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 distinctly different and separated from the mitosis process.
We have studied about how a wriggly caterpillar undergoes a series of developmental steps and turns into a beautiful butterfly. This series of developmental steps is termed as a life cycle. Similarly, every organism and living being has specific lifecycles of their own. The cell cycle can be imagined as “lifecycle of cells.” Therefore, when a cell undergoes many developmental and growth stages between the birth (formation of a cell from mother cells) and reproduction (formation of daughter cell by cell division).
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Cell cycle, also known as 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 starts the beginning of a new interphase cycle. Cell cycle is generally categorized into two types:
a) Cells that lack a nucleus: For example, bacteria lack a cell nucleus. Therefore, the cell cycle is not the same as eukaryotes. In the case of bacteria, the cell cycle is divided into B, C, and D periods. B period refers to the end of cell division to the start of DNA replication. C period indicates the start of DNA replication. D period refers to the end of a DNA replication and division of the bacterial cell into two daughter cells.
b) Cells with a nucleus: Eukaryotes consist of cells with a nucleus. In eukaryotes, a cell cycle can broadly be divided into interphase, the M phase or mitotic phase, and the cytokinesis. Read through this article to know more about different stages of the cell cycle and their role in the cell cycle.
Stages of Cell Cycle
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. All these sequential events take place in a structured and predictable way. The cell cycle is termed a cycle because the events act like a cycle and 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.
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.
M phase or Mitotic phase
In 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).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.
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 four distinct 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 multiple diverse activities. The activities taking place in interphase is very necessary for starting the next mitosis phase. The duration of interphase may vary from 12 to 24 hours in the mammalian tissues.
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 temporary period or long more permanent period. For example neurons after reaching the end stage of development stop dividing and enter into a more permanent resting period.
Gap 1 (G1) Phase
It is also termed as the first gap phase. In this phase, the cell starts growing and enlarges physically. It copies organelles, produces all the necessary molecular building blocks such as RNA and synthesizes proteins that will be required 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.
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 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. The cell’s “nuclear DNA” is condensed into chromosomes. These visible chromosomes are pulled apart with the help of mitotic spindles (specialized structured formed from microtubules). Mitosis can be divided into 4 separate stages including prophase, metaphase, anaphase, and telophase.
Cytokinesis begins after mitosis is completed. Sometimes it is difficult to distinguish between the two due to minute overlap in both the phases. In this phase, the cytoplasm of the cell is divided into two daughter cells. However, cytokinesis differs in plant and animal cells.
Cytokinesis in animals: There is a band of cytoskeletal fibers termed as the contractile ring. During cell division, the contractile ring shrinks inward and pinches the animal cell to divide it into two. This process is known as contractile cytokinesis. The mark "indentation" formed while the ring is contracted is known as "cleavage furrow."
Cytokinesis in plants: We know that the plant cells are stiffer in comparison to animal cells. It is due to the presence of a rigid cell wall. The plant cell is divided in cytokinesis by producing a new structure in the middle of the cell in order to divide the cell into two. The specialized structure formed for separation is called as the cell plate.
Difference in Plant and Animal Cell Cycle
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 divided. There are certain kinds of cells that divide quickly and in these types of cells the newly formed cells immediately enter into another round of cell division cycle. Examples include embryo and tumor.
Alternatively, there are other types of cells that divide at a slow pace and sometimes completely stop dividing. These types of cells stop dividing and enter into another phase called as G0 phase or resting phase. In this phase, the cells stop any kind of activity that will prepare the cell for further division. The cell will continue its function like a neuron after the end stage will stop dividing but continue to conduct signals. Similarly, liver cells will continue storing carbohydrate even though the cell division will stop and the cell will reach its permanent state.
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. Embryo cells just after fertilization last for sometimes 30 minutes or less.
However, in such embryonic cells, the cell growth does not take place. These early embryonic cells quickly divide into smaller cells but lack G1 and G2 phase. DNA replication occurs rapidly in these type of early embryonic cells with very short S phase and M phase.