Cell Cycle and Division - Overview, Structure, Properties & Uses

Cell Cycle: Definition, Phases, Examples, Diagram, Functions, structure

Edited By Irshad Anwar | Updated on Sep 02, 2024 07:26 PM IST

What is a cell cycle?

The cell cycle refers to the entire process by which a single cell grows and then divides into two daughter cells. It comprises four phases or stages: G1, S, G2, and M. During the G1 phase of the cycle, cells increase in size. They also progress with normal cell activities. In the S phase, chromosomes replicate. G2 is the final growth and mitosis stage where the cell checks if everything is ready in a cell for cell division. During mitosis, it undergoes a series of reactions wherein it equitably distributes its chromosomes and cytoplasm into two genetically identical daughter cells. Proper regulation of the cell cycle is crucial for growth, development, and maintaining the health of an organism.

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Cell Cycle: Definition, Phases, Examples, Diagram, Functions, structure

Phases of cell cycle

The cell cycle is a biological process with fundamental activity in the growth and division of cells. It consists of a series of well-defined phases that allow for the precise duplication and distribution of the cell's genetic material into two daughter cells. These phases involve interphase, in which the cell grows and replicates its DNA, and the mitotic phase, in which the cell divides. The proper regulation of cell division is important for developmental work, tissue repair, and the maintenance of genetic stability in organisms.

Interphase

Cells spend most of their time in interphase.
Cell performs its usual functions during this time.
In interphase cell grows in size and prepares itself for the next division.
Interphase is the most active phase of the cell cycle.
The interphase lasts more than 95% of the duration of the cell cycle.
It was earlier regarded as a resting phase because the metabolic activities performed by the cell were not visible under a microscope.

Howard and Pelc classified interphase into three sub-stages:

1. G1 - phase (First Gap phase)

The cell increases in size, doubles its organelles (such as mitochondria and ribosomes), and accumulates materials that will be used for DNA synthesis.
Cells are constantly performing their normal daily functions, including communicating with other cells, secreting substances, and carrying out cellular respiration.

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2. S - phase (DNA synthesis phase)

At the beginning of the S stage, each chromosome is composed of one DNA double helix.
Following DNA replication, each chromosome is composed of two identical DNA double helix molecules.
Each double helix is called a chromatid.

3. G2 - phase (Second Gap phase)

At this stage, the proteins that will be helpful during the cell division are synthesised.
For example, proteins that form microtubules will be synthesised.

DNA replication in Interphase

M Phase

The M phase, or mitotic phase, is the important part of the cell cycle in which the cell undergoes division to give two genetically identical daughter cells. It comprises two central processes:

Mitosis

Mitosis is also called the equational division (because the daughter cells have an equal number of chromosomes as that of parents) and somatic cell division (because it occurs in somatic cells). Mitosis was first observed by Strasburger in 1875. The term mitosis was given by Fleming in 1882.

Different Stages of Mitosis are:

1. Prophase:

During this stage the chromatin condenses and the chromosomes are visible.
The nucleolus disappears and the nuclear envelope fragments.
With the disappearance of the nuclear membrane, the centrioles migrate to the opposite end and the spindle formation occurs.
During late prophase or prometaphase, kinetochores appear on each side of the centromere.
The sister chromatids are attached to the so-called kinetochore spindle fibres with the help of the kinetochore.

2. Metaphase:

Chromosomes become fully condensed and distinct.
Chromosomes move towards the equatorial plane of spindles or metaphase plates.
Chromosomes are arranged with their arms directed towards the pole and centromere towards the equator.

3. Anaphase:

The centromere splits and the two chromatids of the duplicated chromosomes separate from each other.
Each chromatid now becomes a daughter chromosome.
Daughter chromosomes, each with a centromere and single chromatid appear to move toward opposite poles.
Anaphase is the shortest phase of mitosis.

4. Telophase:

During this phase, the spindle disappears and new nuclear envelopes develop around the daughter chromosomes.
Each daughter nucleus contains the same number and kinds of chromosomes as the original parent cell.
The chromosomes become diffuse chromatin and nucleolus reappears.
It is followed by the division of the cytoplasm

Diagrams illustrating each stage of mitosis

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Cytokinesis

Finally, in the cell cycle and at the end of mitosis, cytokinesis is the process by which the cytoplasm of the parent cell divides into two daughter cells. Cytokinesis begins with the formation of a cleavage furrow in animal cells, in which a contractile ring forms just beneath the cell membrane and consists of actin filaments. As this furrow deepens, it will pinch the parent cell completely, dividing it into two cells with a nucleus in each.

Cytokinesis in Animal Cells: Diagram

In animal cells, cytokinesis occurs through the formation of the cleavage furrow.
The cleavage furrow represents the indentation of the membrane between the two daughter nuclei.
A band of actin filament called the contractile ring starts to form the circular constriction between the two daughter nuclei.
Due to the action of the contractile ring, the furrow deepens continuously and ultimately a cell divides into two daughter cells.
Cytokinesis in the animal cell occurs centripetally, that is, from the periphery to the centre

Diagram: cytokinesis in plant cells

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In plant cells, the presence of a rigid cell wall prevents the furrow formation.
Therefore, in plant cells, a new cell wall is formed between the daughter nuclei to complete the cytokinesis.
Many Golgi vesicles and spindle microtubules arrange themselves on the equator to form phragmoplasts.
The membrane of Golgi vesicles fuses to form a plate-like structure called a cell plate.
Golgi vesicles secrete calcium and magnesium pectate.
The further cell plate is modified into the middle lamella.
In plants, cytokinesis occurs in centrifugal order (cell plate formation is from the centre to the periphery).

Cytokinesis provides that each daughter cell receives a complete set of organelles and cytoplasm; thus, the cell cycle is completed, and two daughter cells become produced one genetically identical to the other.

Regulation of the Cell Cycle

The regulation of the cell cycle is important so that faulty or compromised cells cannot divide.
There can be a mutation during DNA replication and other events of the cell cycle.
Therefore, there are checkpoints in the cell cycle that would check for errors and stop the cell cycle if there is any fault and allow the cell cycle to continue if there is no fault.
There are three checkpoints in the cell cycle G1 checkpoint, G2 checkpoint and M checkpoint.