What is the cell cycle?
The cell cycle is refers to the orderly sequence of events that takes place between one cell division and the next cell division that is regulated or controlled within a time period. The regulation and timing is very important in cell cycle to prevent abnormal cells from forming. This means that the cell cycle has an order in which the series of events occur and these events occur in phases or stages that called the Cell Cycle Phases or Cell Cycle Stages. Each of the stages of 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?
- 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).
- The cell cycle helps a cell to know when to divide to another cell when it receives signal and when to rest
- The cell cycle also helps a cell to know when to perform its function when needed
- 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 or 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 explanation for you to understand the little differences.
Definition of Cell Cycle
This includes 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 cell cycle. It includes both nuclear division and cell division. The cell cycle includes even the time that the 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 replaced 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 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 to 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).
Steps of the cell cycle
There are generally two broad steps of the cell cycle which are the step of interphase and step of mitosis. These two steps have other stages under them and will be summarized below; the details of each will be described in details on each article.
The 2 steps of the Cell Cycle
Interphase of cell cycle
During 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.
What are the stages of the cell cycle?
- G1 stage of interphase
- S stage if interphase
- G2 stage of interphase
- M Phase (stage of Mitosis)
- 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
- Letter S stands for stage of Synthesis (that is the phase in which the DNA of the cell is synthesized)
- 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
- Letter M stands for stage of Mitosis
- G1 stage
- S stage
- 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. At 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 active synthesis of ribonucleic acid (RNA) and also the synthesis of protein such as those controlling progress of the cell cycle. It is in 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 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 last for a time period of about 8 hours.
What happens during S phase?
What happens during the S phase of the cell cycle is 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 S phase and before cell division; what happens during the G2 phase of interphase is 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 production of a protein called Tubulin to ensure that microtubules are made. The G2 phase is the shortest phase of interphase and last for about 2 hours 30mins to 3 hours.
M phase of cell cycle (Mitosis Phase or M-Phase)
In the M phase, growth of the cell stops temporarily. The process of mitosis is described in details 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.
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 cycle or it will stop the process unless a certain condition is met. The presence of positive regulator molecules allow 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
- This checkpoints ensures that the cell nutrition is good
- It checks for the size of the cell if it is normal
- This also checks if the environment is favorable
- It checks if the DNA is intact
- This checkpoint monitors to ensure that the DNA is intact
- It also checks if all the chromosomes are attached to the mitotic spindle
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?
The time it takes from one cell cycle to another is dependent on the type of cell involved as epithelial cells in the human gastrointestinal tract divide every 10 hours.