The

Cell Cycle
Cell division
is the process in which one
cell
, called the parent
cell
, divides to form two new cells, referred to as daughter cells. How this happens depends on
whether the
cell
is prokaryotic or eukaryotic.
Cell division
is simpler in prokaryotes than eukaryotes because prokaryotic cells themselves are simpler.
Prokaryotic cells have a single circular
chromosome
, no nucleus, and few other organelles. Eukaryotic cells, in contrast, have multiple chromosomes
contained within a nucleus and many other organelles. All of these
cell
parts must be duplicated and then separated when the
cell
divides.
Cell division
is just one of several stages that a
cell
goes through during its lifetime. The
cell cycle
is a repeating series of events that include growth, DNA synthesis, and
cell division
. The
cell cycle
in prokaryotes is quite simple: the
cell
grows, its DNA replicates, and the
cell
divides. This form of division in prokaryotes is called asexual
reproduction
. In eukaryotes, the
cell cycle
is more complicated.

Eukaryotic Cell

Cycle
Figure 7.2.27.2.2 represents the

cell cycle
of a
eukaryotic cell
. As you can see, the
eukaryotic cell
cycle has several phases. The mitotic phase (M) includes both
mitosis
and
cytokinesis
. This is when the nucleus and then the
cytoplasm
divide. The other three phases (G1, S, and G2) are generally grouped together as
interphase
. During
interphase
, the
cell
grows, performs routine life processes, and prepares to divide. These phases are
discussed below.

Interphase

The

Interphase
of the
eukaryotic cell
cycle can be subdivided into the following phases (Figure 7.2.27.2.2).
• Growth Phase 1 (G1): The
cell
spends most of its life in the first gap (sometimes referred to as growth) phase,
G1. During this phase, a
cell
undergoes rapid growth and performs its routine functions. During this phase,
the biosynthetic and metabolic activities of the
cell
occur at a high rate. The synthesis of amino acids and hundreds of thousands or
millions of proteins that are required by the
cell
occurs during this phase. Proteins produced include those needed for
 DNA replication
. If a
cell
is not dividing, the
cell
enters the G0 phase from this phase.
• G0 phase: The G0 phase is a resting phase where the
cell
has left the cycle and has stopped dividing. Non-dividing cells in multicellular
eukaryotic organisms enter G0 from G1. These cells may remain in G0 for long
periods of time, even indefinitely, such as with neurons. Cells that are completely
differentiated may also enter G0. Some cells stop dividing when issues of
sustainability or viability of their daughter cells arise, such as with DNA damage
or degradation, a process called cellular senescence. Cellular senescence
occurs when normal
diploid
cells lose the ability to divide, normally after about 50
cell
divisions.
• Synthesis Phase (S): Dividing cells enter the Synthesis (S) phase from G1. For
two genetically identical daughter cells to be formed, the
cell
’s DNA must be copied through
DNA replication
. When the DNA is replicated, both strands of the
double helix
are used as templates to produce two new complementary strands. These new
strands then
hydrogen bond
to the template strands and two double helices form. During this phase, the
amount of DNA in the
cell
has effectively doubled, though the
cell
remains in a
diploid
state.
 • Growth Phase 2 (G2): The second gap (growth) (G2) phase is a shortened
growth period in which many organelles are reproduced or manufactured. Parts
necessary for
mitosis
and
cell division
are made during G2, including microtubules used in the mitotic spindle.

Figure 7.2.27.2.2:
Eukaryotic Cell
Cycle. The First Gap (G1), Synthesis (S), and Second Gap (G2) phases make up
interphase
(I). The mitotic phase (yellow M) includes
mitosis
(purple M) and
cytokinesis
. During
cytokinesis
two cells result. Some cells do not divide and they enter into G0 phage.

Mitotic Phase

Before a

eukaryotic cell
divides, all the DNA in the
cell
’s multiple chromosomes is replicated. Its organelles are also duplicated. This happens
in the
interphase
. Then, when the
cell
divides (mitotic phase), it occurs in two major steps, called
mitosis
and
cytokinesis
, both of which are described in greater detail in the concept Mitotic Phase:
Mitosis
and
Cytokinesis
.
• The first step in the mitotic phase of a
eukaryotic cell
 is
mitosis
, a multi-phase process in which the nucleus of the
cell
divides. During
mitosis
, the nuclear envelope (membrane) breaks down and later reforms. The
chromosomes are also sorted and separated to ensure that each daughter
cell
receives a complete set of chromosomes.
• The second major step is
cytokinesis
. This step, which occurs in prokaryotic cells as well, is when the
cytoplasm
divides and two daughter cells form.

Table 7.2.27.2.2:
Cell Cycle
Summary

State Name

Quiescent Senescent Resting phase (G0)

Cells in
1st growth phase (G1)
DNA replication
occurs during this phase.
Interphase Synthesis phase (S)

2ndgrowth phase (G2)

Mitosis
Cell division
(M)
 Table 7.2.27.2.2:
Cell Cycle
Summary

State Name