CELL CYCLE
$\displaystyle \small \bullet$ The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle.
$\displaystyle \small \bullet$ Cell division, DNA replication, and cell growth have to take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genome.

PHASES OF CELL CYCLE
$\displaystyle \small \bullet$ A complete cell cycle requires a cell to grow, duplicate its content and split into two daughter cells.
$\displaystyle \small \bullet$ In eukaryotic cells, this happens in two phases, i.e. Interphase and Mitosis (M phase).


Interphase
$\displaystyle \small \bullet$ Interphase is the phase between the two successive M phases. The actual cell division occurs in the M phase.
$\displaystyle \small \bullet$ This phase is called as resting phase but during this period the cells prepare itself for nuclear division by cell growth.
$\displaystyle \small \bullet$ The interphase is divided into three further phases:
$\displaystyle G_{1}$ phase (Gap 1)
 $\displaystyle \small \circ$ It corresponds to the interval between mitosis and initiation of DNA replication.
 $\displaystyle \small \circ$ The cell is metabolically active and continuously grows but does not replicate its DNA.
 $\displaystyle \small \circ$ Mitochondria, chloroplast, lysosomes, Golgi complex, vacuoles and vesicles are produced
 $\displaystyle \small \circ$ Structural and functional proteins are formed
 $\displaystyle \small \circ$ Nucleus produces all types of RNA, Ribosomes are synthesized.
 $\displaystyle \small \circ$ Metabolic rate of the cell becomes very high
S phase (Synthesis)
 $\displaystyle \small \circ$ It is the period of DNA synthesis or replication.
 $\displaystyle \small \circ$ There is no increase in the chromosome number. If the cell has diploid or 2n number of chromosomes at G1, after S phase also the number of chromosomes remain 2n.
 $\displaystyle \small \circ$ Replication of DNA takes place.
 $\displaystyle \small \circ$ Each chromosome is in the form of two chromatids.
 $\displaystyle \small \circ$ Protein molecules histones are synthesized, which cover the DNA.
$\displaystyle G_{2}$ phase (Gap 2)
 $\displaystyle \small \circ$ Proteins are synthesized in preparation for M phase.
 $\displaystyle \small \circ$ Energy store increases.
 $\displaystyle \small \circ$ Mitotic spindle begins to form, increased metabolic rate and cell growth continues.

M phase (Mitosis phase)
$\displaystyle \small \bullet$ Starts with nuclear division, corresponding to the division of nucleus called karyokinesis and the division of cytoplasm called cytokinesis.

$\displaystyle G_{0}$ or Quiescent phase
$\displaystyle \small \bullet$ Some cells in adult animals do not exhibit division(ex: heart, nerve cell etc) and some other cells divide occasionally when needed to replace worn out or injured cells.
$\displaystyle \small \bullet$ The cells that do not divide further and enter into an inactive phase called G0 or quiescent phase
$\displaystyle \small \bullet$ Cells in this stage remain metabolically active but do not divide.