Cell cycle stages, phases in order (G1, S, G2, M)

This means that the cell cycle has an order in which the series of events occur. They occur in phases or stages that are called the Cell Cycle Phases or Stages. Each of the stages of the cell cycle has various events going on that occur before the next stage or phase occurs. The cell cycle phases will be discussed with the aid of well-labeled diagrams.

What is the purpose of the cell cycle?

1. The purpose of the cell cycle is to regulate the activities of the cell so that a cell does not just keep producing similar cells without control (this is the basis of cancer cells).
2. The cell cycle helps a cell to know when to divide to another cell when it receives signal and when to rest
3. The cell cycle also helps a cell to know when to perform its function when needed
4. Understanding of the cell cycle helps in the production of drugs that will target cancer cells

The cell cycle, therefore, helps a cell to serve the right function and control it as to when to divide and when to stop dividing. It is an uncontrollable growth of cells that leads to abnormal new growth called Neoplasm, tumors, or cancers.

Difference between Cell Cycle, Cell Division and Nucelar Division

The concept of cell division, cell cycle, and nuclear division may sometimes be confusing and needs an explanation for you to understand the little differences.

Definition of Cell Cycle

The cell cycle is the life cycle of the cell from the time it was formed to the time it forms another cell. This cycle of events is termed the cell cycle. It includes both nuclear division and cell division. The cell cycle includes even the time that the eukaryote cell is resting (resting phase); the time the nucleus divides and the time the cell membrane divides to form two separate cells.

Definition of Cell division (Cytokinesis)

The cell contains the nucleus and other organelles such as the mitochondrion, Golgi apparatus, etc. the cells of the body are being destroyed everybody and there is need for the body to produce other cells to replace the damaged cells this is the purpose why a cell needs to divide to form other cells to replace worn-out cells, damaged cells or abnormal cells.

The cell as a whole divides to form another cell (one cell dividing to form another cell makes two cells).

Please note that when the cell divides, there is a need for the new cell to also have the same genetic component as the old cell; therefore, the process through which a cell divides to form another cell is called Cell division while the process of copying the contents of the nucleus into the newly formed cell is called nuclear division.

Nuclear division occurs before cell division as the nucleus needs to copy its contents first before the cell divides its cell membrane to form two separate identical cells.

Definition of Nuclear division (Karyokinesis)

The nucleus of a cell is the part that contains the genetic makeup of the cell. The genes are formed and stored in the nucleus and whenever the cell wants to divide in order to form another cell, the nucleus must copy genetic material to the new cell that is being formed so that all the cells produced will look the same and act alike. The process of copying the genetic material and dividing the copies to each cell is called Nuclear Division (division of the nucleus).
Labeled cell cycle diagram showing the different stages of the cell cycle including the average time it takes for each phase.

Steps of the cell cycle

There are generally two broad steps of the cell cycle which are the step of interphase and the step of mitosis. These two steps have other stages under them and will be summarized below; the details of each will be described in detail in each article.

The 2 steps of the Cell Cycle

1. Interphase
2. Mitosis

Interphase of cell cycle

During the interphase of the cell cycle, the cell grows to its normal size after cell division and then continues to carry out its normal functions until a signal to divide is received. Some needed proteins are then synthesized at this step too.

Stages of the cell cycle

1. G1 stage of interphase
2. S stage if interphase
3. G2 stage of interphase
4. M Phase (stage of Mitosis)
5. Cell division (cytokinesis)

When cells are undergoing division, they are called competent cells but when a cell is not progressing to mitosis, it will remain in phase G0 (G zero which is the quiescent phase)

Meaning of the letters M, G1, G2 and S in cell cycle

1. Letter S stands for stage of Synthesis (that is the phase in which the DNA of the cell is synthesized)
2. The letter G stands for Gap; the Gap after cell division and before S phase is called the G1 phase the gap after S phase and before the next cell division is called the G2 phase
3. Letter M stands for stage of Mitosis

Interphase stages

1. G1 stage
2. S stage
3. G2 Stage

There are four (4) main stages which are the Prophase, Metaphase, Anaphase and Telophase; some authors may add another stage before Metaphase known as the Prometaphase.

