is the basic fundamental process every cell in the body undergoes for its growth, replication, and formation of new cells. It is a continuous process that maintains the proper functioning and growth of the body. The primary phases of the cell cycle are- Interphase and M-phase. During the Mitotis phase (M-Phase), the cell division takes place. There are many checkpoints during the cell cycle and any abnormality during the cycle will lead to negative consequences in the organism.

What is the Cell Cycle?

A cell is the basic structural and functional unit of an organism, it undergoes a series of events called the cell cycle that results in the formation of new cells called daughter cells along with the duplication of DNA. The cycle was discovered by Prevost and Duman in the year 1984 while studying the cleavage of the frog egg.

This process is very important in an organism as it is responsible for growth, development, asexual reproduction in unicellular organisms, and cell repair in multicellular organisms.

Phases of Cell Cycle

The cell cycle is divided into two major phases:

• Interphase
• Mitotic phase (M-Phase)

Interphase

In this phase, the cell is prepared for cell division and DNA replication and is also known as the resting phase as it occupies 95% time of the overall cycle. The interphase is further divided into three phases:

• G1 phase
• S phase
• G2 phase

G1 phase

The first gap phase of the cell occurs between the mitosis and initiation of DNA replication and is called the G1 phase or Gap 1 phase. The main characteristics of this phase are:

• Cell growth: the main function in this phase is cell growth, the cell increases in size and also synthesizes enzymes that are required for DNA replication and cell division.
• Biosynthesis: the synthesis of RNA and proteins that are required for cell growth.
• Energy storage: ATP is accumulated during this phase that is required for the synthesis of DNA.
• Checkpoint is monitored; A crucial checkpoint called the G1 checkpoint is monitored during this phase

S Phase (Synthesis Phase)

The replication of DNA occurs in this phase, the chromosomes duplicate themselves and form two sister chromatids that are held together in the center by the centromere so that the resulting daughter cells will get identical sets of chromosomes. This phase is followed by the G1 Phase and preceded by the G2 phase.

The replication forks are formed this is where the replication begins. The DNA unwinds at these points and forms the replication fork where the new DNA strand is synthesized. The type of replication that takes place is called semi-conservative replication as it has one old strand and one new strand.

A variety of enzymes are required for the replication, DNA polymerase adds the nucleotides to the growing DNA chain. The helicase enzymes help to unwind the DNA and the ligase enzymes join Okazaki fragments on the lagging strand. Accurate replication of DNA is required to maintain the integrity of the genetic material and also to prepare the cell for mitosis.

G2 phase

This is the second gap phase which is a crucial stage that follows the S phase and proceeds the actual cell division. In this phase the final preparation like protein synthesis and formation of microtubules that are required for spindle fiber formation during mitosis takes place. The newly replicated DNA is checked for errors so that the transmission of the damaged DNA can be prevented.

Mitotic phase (M-phase)

This phase is where cell division takes place which includes mitosis and cytokinesis.

Mitosis is divided into four stages:

• Prophase: the chromosomes condense and become visible, the nuclear envelope breaks down and spindle fibres start forming during this stage
• Metaphase: in this stage, the chromosome aligns the equator region of the cell and gets attached to the spindle fiber at the centromere region
• Anaphase: the sister chromatids are pulled apart in this stage and they move toward the opposite poles of the cell
• Telophase: the chromosome begins to decondense and the nuclear envelope is formed around the chromosome sets while the spindle fibers begin to disintegrate.

Cytokinesis is the division of the cytoplasm that takes place immediately after the telophase resulting in the formation of two daughter cells.

Regulation of Cell Cycle

The regulation of the cell cycle takes place by many checkpoints, enzymes, and proteins to ensure that the incomplete or damaged DNA is not passed to the daughter cells and also to maintain genetic stability and prevent certain diseases like cancer which is caused due to uncontrolled cell division.

There are three checkpoints in the cell cycle:

• G1 checkpoint: this checkpoint ensures whether the cell is in sufficient size with required nutrients and also checks if the DNA is undamaged and ready for replication
• G2 checkpoint: this checkpoint checks that the replication of DNA is completed in the S phase without any damage
• M phase checkpoints: this checkpoint ensures that the chromosomes are properly aligned and distributed to the daughter cells without any errors.

Apart from the checkpoints enzymes like CDKs, Cyclins,p53, and tumor suppressors contribute to the regulation of the cell cycle.

Disease Caused by the Malfunction of Cell Cycle

The disease is caused due to improper cell cycle cancer which is mainly related to uncontrolled cell growth and cell division. The uncontrolled cell division occurs due to mutation and it leads to the overactivation of proteins that promote the oncogene and inactivation of the tumor suppressor gene.

The genomic instability is caused due to errors during DNA replication and mitosis that lead to mutations and chromosome abnormalities. Other conditions caused due to cell cycle dysregulation are:

• Developmental disorders
• Autoimmune disease
• Degenerative disease
• Benign tumors