Cell Division And Cell Cycle : 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 cell and is essential for the perpetuation of the species.
Discovery : Prevost and Dumas (1824) first to study cell division during the cleavage of zygote of frog.
Nagelli (1846) first to propose that new cells are formed by the division of pre-existing cells.
Rudolf virchow (1859) proposed “omnis cellula e cellula” and “cell lineage theory”.
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.
(a) Cell growth : (Period of synthesis and duplication of various components of cell).
(b) Cell division : (Mature cell divides into two cells).
What is 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:
(ii) M-phase/Dividing phase
(i) Interphase :
It is the period between the end of one cell division to the beginning of next cell division. It is also called resting phase or not dividing phase. But, it is actually highly metabolic active phase, in which cell prepares itself for next cell division. In case of human beings it will take approx 25 hours. Interphase is completed in to three successive stages.
G1 phase/Post mitotic/Pre-DNA synthetic phase/Gap Ist : In which following events take place.
(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.
(a) DNA replicates and its amount becomes double (2C – 4C).
(b) Synthesis of histone protein.
(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.
(ii) M-phase/Dividing phase/Mitotic phase
(a) Nuclear division i.e. karyokinesis occurs in 4 phases – prophase, metaphase, anaphase and telophase. It takes 5-10% (shortest phase) time of whole division.
(b) Cytokinesis : Division of cytoplasm into 2 equal parts. In animal cell, it takes place by cell furrow method and in plant cells by cell plate method.
Duration of cell cycle :
It depends on the type of cell and external factors such as temperature, food and oxygen. Time period for , S, and M-phase is species specific under specific environmental conditions. e.g. 20 minutes for bacterial cell, 8-10 hours for intestional epithelial cell, and onion root tip cells may take 20 hours.
Regulation of cell cycle:
Stage of regulation of cell cycle is 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 phase.
(iii) It may stop division and enter quiscent stage. But when conditions change, cell in phase can resume the growth and reenter the phase.
Types of cell division : It is of three types, Amitosis, Mitosis and Meiosis.
- G0 – phase : The cells, which are not to divide further, do not proceed beyond the G1 phase and start undergoing differentiation into specific type. such cells are said to be in G0
- Generation time : Period between 2 successive generation (range 8 hr – 100 days).
- Mitogens : Chemicals which enhance or stimulate cell division e.g. lymphokinase (in man)
- Cell cycle duration : 20 minutes in bacteria , 20 hrs in root tip of onion, 2-3 hrs in yeast, 24 hrs in man.
- G0 phase : Cell only starts dividing when the period is favorable otherwise, it remain viable for months or years as such in G0
- During the mitosis of He-La cells, the longest period is gap I phase or G1.
- DNA replication occurs in S-phase.
- In a cell cycle the condensation of chromosome with visible centromere occurs during M-phase.
- Sequence in cell cycle is .
- M-phase is of shortest duration of cell cycle.
- In , the damaged DNA is repaired.
- Histone protein and RNA synthesis occurs in S-phase.
- Duplication of chromosome occurs at S- phase.