G1 phase of cell cycle

This is part of the interphase of the cell cycle and represents the time gap between mitosis and DNA replication. The G1 phase is usually the longest phase of the cell cycle (lasting for about 25 hours) and also the most variable phase of the cell cycle. In the end, G1 phase, the cell either becomes committed to dividing or not dividing.

What happens during G1 phase?

What happens during the G1 phase is an active synthesis of ribonucleic acid (RNA) and also the synthesis of protein such as those controlling progress of the cell cycle. It is in the G1 phase that the regulatory proteins (control systems) are produced. It is also in the G1 phase that the cell volume (or the size of the cell) which was initially reduced by half will now return to its original size.

S phase of interphase

When the cell receives a signal to divide, it triggers the replication of the DNA in the nucleus of the cell so that each chromosome will have two identical chromatids. This phase of the cell cycle is called the S phase which means the Stage of synthesis of DNA and it is a short phase compared to other phases. The S phase lasts for a time period of about 8 hours.

What happens during S phase?

What happens during the S phase of the cell cycle are the replication of the Deoxyribonucleic acid (DNA) and the synthesis of histones. It is also in the S phase that the duplication of the centrosome begins.

G2 phase of Interphase

This is the gap after the S phase and before cell division; what happens during the G2 phase of interphase is the continuous growth of the and checking of the new DNA to ensure there are no errors as the DNA is formed if there are errors, they are repaired and the process of division is started. When microtubules are needed to form the mitotic spindle, there will be a sharp increase in the production of a protein called Tubulin to ensure that microtubules are made. The G2 phase is the shortest phase of interphase and lasts for about 2 hours 30mins to 3 hours.

M phase of cell cycle (Mitosis Phase or M-Phase)

In the M phase, the growth of the cell stops temporarily. The process of mitosis is described in detail under Mitosis (where all the stages under mitosis have been discussed with diagrams and pictures). The process of Mitosis is continuous but has been divided into 4 stages to help in easy understanding: the four stages are prophase, metaphase, anaphase, and telophase.

M Stages

1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase

Diagram showing the checkpoints of the cell cycle that help in the regulation of the cell cycle

Cell cycle checkpoints (Cell Cycle Control system)

Each step of the cell cycle is checked or monitored by mechanisms called cell cycle checkpoints. These checkpoints are mainly 3 in the cell cycle: there is one checkpoint near the end of G1 phase, a second checkpoint at the point of transition of G2 phase to M phase, and the third checkpoint during the metaphase stage of Mitosis.

The checkpoints help to check that the timing is right and that the signals are correct. A cell that divides quickly is abnormal and the one that is slowly is also abnormal.

The cell cycle control system ensures that a cell proceeds to the next stage of the cycle or it will stop the process unless a certain condition is met. The presence of positive regulator molecules allows the cell cycle to advance to the next stage whereas negative regulator molecules ensure the cell cycle is halted until the criteria for proceeding are met.

G1/S checkpoint of cell cycle

1. This checkpoints ensures that the cell nutrition is good
2. It checks for the size of the cell if it is normal
3. This also checks if the environment is favorable
4. It checks if the DNA is intact

Metaphase/Anaphase Checkpoint

1. This checkpoint monitors to ensure that the DNA is intact
2. It also checks if all the chromosomes are attached to the mitotic spindle

G2/M checkpoint

This checkpoint ensures the DNA is completely replicated.

When a problem encountered at any checkpoint cannot be corrected, the proteins encoded by tumor suppressor genes are activated and Apoptosis (cell will cause itself to die).

How long does it take for a cell to go through the cell cycle?