Cell cycle: Definition, Control of Cell Cycle, Environmental Factors and More

Overview of Cell Cycle

The cell cycle is the sequential series of changes that involves the non-dividing and dividing phase. Each phase is further sub-divided. The function of the cell cycle is basically to duplicate the DNA as in the parent cell and then segregate it into two identical daughter cells. The first phase in this is the interphase which is sometimes inappropriately termed as the resting phase. It consists of G1, S – phase and G2 phase. G1 is the time of extensive growth of cells i.e., organelles, enzymes, and cell machinery, etc., S phase is the phase of duplication of DNA.

There is a synthesis of DNA as chromosomes are replicated and appeared as sister chromatids. Following the S phase, there is the G2 phase, the preparatory phase for mitosis. In this mitotic apparatus organized for the movement of chromosomes to opposite poles. Then there is the second phase of the cell cycle, the mitotic phase. In which chromosomes are equally distributed and two identical daughter cells are produced.

At each phase, there are specific checkpoints for determining the fate of the new phase according to cell makeup. Some cells actively divide in favorable conditions. Here cell size is the vital condition passing the restriction point or check post. There are some signals in the form of proteins called MPF. It induces mitosis. It has two components 1) Cdc2 – a protein kinase enzyme and 2) Cyclin – for starting the division, without this cell cycle will enter the G phase.


The cell goes through a series of changes, which involves a period of growth, duplication of DNA, followed by cellular division. This series of changes is called the cell cycle.

It consists of two phases viz., interphase which is the period of non-apparent division as well as the period of the division known as the mitotic stage. Each phase is further divided into different sub-phases.


The duration of the life cycle of the cell (cell cycle) between two consecutive stages is described as the interphase or misleadingly called resting phase. It is the period of the immense biochemical tasks as well as can further be divided into G1-phase, S-phase, and also G2-phase.

cell cycle

G1 phase (Initial growth phase or Post mitotic gap phase)

G1 (Gap 1) is the period of extensive metabolic activity, in which cell generally grows in size, specific enzymes, are synthesized and DNA base units are gathered for the DNA synthesis. A post-mitotic cell can leave the cell cycle during G1 entering a phase called G, and continue to be for days, weeks, or in some cases (e.g., nerve cells and also cells of the eye lens) for the lifetime of the organism without dividing further.

S phase (DNA duplication)

Following the G1, S phase starts when DNA synthesis starts; after its completion, all of the chromosomes have actually been replicated, i.e., each chromosome consists of two sister chromatids. Thus, during this stage, the amount of DNA in the cell has doubled, though the ploidy and also a number of chromosomes are unchanged.

The speed of RNA transcription and protein synthesis are really low during this stage. An exception to this is histone production, the majority of which happens throughout the S stage.

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G2 phase (pre-mitotic stage)

Chromosomes are duplicated which initiates the G2 stage (pre-mitotic phase), hence preparing the cell for division e.g., power storage for chromosome movement to center and then to opposite poles, mitosis specific proteins, RNA and microtubule subunits (for spindle fibres) manufacture. Cells after this move forward to the stage which is the period of division.

At each phase, there are specific checkpoints, which determine the fate of the new phase according to the cell’s internal makeup.

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The span of each phase varies. In the case of human cell, the typical cell cycle has to complete within 24 hours, mitosis takes thirty minutes, G1 9 hours, the S-phase 10 hours, and also G2 4.5 hours whereas the complete cell cycle in yeast is only 90 minutes.

Control of Cell cycle

Different cells have a various duration of the cell cycle. Some cell actively divides like liver cells. However, some cells never divided again in a lifetime like nerve cells. G1 stage is a key phase in the control of cell cycle as well as cellular division. A crucial checkpoint occurs late in the G1 stage before DNA synthesis in the S stage. This is called Restriction Point. If all the internal and external factors are favorable, the cells duplicate their DNA. After that, it divides.

The cell may leave the cell cycle at the restriction point. It may switch to a non-dividing state. This state is called G stage. The conversion of G1 to G is regulated by the following factors.


Environmental factors

Most cells of the body are actually in Gphase. The most specialized cells never divide again for example nerve and muscle cells. Some other cells like liver cells can be boosted by certain environmental aspects. These environmental elements are nutritional status, cell population, and also the development status of the cells.

Growth factor

The growth factor additionally controls the cell cycle. As an example, the fibroblast called platelet-derived growth factor(PDGF) is required. PDGF is released during injury.

Size of the cells

Some cells actively divide in favourable conditions. Right here cell size is one of the most vital conditions passing the restriction point. The cell must grow to a particular size during the G1 phase prior to DNA synthesis begin. The ratio of cytoplasmic volume to genome size is an essential indicator. The cell grows by adding cytoplasm. Yet the amount of DNA in the core remains constant. When the cell’s volume to genome proportion reaches a certain threshold value the cell passes the restriction factor and copy DNA.

Control by MPF(maturation promoting factor)

Scientists have actually identified signals. These signals are in the form of complex proteins called MPF. It stimulates the cell to become part of mitosis from interphase G2. The amount of MPF in the cell fluctuates in the cell cycle. MPF appears in late interphase as well as reaches its greatest concentration during mitosis. It vanishes at the end of mitosis. When MPF reaches a specific limit concentration in a G2 stage, prophase starts.

