Question

Explain in brief "Cell cycle" which includes G1 phase, S phase, G2 phase and M phase.

Answer 1

cell cycle is a sequence of events by which a cell duplicates it's genome and synthesis other constituents of cell and eventually divide into 2 daughter cells....

It consists of 2 phases : Interphase
Mitotic phase

Interphase consists of 3 phases :
G1 : Here, no synthesis of DNA takes place. Most of the cell organelles duplicate in this phase...

S : Synthesis of DNa takes place. Amount of DNA gets doubled while no. of chromosomes remains same.

G2 : Mitochondria, Chloroplast, Golgi body duplicate themselves. no synthesis of DNA. Synthesis of Tubulin protein takes place.

M phase is mitotic phase where actual division takes place......

Answer 2

Cell cycle (Given by Howard and Pelc in 1953) is a controlled series of changes which are genetic and occur in a newly formed cell by which it duplicates its own genome, synthesis of other constituents, undergoes growth no divides or splits into two daughter cells. These all events are performed in a coordinated manner and all are under the influence of genetic control.

1) G1 phase: It is also known as "First growth phase" or "Post mitotic gap phase". It is a phase between end of Mitotic phase of previous cell cycle and is a initiation of the DNA replication. G1 is the longest phase of interphase on the cell cycle. The cell is metabolically active and grows in a continuous formation. The Nucleus, However, grows only to a small extent. RNA and all other proteins are synthesized. A large number of nucleotides, amino acids for histone (type of protein in chromosomes) synthesis and energy rich compounds are also formed here. Cell organelles increase in their numbers. The Duration of G1 phase can be variable. It is longer for the cells to divide infrequently. A shorter G1 phase occurs in cells undergoing frequent division. In G1 phase, ususally, the cell has three options.
1) Continue to the formation itself and undergo complete cell cycle by entering S-phase.
2) Stop The cell cycle and enter G0 phase for undergoing differentiation.
3) Get arrested in G1 phase itself hence it may enter G0 phase or re-enter the whole cell cycle.
The deciding factor is the availability of mitogens and storage of energy rich compounds at the deciding point called as "check point". This check point is called as G1 cycling or CG1 (G and 1 in suffix respectively). It causes the transition of G1 phase to the S-phase. Once the check point has been completed the G1 phase is crossed, cell cycle will go on uninterrupted till it has been completed.

2) S-phase: In S-phase (S stands for synthetic) the chromosomes will replicate themselves. For this their DNA molecules functions as a templates and form carbon compound copies. The content of DNA doubles, that is, from 1C to 2C for haploid cells and from 2C to 4C for diploid cells (C stands for carbon components). As a result of this, the duplicate sets of genes are formed. Along with replication of DNAs, new chromatin fibres are formed which, however are going to remain attached in pairs and the number of chromosomes will not increase, that is, 1n to 1n and 2n to 2n (haploid and diploid respectively). As chromatin fibres are elongated chromosomes, each chromosome comes to have two chromatin threads or sister chromatids which still remain attached at a commonly known point called as centromere. S-phase is also called as invisible phase of Mitotic stage since it is in this phase that the chromosomes prepare themselves for equitable distribution later on the cell cycle. Subunits of kinetochores are synthesized. Centrosome also begins to divide in centriole contains the cells which form two centrosomes or centriole pairs.

3) G2-phase: In the G2 phase (also known as second growth phase or pre-Mitotic gap phase) synthesis of DNA completely stops. However, formation of the RNAs and proteins continue to take place. They are required for the multiplication of cell organelles, spindle formation and cell growth. It prepares the cell to undergo their division. The second check point starts here and is called as Mitotic cyclin (CM, M is in suffix) lies between G2 and M-phase or Mitotic phase and or ultimately causes transition from G2-phase to M-phase.

4) Mitotic phase or M-phase: M-phase is the final and last phase of the cell cycle. It represents the phase of some actual divisions. Prior to it, the cell will have components which have undergone duplication. M-phase is, therefore, the stage separation in a compelete and already duplicated components. It consists of karyokinesis (division of the nuclei and nucleus) and cytokinesis (division of the cell protoplast). After the M-phase a cell may re-enter a fresh cell cycle and start again or pass into G0 phase. G0 phase (concept given by the scientist Lajtha in 1963) is the stage when cell cycle is arrested. Therefore, further divisions will stop. The cell may then grow in size and get differentiated subsequently forming a reserve cell.