The Cell Cycle
We have already discussed how the two main events of cellular reproduction are
the copying of cellular components and the cleavage of the cell. These two
events, copying and cleaving, represent the two larger phases of the cell cycle,
interphase and Mitosis. Mitosis is the part of the cell cycle when
the cell prepares for and completes cell division. During interphase,
appropriate cellular components are copied. Interphase is also a time of
checkpoints to make sure that the cell is ready to proceed into mitosis. Both of
these two phases have further sub-divisions. Since the cell cycle is a "cycle"
it has no distinct beginning or ending. Cells are continually entering and
exiting the various phases of the cycle. Mitosis will be covered in depth in
the next SparkNote; here we will focus on
what goes on during interphase.
G1 phase
G1 is an intermediate phase occupying the time between the end of cell division
in mitosis and the beginning of DNA
replication
during S phase. During this
time, the cell grows in preparation for DNA replication, and certain
intracellular components, such as the
centrosomes
undergo
replication. Before a cell begins DNA replication, it must ensure that it is
biologically ready to take on such a process. G1 is the phase when this
cellular monitoring takes place.
During G1, the cell reviews the cellular environment and the cell size to ensure
that the conditions are appropriate to support DNA replication. Not until the
cell is ready does it leave G1. If all is not ready to undergo DNA replication,
cells can pause during G1 and enter a phase called G0. Depending on a
cell's preparedness to continue in the cell cycle, G0 can last days, weeks, or
even years. When the cell has reached an appropriate size and is in a
supportive environment for DNA replication, it will exit either G1 or G0 and
enter the next phase of interphase called S phase.
S phase
S phase, or synthesis, is the phase of the cell cycle when DNA packaged into
chromosomes is replicated. This
event is an
essential aspect of the cell cycle because replication allows for each cell
created by cell division to have the same genetic make-up. (The specifics of
how this replication takes place is covered in the DNA
replication
SparkNote in the series of
SparkNotes dedicated to Molecular
Biology.) During S
phase a number of events additional to chromosomee replication take place. Cell
growth continues through S phase, as does the rate of synthesis of a number of
proteins and enzymes that are involved in DNA synthesis. Once DNA replication
is complete the cell contains twice its normal number of chromosomes and becomes
ready to enter the phase called G2.
G2 phase
Similar to G1, G2 is an intermediate phase, a time for the cell to ensure that
it is ready to proceed in the cell cycle. Occurring between the end of DNA
replication in S phase and the beginning of cell division in mitosis, G2 can be
thought of as a safety gap during which a cell can check to make sure that the
entirety of its DNA and other intracellular components have been properly
duplicated. In addition to acting as a checkpoint along the cell cycle, G2 also
represents the cell's final chance to grow before it is split into two
independent cells during mitosis.
Cell Cycle Summary
Interphase is made up of three distinct phases: G1, S phase, and
G2. The G1 and G2 phases serve as checkpoints for the cell to make sure that it
is ready to proceed in the cell cycle. If it is not, the cell will use this
time to make proper adjustments that can include cell growth, correction or
completion of DNA synthesis, and duplication of intracellular components. S
phase involves the replication of chromosomes. All three stages of interphase
involve continued cell growth and an increase in the concentration of proteins
found in the cell.
