• These processes are:
- Replication of DNA, along with the synthesis of basic
nuclear proteins such as the histones. - In animal cells, duplication of a centriole takes place
by the outgrowth of daughter centrioles from the parent
centrioles, which are at right angle to each other. - Synthesis of energy rich compounds, which provide
energy for mitosis, and synthesis of rNa and proteins at
the end of interphase.
• Interphase lasts more than 90% of the duration of the
cell cycle. However, the non-dividing state of the mature cell
lasts throughout the life of the cell. Human nerve cells do
not divide after birth. Therefore, interphase period of human
nerve cells, lasts throughout the life of a person. Interphase
of a dividing cell has three stages– G 1 , S and G 2.
G 1 -phase
• G 1 phase or post-mitotic phase or pre-DNA synthesis
phase or first growth period or first gap period
corresponds to the interval between M-phase and initiation
of DNa replication.
• During G 1 phase, the cell is metabolically active and
continuously grows to its maximum size but does not
replicate its DNa. In this phase synthesis of new proteins
and rNa takes place.
• Nucleotides, amino acids and energy rich compounds (e.g.,
aTP) are formed.
• It is marked by synthesis of various enzymes that are required
in S phase, mainly those needed for DNa synthesis.
• Duration of G 1 is highly variable, even among different cells
of the same species.
• Duration of G 1 phase is longer in cells dividing infrequently
and shorter in cells undergoing frequent divisions.
• In G 1 phase a cell has three options: to continue cell cycle
and enter S phase, stop cell cycle and enter G 0 phase for
undergoing differentiation and get arrested in G 1 phase
whence it may enter G 0 phase or re-enter cell cycle.
• There is a check point called G 1 cyclin or CG 1 where decision
about entry in G 0 or S stage is made. Once the check
point is crossed, cell cycle will go on uninterrupted till it is
completed.
• G 1 can be terminated by various stimuli, but once a cell has
completed G 1 phase and has entered the ‘S’ phase it cannot
be terminated.
• Some cells in the adult animals do not appear to exhibit
division (e.g., heart cells) and many other cells divide only
occassionally. These cells do not divide further, they exit G 1
phase to enter an inactive stage called quiescent stage
(G 0 ).
S-phase
• G 1 -phase is followed by S-phase or DNA synthesis
phase.
• In S-phase, the chromosomes replicate. The DNa content
doubles.
• as a result, duplicated set of genes are formed. When it is
complete each chromosome has two sister chromatids which
remain attached at a common point called centromere.
• Thus, during this phase, the amount of DNa in the cell has
effectively doubled, though the ploidy of the cell remains
the same i.e., if the initial amount of DNa is denoted as 2C
then it increases to 4C. However, there is no increase in the
chromosome number; if the cell had diploid or 2n number
of chromosomes at G 1 , even after S-phase the number of
chromosomes remains the same, i.e., 2n.
• In this phase, the chromosomes prepare themselves for
equitable distribution later on.
• Subunits of kinetochores are synthesised.
• Centrosome also begins to divide in centriole containing
cells to form two centrosomes or centriole pairs.
• S-phase is called invisible phase of cell cycle as replicated
chromosomes are not visible at this stage.G 2 -phase
• G 2 phase or post DNA synthesis phase or pre-mitotic
phase or second gap period or second growth period
follows S-phase.
• In this phase, synthesis of DNa stops, however, formation of
rNas and proteins continues.
• They are required for multiplication of cell organelles, spindle
formation, aster formation and cell growth.
• It prepares the cell to undergo division.
• The second check point called mitotic cycle (CM) lies between
G 2 and M phase and causes transition from G 2 to M phase.M-phase (Mitotic phase)
• M-phase or mitotic phase (or meiotic phase in meiosis)
represents the phase of actual division.
• Prior to it, the cell components have undergone duplication.
M-phase is, therefore, the stage of separation of already
duplicated components.
• It consists of karyokinesis (division of nucleus) and
cytokinesis (division of cell protoplast).
• after M-phase, a cell may re-enter fresh cycle or pass into G 0 -
phase.
• G 0 -phase is the stage when cell cycle is arrested. Therefore,
further divisions stop.
• The cell may then grow in size and become differentiated.G 0 -phase (Quiescent stage)
• It is the stage of inactivation of cell cycle due to non-
availability of mitogens and energy rich compounds.