Cell Division: Definition, and Different Phases of Cell Cycle

The ability to grow and reproduce is the fundamental property of all living organisms. These organisms grow by the addition of new cells which arise by the division of pre-existing cells. Cell division or cell reproduction is the fundamental process that helps in continuing life. Here, we will discuss the concept and different phases of cell cycle along with some important questions.

Cell division is defined as a process in which a single parent cell divides into two daughter cells. The two daughter cells are identical to each other and also identical to the parent cell. 
Read more: Cell Cycle and Cell Division

Define Cell Division?

When a parent cell divides into two or more cells, which is called daughter cell, cell division takes place. It generally occurs as a part of a larger cell cycle. All the cell reproduces by splitting into two, wherein each parental cell gives rise to two daughter cells.

What is Cell Cycle?

The sequence of events by which a cell duplicates itself synthesizes other constituents of the cell and eventually divides into two daughter cells is termed the cell cycle. The cell synthesizes all its constituents in a sequential pattern to ensure the proper division and distribution of components occurs between the two daughter cells. Hence we can say that the cell cycle is the period of time between the formation of new daughter cells and their further division.

Different Phases of Cell Cycle

The cell cycle consists of two phases namely 

• Interphase refers to a phase which is between two successive m phases and here the cell prepares itself for cell division. It is a phase in which the cell duplicates its organelles and replicates its genetic material. Interphase constitutes off more than 95% duration of the whole cell cycle.
• M-phase or Mitosis is the phase where the actual cell division occurs; in this phase the cell divides its duplicated constituents and forms identical daughter cells.

Interphase

Interphase is said to be the most active stage of the cell cycle.

• The G1 phase implies the interval between mitosis and the initiation of DNA replication. Here in this phase, most of the organelles duplication occurs.
• G0 phase is the phase in which the cell remains metabolically active but does not start to replicate until they are asked to do so.
• S phase is a phase in which the synthesis or replication of DNA takes place.
• G2 phase is the second gap phase between S and M phases. Mitochondria, Chloroplast and Golgi bodies duplicate in this phase.

M Phase

M phase or mitotic phase is the phase where the actual cell division occurs. It consists of two processes:

• Karyokinesis - where the division of nucleus occurs.
• Cytokinesis - where the division of cytoplasm takes place.

Mitosis

Mitosis is a process wherein a parent cell divides to form two identical daughter cells. Mitosis is called equational division because the two daughter cells have the same number of chromosomes as those present in the parent cell. 

1.Karyokinesis is the division of the nucleus.

• Prophase: It is the first stage of mitosis. Prophase is marked by the initiation of condensation of chromosomal material. The completion of prophase can be by chromosomal material condensation and initiation of assembly of the mitotic spindle. 
• Metaphase: It is the second stage of Mitosis. The nuclear envelope has completely disintegrated. Chromosomes are spread throughout the cytoplasm of the cell. The metaphase chromosome is made up of two sister chromatids which are held together by the centromere. Small disc-like structures at the surface of the centromere are called Kinetochores, these structures serve as the site of attachment of spindle fibres. Metaphase is characterized by all the chromosomes lined at the equator with one chromatid of each chromosome connected by its kinetochores to the spindle fibre from one pole and its sister chromatid attached by its kinetochore to spindle fibres from the opposite pole.
• Anaphase: On the onset of anaphase, each chromosome that was arranged in the metaphase plate is played simultaneously and the two daughter chromatids begin their migration towards opposite poles.
• Telophase: This is the final stage of Mitosis. The chromosome has reached their respective pole and has condensed and lost their individuality. The nuclear envelope assembles around the chromosome clusters. The nucleolus, Golgi Complex and ER reform.

2.Cytokinesis is the division of the cytoplasm. This phase marks the end of cell division. Cytokinesis occurs differently in animal cells and plant cells. In animal cells, cytokinesis is achieved by the formation of a furrow. In-plant cells, cytokinesis is achieved by the formation of the cell plates.

Meiosis

Meiosis is a double division in which a diploid cell forms four haploid cells, each having half the number of chromosomes. Meiosis consists of two divisions: meiosis I and meiosis II.

• Meiosis I is called reductional division because the chromosome number is reduced to have in this process.
• Meiosis II is called equational division because during this division the number of chromosomes remains the same as produced at the end of meiosis II.

Meiosis I

1.Prophase I: It is elaborate, prolonged and complex.

• Leptotene: chromosomes become gradually visible under the light microscope during this stage 
• Zygotene: during this stage chromosomes start pairing together and this process of association is called synapsis. The complex formed by a pair of the synapsed homologous chromosomes is called bivalent. 
• Pachytene: bivalent chromosome now clearly visible as tetrads. This stage is characterized by the appearance of recombination nodules, the site at which crossing over occurs between non-sister chromatids of the homologous chromosome. Crossing over is the exchange of genetic material, which is an enzyme-mediated process and the enzyme involved is called recombinase. Recombination between homologous chromosomes is completed by the end of pachytene.
• Diplotene: it is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosome of the bivalents to separate from each other except at the sides of crossovers. The X-shaped structures are called chiasmata.
• Diakinesis: Termination of chiasmata. 

2.Metaphase I: Bivalent chromosomes align themselves on the equatorial plate. The distribution of bivalent chromosomes is at random. The microtubules of spindle fibres from opposite poles are attached to the centromere of the chromosome facing towards it. 

3.Anaphase I: the two homologous chromosomes separate from each other and start moving towards the poles. Only one chromosome out of a pair reaches the pole, the number of chromosomes becomes half in the daughter cell.

4.Telophase I: the chromosomes reach the poles. Nucleolus and nuclear membrane reappear. It produces two daughter cells each containing a single nucleus. 

Cytokinesis: two daughter cells are formed which are haploid. 

Interkinesis: metabolic stage between telophase I and prophase II. Chromosomes elongate during this phase and no replication of DNA takes place.
Meiosis II

It is similar to Mitosis.

1. Prophase II: is not long or complicated. Chromatin material becomes compact. The nucleolus and nuclear envelope disintegrate and disappear.
2. Metaphase II: chromosomes align themselves at the equator. Kinetochores attached to the microtubules have been extended towards the equator by the opposite poles. 
3. Anaphase II: centromere holding the two chromatid split and allow the separation and movement of the to chromatic towards the opposite poles
4. Telophase II: chromatids reach the pole and decondense. The nuclear membrane and nucleolus reappear and four haploid nuclei are formed.

Cytokinesis: Each daughter cell of meiosis I divide to form two daughter cells hence at the end of meiosis, a diploid cell gives rise to four haploid daughter cells.

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Things to Remember

• Cell division is the basis of life itself; it is the process of how animals grow and reproduce. 
• Cell division is critical for both single-cellular as well as multicellular organisms. 
• When cells divide, two daughter cells are produced from one mother cell.
• Each new cell has exactly the same genetic material (DNA) as the original cell.
• Cell division is important for the growth of organisms, repair of damaged tissues, healing, regeneration, etc. 
• The human body also repairs injuries by the means of cell division.