Cell Cycle
is the series of events that take place in a cell
leading to its division and duplication of its DNA
to produce two daughter cells
.
Transmission of genetic information from one cell
generation to the next requires genome replication
during the
S-phase
, and its segregation to the two
new daughter cells during mitosis or
M-phase
.
The cell cycle has four distinct phases:
1- M
mitosis
,
2-
three interphase periods
termed:
• G
1
(the time gap between mitosis and DNA replication),
• S (the period of DNA synthesis),
• G
2
(the gap between DNA duplication and the next
mitosis).
•
During the G
1
phase :
there is active synthesis of RNA
and proteins, including proteins that control the cell cycle,
and the cell volume, reduced to one-half by mitosis, grows
to its previous size.
• The S phase :
is characterized by the synthesis of DNA
and histones and by the beginning of centrosome
duplication.
• G
2
phase:
relatively short, proteins required for mitosis
accumulate..
G1 : tRNA, mRNA,
ribosomes, and
enzymes are
produced
S: DNA replication
G2: Proteins required
for the spindles are
synthesized
M: The nucleus is
replicated
• G0 phase
: cell cycle activities may be temporarily or
permanently
suspended
and the new cells begin to
specialize
and
differentiate
, some differentiated cells, such as those of the
liver,
renew cycling
under certain conditions; others, including
most muscle and nerve cells are
terminally differentiated
.
In a normal cell cycle, S-phase is always followed by M-
phase and M-phase does not occur until S-phase is
complete.
➢ Between the S- and M-phases, there are two preparatory
gaps.
▪ G1 separates M from S, and
▪ G2 is between S and M ( new mitosis).
❖ When the cell undergoes differentiation, it exits from the
G1 phase of the cell cycle to enter into a
quiescent state
referred to as G0.
Cell cycle checkpoints
• a series of control systems enabling
proliferation only in the presence of
stimulatory
signals
such
as
growth
factors.
• The timing and order of cell cycle events
are
monitored
during
cell
cycle
checkpoints
1. G1/S boundary
, also knowing as
restriction
point
, As the cell progresses through G1,
depending on internal and external conditions,
it can either delay G1, enter a quiescent state
(G0), or cross the restriction point to enter S
phase.
The main checkpoints are:
2-
In S-phase
, also known as the
DNA
damage checkpoint
, ensures that the cell
underwent all of the necessary changes
during the S and G2 phases and is ready to
divide.
3. G2/M-phases
: ensures that DNA replication is
complete
4. M-phase
: (
The
mitotic spindle checkpoint
),
Check that all the chromosomes should be
aligned at the mitotic plate and be under bipolar
tension.
❖ The checkpoints also are activated by:
1.
DNA damage
2. Mis-aligned chromosomes at the mitotic spindle.
In this case, the growth arrest caused by checkpoints
allows the cell to
repair the damage
. After damage
repair, progression through the cell cycle resumes. If the
damage cannot be repaired, the cell is
eliminated
through apoptosis.
Fig. 1 The cell cycle checkpoints
DNA replication
Apoptosis
Programmed cell death serves as a major mechanism for the
precise regulation of cell numbers and as a defense
mechanism to remove unwanted and potentially dangerous
cells.
➢ the execution of the death program is often associated
with
characteristic
morphological
and
biochemical
changes, and this form of programmed cell death has
been termed
apoptosis
.
❖ Apoptosis is an important means of eliminating cells
whose survival is blocked by
❖lack of nutrients
,
❖damage caused by free radicals or radiation
,
❖action of tumor suppressor proteins
.
❖ In all examples studied apoptosis occurs very rapidly, in
less time than required for mitosis, and the affected cells
are removed without a trace.
❑ Loss
of
mitochondrial
function:
Mirochondrial
membrane integrity is not maintained, causing the end of
normal activity and release of cytochrome c into the
cytoplasm where it activates proteolytic enzymes called
caspases .
The initial caspases activate a cascade of other
caspases, resulting in protein degradation throughout the
cell.
The main important features of apoptosis are summarized as:
❑ Fragmentation of DNA
: Endonucleases are activated which
cleave DNA between nucleosomes into small fragments.(The
new ends produced in the fragmented DNA allow specific
histochemical
staining
of
apoptotic
cells
using
an
appropriate enzyme that adds labeled nucleotides at these
sites.)
❑ Shrinkage of nuclear and cell volumes
: Small dark-stained
(pyknotic) nuclei can sometimes be identified with the light
microscope
❑ Cell membrane changes
: the cell undergoes dramatic
shape changes, such as "blebbing", as membrane
proteins and cytoskeleton are degraded. Phospholipids
normally found only in the inner layer move to the outer
layer, serving as signals to induce phagocytosis.
❑ Formation and phagocytic removal of these apoptotic
bodies
.
The study of apoptosis under fluorescence microscopy is
classified into five different categories, which are;
• viable non-apoptotic (viable),
• viable apoptotic (early apoptotic),
• non-viable apoptotic (late apoptotic),
• necrotic and
• chromatin free (ghost) cells
as shown in (Fig. 1).
Acridine orange (AO)
and
propidium iodide (PI)
are
nucleic acid specific fluorochromes which emit green
and orange fluorescences, respectively, when they are
bound to DNA. Only AO can cross the plasma
membrane of viable and early apoptotic cells. Late
apoptotic cells and necrotic cells will stain with both AO
and PI.
Figure 1 showed the fluorescence microscopy images of cells.
where V: viable cells; E: early apoptotic cells; L: late apoptotic
cells; AB: apoptotic bodies; D: dead cells
