Cell Cycle

A cell cycle is a set of events that occur in a cell as it divides and grows. A cell spends the majority of its time in interphase, which is when it grows, duplicates its chromosomes, and prepares to divide. After that, the cell exits interphase, goes through mitosis, and completes its division. The daughter cells that result go through their own interphase and start a new round of the cell cycle.

The organised series of events that occur in a cell in preparation for cell division is known as the cell cycle. On their way to cell division, cells go through a series of carefully controlled and regulated stages of growth, DNA replication, and cell division that result in two genetically identical cells. The cell cycle is divided into four stages, during which the cell develops in bulk, prepares for cell division, and divides.  The four stages of cell cycle are:

• G1 phase
• G0 phase
• S phase
• G2 phase

All of these stages are part of the cell cycle’s interphase stage, which occurs before the mitotic phase.

Cell Cycle Interphase 

The period between the end of one mitotic cell division and the start of the next cycle is known as interphase. It is the most time-consuming step of cell division. It is the period that occurs between the conclusion of telophase and the start of the next prophase. During the interphase, DNA synthesis does not take place. It only happens during the S phase of interphase, which is preceded and followed by two gap phases of interphase (G1 and G2) during which no DNA synthesis occurs. The cell cycle’s interphase stages are split into four major stages.

G1 Phase of Cell Cycle: It is the first phase of the interphase, beginning at the end of the preceding M phase and lasting until DNA synthesis begins. The G1 phase is also known as the growth phase since the cell’s biosynthetic activities are at their peak at this time. The cell’s protein supply grows, as does the number of cell organelles. Following the completion of these actions, a cell has three options:

• It has the ability to go through the cell cycle and enter the S phase.
• It has the ability to enter the G0 phase of the cell cycle and differentiate.
• In the G1 phase, it can be arrested.

The checkpoint, also known as the restriction point, is regulated by G1/S cyclins and is responsible for the transition from the G1 to the S phase.

G0 Phase of Cell Cycle:  The G0 phase of the cell cycle is known as the resting phase, and cells in this phase are known as resting cells. Neurones, for example, enter this phase after exiting the cell cycle and never divide again. Cells enter a resting state when they are deprived of nutrition or growth stimuli.

G0 cells typically have fewer ribosomes and RNA than their cycling G1 counterparts. They also produce protein at a pace that is less than half that of G1. When a G0 cell is stimulated to expand by growth hormones or nutrients, the rate of protein synthesis changes, and this usually coincides with an influence on the chromosomal cycle.

S Phase of Interphase: This is the third stage in the cell cycle sequence, and it occurs between the G1 and G2 phases. The word S stands for synthesis, and it is a highly specialised phase of interphase. In this phase, DNA synthesis occurs. A cell must make a fresh copy of its chromosomes before it may divide.

It takes both the replication of the lengthy DNA molecule in each chromosome and the assembly of a new set of chromosomal proteins onto the DNA to create chromatin or chromatid to make a new copy of the chromosome.

By the end of the phase, each chromosome has been cloned into two complete chromatids, which are linked at their centromeres until the M phase begins.

G2 Phase of Cell Cycle: The G2 phase lasts from the end of the S phase to the beginning of mitosis. Interphase is usually the shortest component of the process. During this period, there is a lot of cellular synthesis going on. Mitochondria and chloroplasts divide, resulting in an increase in energy storage.

The mitotic spindle begins to take shape. There are two control points in the interphase, G1/S and G2/M, at which the cell decides whether or not to proceed to the next stage. Checkpoints are two control locations that are also known as checkpoints.

M Phase of the Cell Cycle: Duplicated chromosomes are aligned, separated, and moved into two identical daughter cells in a multi-step process. Karyokinesis is the initial part of the mitotic phase, and cytokinesis is the second part of the mitotic phase. There are four types of karyokinesis:

Prophase

• The chromosomes become visible when they compress.
• Centrosomes give rise to spindle fibres.
• The nuclear envelope is breached.
• The nucleolus vanishes.

Prometaphase

• The kinetochores form at the centromeres as the chromosomes continue to condense.
• Centromeres migrate in opposite directions.

Metaphase

• Microtubules adhere to kinetochores and the mitotic spindle is fully grown.
• On the metaphase plate, the chromosomes organise themselves.
• Each sister chromatid is connected to a spindle fibre that comes from opposite poles.
• Cohesin proteins that bind all sister chromatids break down during anaphase.
• The sister chromatids are then dragged toward the cell’s opposite poles.

Telophase

• At opposite poles, the chromosomes arrive.
• Around chromosomal sets, a nuclear envelope forms.
• The mitotic spindle disintegrates.

Cytokinesis

In animal cells, cytokinesis begins when a ring of actin filaments develops at the metaphase plate. The ring contracts, creating a cleavage furrow that splits the cell in half.

Golgi vesicles in plant cells clump together near the old metaphase plate to create a phragmoplast.

The phragmoplast’s Golgi vesicles fuse to form a cell plate that grows from the centre toward the cell walls. The vesicle membranes fuse to produce a plasma membrane, which divides the cell in half.

The eukaryotic cell cycle is then restarted, beginning with the G1 phase of interphase.

Cell Cycle and Its Regulation

Cell fusion in distinct stages of the cycle can be used to assess the presence of different regulators at different stages of the cell cycle. A hybrid cell is created by fusing two cells together. Hybrid cells are heterokaryons with two nuclei that share the same cytoplasm.

Various combinations can fuse interphase nuclei during the phases of the cell cycle:

When a S phase cell is joined with a G1 cell, the heterokaryon indicates that both nuclei replicate DNA. This suggests that the S phase cell’s cytoplasm contains a DNA replication activator or regulator. S phase activator is the name of the regulator. It could be a regulator whose activation is determined by the readiness of cells in G1 to initiate a replication cycle.

When a S phase cell is united with a G2 cell, the S phase nucleus continues to replicate but the G2 nuclei do not. It suggests that the effects of the S phase activator cause already duplicated DNA to become resistive.

When a cell in the mitotic phase or M phase fuses with a cell in the G1 or G2 stage of interphase, the interphase nucleus enters pseudo mitosis, which is marked by premature chromosomal condensation in the interphase nuclei. This suggests that dividing cells contain a mitotic phase inducer.

When S phase nuclei are joined to mitotic cells, a complicated pattern emerges with fragmented S phase chromosomes.

Conclusion

The process through which cells multiply and divide into two new cells is known as the cell cycle. G1, S, G2, and M are the phases of the cell cycle. The cell is in the G1 stage when it is prepared to divide. To accomplish this, the cell enters the S phase, during which it copies all of the DNA. DNA synthesis is denoted by the letter S. The cell enters the G2 stage once the DNA has been duplicated and there is a complete additional set of all genetic material, where it organises and condenses the genetic material, or begins to condense the genetic material, and prepares to divide. M is the next stage. Mitosis is represented by the letter M. The cell divides the two copies of the genetic material into the two daughter cells at this point. After the M phase, cell division occurs, leaving two cells, and the cell cycle can resume.