Cell cycle
The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed as cell cycle.
Phases of cell cycle
Eukaryotic cell cycle is studies by human cell culture. Human cells divide in 24 hours and yeast cell cycle completes in 90 minutes.
The cell cycle is divided into two basic phases:
1)Interphase
2)M Phase(Mitosis Phase)
1) Interphase:
This phase of cell cycle is known as resting phase, is the time during which cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner.
Interphase is divided into fallowing further phases
a) G1 Phase(Gap 1)
b) S Phase(Synthesis)
c) G2 Phase(Gap 2)
a) G1 Phase(Gap 1):
G1 phase present between mitosis and initiation of DNA replication. During G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA.
b) S Phase(Synthesis Phase):
In S phase of the interphase DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number. Even after S phase the number of chromosomes remains the same(2n). In animal cells, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
c) G2 Phase(Gap 2):
In this phase, proteins are synthesized in preparation for mitosis while cell growth continues.
Some cells in the adult animals do not exhibit division and many other cell divide occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage(G0) of cell cycle. Cells in G0 phase remain metabolically active but no longer divide until it is necessary.
After the interphase the cell enters into the M Phase or Mitosis Phase.
Mitosis(M Phase)
M phase is the phase where actual division of cell or mitosis occurs. The M phase starts with the nuclear division, corresponding to the separation of daughter chromosomes called karyokinesis and usually ends with the division of cytoplasm called cytokinesis.