The concentration of MPF during mitosis is required for the starting of mitosis.

The decrease in MPF signals end the mitosis and the G1 stage of the next cycle begins.

MPF induces mitosis by serving as an enzyme. MPF belongs to a family of enzymes called protein kinases. A protein kinase is an enzyme that activates other proteins as well as enzymes. It catalyses the transfer of a phosphate group from ATP to every target protein. MPF has two components:

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  1. Cdc2: it is a protein kinase enzyme. It is in charge of transition from interphase to mitosis. it stays inactive until attached by cyclin. The concentration of cdc2 stays consistent throughout the cell cycle.
  2. Cyclin: it is attached with cdc2 as well as activates it. Its concentration varies according to the different stages of the cell cycle it is continuously produced throughout interphase. Its concentration is boosted at the G2 stage. Therefore, it is attached with cdc2 and also makes the MPF active. So, cell division starts. Without cyclin the cell cycle enters into G Cyclin is destroyed at the end of the cell cycle.

FAQs about Cell Cycle

Q1: What is the cell cycle, and what are its main phases?

The cell cycle is a series of changes involving the non-dividing and dividing phases in which DNA is duplicated and segregated into two identical daughter cells. It consists of interphase (G1, S, and G2 phases) and the mitotic phase. Interphase is further divided into sub-phases, and the mitotic phase involves chromosome distribution and the production of two identical daughter cells.

Q2: What happens during the G1 phase of interphase?

The G1 phase is the initial growth phase or post-mitotic gap phase. During this phase, the cell undergoes extensive metabolic activity, grows in size, synthesizes specific enzymes, and gathers DNA base units for DNA synthesis. Some cells may leave the cell cycle during G1 and enter a non-dividing state.

Q3: What characterizes the S phase of interphase?

The S phase, or DNA duplication phase, follows G1 and involves the synthesis of DNA. Chromosomes are replicated, resulting in the appearance of sister chromatids. The amount of DNA in the cell doubles during this stage.

Q4: What is the G2 phase, and what preparations occur during this phase?

The G2 phase, or pre-mitotic stage, is initiated after chromosome duplication in the S phase. During G2, chromosomes are duplicated, preparing the cell for division. This phase involves power storage for chromosome movement, synthesis of mitosis-specific proteins, RNA, and microtubule subunits for spindle fibers.

Q5: Are there specific checkpoints during the cell cycle, and what is their role?

Yes, there are specific checkpoints at each phase of the cell cycle that determine the fate of the new phase based on the cell’s internal makeup. Checkpoints play a crucial role in controlling the progression of the cell cycle.

Q6: How is the cell cycle controlled, and what is the significance of the restriction point?

The cell cycle is controlled by factors such as the restriction point or checkpoint in the late G1 stage. If internal and external factors are favorable, cells duplicate their DNA and proceed to division. Otherwise, cells may leave the cell cycle and enter a non-dividing state.

Q7: What factors influence the cell cycle, and how does environmental status affect cell division?

The duration of the cell cycle varies among different cells. Environmental factors such as nutritional status, cell population, and the developmental status of cells influence the cell cycle. Specialized cells like nerve and muscle cells may never divide, while others like liver cells can be influenced by environmental factors.

Q8: How is cell division induced, and what is the role of Maturation Promoting Factor (MPF)?

Cell division is induced by Maturation Promoting Factor (MPF), a complex protein. MPF, consisting of Cdc2 (protein kinase enzyme) and Cyclin, stimulates the transition from interphase to mitosis. MPF concentration fluctuates in the cell cycle, reaching its peak during mitosis. The decrease in MPF signals the end of mitosis, initiating the G1 stage of the next cycle.

Q9: What is the significance of cyclin in cell cycle regulation?

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Cyclin is a protein that activates Cdc2, forming MPF and initiating mitosis. Cyclin concentration varies in different stages of the cell cycle, with its peak in the G2 stage. Without cyclin, the cell cycle enters G0, a non-dividing state.

Wrap Up: Understanding the Cell Cycle

In summary, the cell cycle is a dynamic and orchestrated series of events that encompasses both non-dividing and dividing phases in a cell’s life. The primary phases, interphase (G1, S, G2) and the mitotic phase, play crucial roles in DNA duplication and cell division. Specific checkpoints ensure the cell’s internal makeup aligns with favorable conditions for progression.

During the G1 phase, cells undergo extensive metabolic activity and growth, and some may enter a non-dividing state. The S phase involves DNA duplication, resulting in sister chromatids. G2, the pre-mitotic stage, prepares the cell for division by duplicating chromosomes and synthesizing essential components.

Checkpoints regulate the cell cycle, with the restriction point in G1 determining whether cells proceed to duplicate DNA. Environmental factors, such as nutritional status and cell population, influence cell division. Specialized cells may never divide, while others respond to specific environmental cues.

Cell division is induced by Maturation Promoting Factor (MPF), a complex protein comprising Cdc2 and Cyclin. MPF’s concentration fluctuates, peaking during mitosis and signaling the end of mitosis when reduced. Cyclin, a key regulator, activates Cdc2, initiating mitosis. Without cyclin, cells may enter a non-dividing state (G0).

These insights into the cell cycle, its phases, checkpoints, and regulatory factors deepen our understanding of cellular processes, emphasizing the intricate balance required for successful DNA duplication and cell division.