Phases of Cell Cycle and Cell Division

​Phases of Cell Cycle and Cell Division

Rudolf Virchow (1855,1859) was the first to suggest that new cells are formed from the division of the pre-existing cells- omnis cellula e cellula (every cell is derived from a cell).

In 1873, strasburger similarly proposed that nuclei are formed from pre-existing ones. Boveri (1879) and Flemming (1879, 1880) studied details of somatic cell division.

It is the process by which a mature cell divides and forms two nearly equal daughter cells which resemble the parental cell in a number of characters.

"Continuity of life" is an important intrinsic characteristic of living organisms and is achieved through the process of reproduction. The reproduction may be asexual or sexual. Both of these involve the division and replication of cells. Even the growth and development of every living organism depends on the growth and multiplication of its cells.

In unicellular organisms, cell division is the means of reproduction by which the mother cell produces two or more new cells. In multicellular organism also, new individual develop from a single cell. The zygote, by the cell division. Cell division is central to life of all cells and is essential for the perpetuation of the species.

A cell divides when it has grown to a certain maximum size which disturb the karyoplasmic index (KI)/Nucleoplasmic ratio (NP)/Kernplasm connection. Two processes take place during cell reproduction.

Phases of cell cycle

(a) Cell growth : (Period of synthesis and duplication of various components of cell).

(b) Cell division : (Mature cell divides into two cells).

(3) Cell cycle : Howard and Pelc (1953) first time described it. The sequence of events which occur during cell growth and cell division are collectively called cell cycle. Cell cycle completes in two steps:

(i) Interphase

(ii) M-phase/Dividing phase

G1 Phase/Post Mitotic/Pre-DNA Synthetic Phase/Gap Ist

In which following events take place:G1 phase of interphase

(a) Intensive cellular synthesis.

(b) Synthesis of rRNA, mRNA ribosomes and proteins.

(c) Metabolic rate is high.

(d) Cells become differentiated.

(e) Synthesis of enzymes and ATP storage.

(f) Cell size increases.

(g) Decision for a division in a cell occurs.

(h) Substances of G stimulates the onset of next S – phase.

(i) Synthesis of NHC protein, carbohydrates, proteins, lipids.

(j) Longest and most variable phase.

(k) Synthesis of enzyme, amino acids, nucleotides etc. but there is no change in DNA amount.

S-phase/Synthetic Phase

(a) DNA replicates and its amount becomes double (2C - 4C).

(b) Synthesis of histone proteins.

(c) Euchromatin replicates earlier than heterochromatin.

(d) Synthesis of NHC (non-histone chromosomal proteins).

(e) Each chromosome has 2 chromatids.

G2-Phase/Pre Mitotic/Post Synthetic Phase/Gap-IInd

(a) Intensive cellular synthesis.

(b) Increase in energy store.

(c) Mitotic spindle protein (tubulin) synthesis begins.

(d) Chromosome condensation factor appears.

(e) Synthesis of 3 types of RNA and NHC proteins.

(f) Synthesis of ATP molecule and storage.

(g) Duplication of mitochondria, plastids and other cellular macromolecular complements.

(h) Damaged DNA repair occur.M phase of cell cycle

M-Phase/Dividing Phase/Mitotic Phase

(a) M –phase is the final phase of cell cycle.

(b) It represents the phase of actual division.

(c) Prior to it, the cell components have undergone duplication.

(d) M – phase is, therefore, the stage of separation of already duplicated components.

(e) It consists of karyokinesis (division of nucleus) and cytokinesis (division of cell protoplast.).

G0 – Phase (Quiescent Stage)

(a) It is the stage of inactivation of cell cycle due to non-availability of mitogens and energy rich compounds.

(b) The cells remain metabolically active.

(c) They do not grow or differentiate.

(d) The cells function as reserve cells which can join cell any time.

Differentiation Phase

(a) Most of the cells leave the G1-phase midway.

(b) They grow in size, assume particular shape and come to have a particular function. The phenomenon is called cell differentiation.

Duration of Cell Cycle

It depends on the type of cell and external factors such as temperature, food and oxygen. Time period for G1, S, G2 and M-phase is species specific under specific environmental conditions. e.g. 20 minutes for bacterial cell, 8-10 hours for epithelial cell, and onion root tip cells may take 20 hours.

Regulation of Cell Cycle

Stage of regulation of cell cycle is G1 phase during which a cell may follow one of the three options.

(i) It may start a new cycle, enter the S-phase and finally divide.

(ii) It may be arrested at a specific point of G1 phase.

(iii) It may stop division and enter G0 quiscent stage. But when conditions change, cell in G0 phase can resume the growth and reenter the G1 phase.