CELL DIVISION

 CELL DIVISION

For any cell to reproduce successfully, three fundamental events must take place:(1) Its genetic information must be copied,(2) The copies of genetic information must be separated from one another and (3) The cell must divide. All cellular reproduction includes these three events, but the processes that lead to these events differ in prokaryotic and eukaryotic cells. The division of chromosomes and cytoplasm of a cell into two cells is known as cell division. The cell that undergoes division is known as parent cell, while the cell derived from the division of a parent cell are known as daughter cells. 

In prokaryotes, the cell division occurs by an asexual division called as binary fission.. In bacteria, the circular bacterial chromosome replicates, and the two new genomes move toward opposite ends of the cell. A new plasma membrane is formed between them and the the cytoplasm divides into two leading to cell division. The new daughter cells absorb nutrients for growth; replicate their bacterial chromosome, and divide again. In eukaryotes, chromosomes are separated from the cytoplasm by the nuclear envelope. The cell reproduction requires the processes of DNA replication, copy separation, and division of the cytoplasm. However, the presence of multiple DNA molecules requires a more complex mechanism.  

CELL CYCLE

The entire sequence of repeating events from one mitotic division to the next is referred to as the cell cycle. The cycle has two major phases, viz., (1) Interphase, the period between cell divisions and (2) M phase, which includes mitosis and cytokinesis. In plants, cell division occurs in meristems. In dormant meristems, the cell rest in Go phase.  When conditions are correct, the cell enters into the cell cycle. 

I. INTERPHASE mainly consists of three phases namely, G1, S and G2 phases. G1 and G2 are resting phase called gap 1 and  

2 respectively, S phase is the period of DNA replication. A fourth interphase stage called Go phase occurs in plants during adverse growth conditions. The Go phase is a non-dividing stage during which cells usually maintain a constant size. The cells can remain in Go phase for a long period, even indefinitely, or they can reenter into G1 pagain. Many cells never enter Go phase

G1 phase :

 The period between telophase and S phase is called as G1 phase where synthesis of proteins and RNA occurs. During this stage, the nucleus migrates to the centre of the cell and is surrounded by phragmosome. A critical point termed the G1/S checkpoint occurs in G1 phase after which the cell is committed to divide. Before reaching the G /S checkpoint, cells may exit and pass into Go, a non-dividing phase. S (Synthetic) Phase: The phase between G1 and G2 where DNA synthesis and chromosome replication occurs is called as S phase. Before S phase, each chromosome is composed of one chromatid and after S phase, each chromosome is composed of two chromatids i.e. two copies of its genetic information.  

G2 Phase : 

The phase after DNA replication and beginning of prophase where synthesis of protein and RNA necessary for cell division take place is termed as G2 phase. The important point called G2/M checkpoint occurs in G2 phase. Only after passing this checkpoint, the cell which is ready to divide can enter into M phase.  

REGULATION OF CELL CYCLE

The plant hormones like auxin and cytokinins are known to initiate cell cycle. Auxin stimulates DNA replication while cytokinin initiates the mitotic cell division events.  

During the cell cycle, the cell has check points to check for problems during DNA synthesis and chromosome segregation. The cell cycle is controlled by activity of proteins called as cyclins, and cyclin-dependent protein kinases (CDPK) enzymes. A kinase is an enzyme which will phosphoryate another protein. One group of cyclins called the G1 cyclins, are manufactured and activate CDPKs which stimulate DNA synthesis at the G /S check point. If sufficient G1 cyclins are not formed, the cell will not progress to the S phase. After passing this point, the G1 cyclins are degraded and a new class of cyclins called M cyclins are produced. These activate a second set of CDPKs which permit the cells to pass the G2/M check point into mitosis.  

After passing through G2/M check point, the animal cells will divide whereas the cells plant cells will not divide. This means that the plant cells continue to replicate DNA without dividing. This phenomenon known as endoreduplication, occurs in more than 80% of the plant cells. These mechanisms arrest the cell cycle and allow the cell to repair defects so that it is not transmitted to the next generation.  

DURATION OF CELL CYCLE

Although the length of interphase varies from cell type to cell type, a typical dividing mammalian cell spends about 10 hours in G1, 9 hours in S, and 4 hours in G2 phase. In Vicia faba, the duration is about 12 hours in G1, 4 hours in S, and 12 hours in G2 phase.

II. MITOSIS

Prophase

1. The chromosomes appear as thin thread like structure.

2. Each chromosome possesses two chromatids as the chromosome was duplicated in the preceding S phase.  

3. In animal cells, the spindle grows out from a pair of centrosomes which migrate to opposite sides of the cell. Each centrosome has a centriole, which is also composed of microtubules. Higher plant cells do not have centrosomes or centrioles, but they do have mitotic spindles.  

4. Chromosomes begin to condense, while nuclear envelop and nucleolus are present. 

Prometaphase

1. Nuclear envelop begins to disintegrate marking the beginning of prometaphase.  

2. Spindle microtubules, which are outside the nucleus, enter the nuclear region and connect to the kinetochore of the sister chromatids.  

3. Some microtubules do not attach to a chromosome and remain at the center of the cell.  

4. The two chromatids of each chromosome further condense and become visible due to matrix coating and relational coiling. 

Metaphase

1. During metaphase, the chromosomes arrange themselves in the metaphase plate, between the two centrosomes.  

2. The movement of chromosome to the equatorial plate and their orientation is termed as metakinesis. 

3. The chromosomes are shortest and thickest only during metaphase. 

4. The chromatids are held together at the centromeres and relational coiling is absent during this stage.  

Anaphase

1. Anaphase begins when the centromere of each chromosome divides longitudinally leading to separation of chromatids. After separation, each one is considered as a separate chromosome.  

2. Centromere is the first portion of each of the chromosome to begin to move towards the poles. 

3. Depending on the position of the centromeres viz., metacentric, acrocentric ortelocentric, the chromosomes resemble ‘V’, ‘L’ or ‘I’ shapes respectively.

4. The sister chromatids move towards the poles. 

Telophase

1. Telophase begins with the arrival of chromosomes at the poles.

2. The chromosomes unwinds and lengthens to become a mass of chromatin fibers and the nucleus will be reorganized from the chromatin.

3. Nuclear envelop is formed around each set of chromosomes producing two separate nuclei within the cell. Nucleolus also reappears.

Cytokinesis

The division of cytoplasm into two halves is called as cytokinesis. It usually occurs during late anaphase or occurs simultaneous with telophase. In plants, cytokinesis takes place through the formation of cell plate at the centre of the cell which gradually moves outwards towards the periphery dividing the cytoplasm into two daughter cells. In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow. The two daughter cells produced by mitosis has the same number of chromosomes as the parent cell. Each daughter cell later enlarges in size till it becomes comparable to the parent cell.