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Cell Cycle and Cell Division

Cell Cycle

Do you know the cause of cancer is related to cell cycle? The cancer cells cannot be regulated by the cell cycle and therefore constantly divides even in the absence of the required growth factors. This is the reason why it is difficult to kill cancer cells. Let us learn more about the cell cycle.

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Introduction to Cell Cycle and Cell Division
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Cell Cycle

Cell cycle and cell division form the basis of life. Growth, regeneration, and reproduction depend upon cell cycle and cell division. However, what happens when this cell cycle is disturbed or when the cells fail to regulate the cell cycle? It can lead to uncontrolled cell growth and the division which can finally lead to cancer

Scientists and microscopists have known cell cycle and cell division for over hundred years. However, it was the pioneering and revolutionary work of Alma Howard and Stephen Pelc in the 1950s that revealed DNA replication took place at a particular phase of the cell cycle. They demonstrated that cell cycle is distinct and separate from the mitosis process.

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Definition

Cell cycle (cell division cycle) is an ordered sequence of events occurring in a cell. Cell cycle results in cell growth and DNA replication thereby forming two daughter cells. It is an essential process for the formation of a mature organism from single-celled fertilized eggs. The process of cell cycle promotes renewal and regeneration of hair, blood cells, skin, and certain internal organs. The formation of daughter cells by cell division indicates the start of a new interphase cycle.

Different Stages of a Cell Cycle

Cell Cycle

Stages of Cell Cycle
I = Interphase, M = Mitosis; inner ring: M = Mitosis, G1 = Gap 1, G2 = Gap 2, S = Synthesis; not in ring: G0 = Gap 0/Resting.
(Source Credit: Wikipedia)

A cell splits after completing a sequence of events. For example, a cell grows, replicate its DNA (genetic material), and then finally divides into two daughter cells. The cell cycle is termed a cycle because the events repeat itself. After completing one complete round the newly formed daughter cells begin the same process all over again. The two key phases of a cell cycle are interphase and M phase or the mitotic phase.

Every phase will be successfully activated on proper progression and completion of the previous phases. However, if a cell is temporarily stopped progressing or somehow stopped dividing then the cell enter into another state termed as G0 phase, also called a “state of quiescence.” Let’s discuss the steps or phases in details.

Interphase

In this phase, the cell grows and produces a copy of the genetic material (DNA). Interphase can be further subdivided into three distinct phases: G1 phase, S phase (synthesis), G2 phase. The cell cycle begins after the division of mother cell into two new daughter cells. If the newly formed cell wants to move on then it must divide itself.

However, there are certain initial steps that occur before the actual division. In these phases, the new daughter cell prepares itself for the division. The interphase usually seems like a resting phase between the cell divisions but on contrary, it is a phase with a number of diverse activities. The duration of interphase may vary from 12 to 24 hours in the mammalian tissues.

Sub-phases of Interphase

  • Gap 0 (G0) Phase: At times the cell will leave the cycle and temporarily stop dividing. This is called a resting period. It can be for a short time or long more permanent period. For example neurons after reaching the end stage of development stop dividing and enter into a more permanent resting phase.
  • Gap 1 (G1) Phase: It is also termed as the first gap phase. In this phase, the cell starts growing and enlarges physically. It forms the copy of organelles, produces all the necessary molecular building blocks such as RNA and also synthesizes proteins that are essential in later stages. At this point, a control mechanism is activated to ensure proper DNA synthesis. The control mechanism is termed as the G1 checkpoint.
  • S Phase: In this phase, a cell produces a complete copy of DNA in the nucleus to produce two similar daughter cells. DNA replication begins in the S phase or the synthesis phase. The microtubule-organizing structure (centrosome) is also copied in this phase. The centrosome is the structure that helps in dividing the DNA during M phase.
  • Gap 2 (G2) phase: In G2 phase the cell grows further, produce proteins and organelles and starts rearranging the constituents of the cell for mitosis phase. At the end of the G2 phase, another checkpoint is activated called as G2 Checkpoint. G2 Checkpoint ensures everything is ready for division and M phase. The end of the G2 phase ends when the mitosis process begins.

Interphase can be summarized as the phase between two M (mitotic) phase.

M phase

n this phase, the cell splits its DNA into two copies. Additionally, the division of the cytoplasm takes place thereby forming two daughter cells. M phase can be categorized into karyokinesis (the division of cell chromosome) and cytokinesis (the division of cell cytoplasm to form new daughter cells). M phase is categorized into two distinct phases: mitosis and cytokinesis. In this phase, the cell divides the duplicated DNA and the cytoplasm into two new daughter cells.

  • Mitosis: The cell’s “nuclear DNA” is condensed into chromosomes. These visible chromosomes are pulled apart with the help of mitotic spindles (the special structures formed from microtubules). Mitosis is further subdivided into 4 separate stages including prophase, metaphase, anaphase, and telophase.
  • Cytokinesis: Cytokinesis begins after mitosis is complete. In this phase, the cytoplasm of the cell is divided into two daughter cells.

Cell Cycle Exit and Gap 0 Phase

By now we know the cell cycle results in the formation of two new daughter cells. Now the question arises what happens to the newly formed daughter cells after one complete round of cell cycle? This is entirely dependent on what cells are dividing. There are certain kinds of cells that divide quickly and in these types of cells, the new daughter cells immediately enter into another round of cell division cycle.  Examples include embryo and tumour.

Alternatively, there are other types of cells that divide at a slow pace and sometimes completely stop dividing and enter into another phase called as G0 phase or resting phase. The cell will continue its usual function. Such as neuron after the end stage does not divide but continue to conduct signals.

Duration of the Cell Cycle

The cell cycle duration will vary in different types of cells. The G1 phase will continue for approximately 11 hours, S phase will continue for 8 hours, G2 phase for nearly 4 hours and the M phase for nearly one hour in a rapidly dividing human cell with cell cycle duration of 24 hours. Some cells may divide faster than human cells whereas some cells may take more time to complete an entire cell cycle. For example “budding yeast” will complete the entire cell cycle (4 stages of the cell cycle) in about 90 minutes.

Solved Example for You

Q: Which is the longest phase of the cell cycle?

  1. G2
  2. G1
  3. S
  4. G0

Sol: The correct answer is option “B”. G1 phase is usually the longest phase of cell cycle. G1 phase is the first gap phase where the cell is preparing for the other stages of cell cycle. Moreover, G1 phase follows the mitosis cell division. It is the time for the newly formed cells to grow before the DNA replication. So, the G1 phase is the longest. G1 phase can vary in the different type of cells. It can last for minutes such as prokaryotic cells, hours such as yeast or sometimes for years such as liver cells.

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