Read this article to learn about Cell Cycle: Definition and Control of Cell Cycle !
Cell Cycle is an orderly sequence of events by which a growing cell duplicates its contents and divides into two.
The period required to complete one cell cycle is called generation time, e.g., 20 minutes for bacterial cell, 90 minutes in yeast, 19 hours in beans, 20 hours for onion root tip cells, 22 hours for human cells growing in culture etc. 
Howard & Pelc (1953) studied the details of cell cycle. The entire cell cycle may be divided into four phases: G1 S, G2 & M. The phases G1, S and C2 are together known as interphase (I-phase) which lasts more than 95% of the generation time. Though the interphase is called resting phase, it is actually an active or energy phase which is preparatory to cell division.
i. G1– phase (= Gap-I or First growth phase or post-mitotic gap phase):
It is the most variable as well as longest phase of cell cycle during which RNA and proteins are synthesized. In a specific point of G1 a cell decides whether to start anew cycle or to withdraw from the cycle. This point is called G1 checkpoint. The cell that leaves the cell cycle to remain in a resting stage is said to be in Go state or quiscent phase (Lajtlia, 1963).
ii. S-phase (= synthetic phase):
During this phase DNA synthesis or replication takes place. As a result the DNA content per cell doubles. In animal cells, centrioles duplicate in the cytoplasm. S-phase is also known as invisible stage of M-phase.
iii. G2-phase (= Gap II or Second Growth Phase or Pre-mitotic phase).
In this phase, cell growth continues due to synthesis of RNA and proteins. However, DNA synthesis stops. Cell organelles like mitochondria and chloroplast divide.
iv. M-Phase (= Mitotic phase).
It is the final phase of cell cycle. It starts with nuclear division (Karyokinesis) and ends with division of cytoplasm (cytokinesis or C-phase). M-phase is of 3 types amitosis, mitosis & meiosis.
Control of Cell Cycle:
The events of cell cycle are genetically controlled and highly conserved through evolution. The control mechanism operates in the same manner in yeast, plants and animals.
Usually the cell cycle is controlled at three main checkpoints viz., G1/S, G2/M and spindle checkpoint (late metaphase). Checkpoints are the stages where a cell cycle may be stopped if the circumstances are not right for cell division.
i. G1/S Checkpoint.
It is the primary checkpoint commonly known as ‘start’ (in yeast), restriction point or R-point (in animals). This checkpoint assesses extracellular growth factors or mitogens and intracellular nutritional state. Starvation, lack of mitogens can halt the cell cycle at this point and the cell enters the G0-phase
ii. G2/M Checkpoint:
It ensures the success of DNA-replication. DNA-dam aging agents can stop the cycle at this point.
iii. Spindle checkpoint:
It operates at the beginning of anaphase. It assesses whether all chromosomes are attached to the spindle or not. Passage through the above checkpoints is controlled by specific protein kinases that take part in phosphorylation and dephosphorylation. For the activity of protein kinases a number of protein stimulators are required which are called cyclins. The protein kinases are thus called Cyclin-dependent Kinases (Cdks) or mitosis promoting factor (MPF).