cell cyles,stages,regulation

1. Cell cycles and its regulation
Presented by Dr.SIBI P ITTIYAVIRAH,
PROFESSOR,
DIVISION OF PHARMACOLOGY
DEPARTMENT OF PHARMACEUTICAL SCIENCES,CPAS,CHERUVANDOOR,KERALA,INDIA.

2. The cell cycle is the life cycle of a cell.
Definition:-Cell cycle can be defined as the ordered sequence of events
that occur in a cell in the preparation for cell division.
In other words, it is the series of growth and development steps
a cell undergoes between its “birth”—formation by the division
of a mother cell—and reproduction—division to make two new
daughter cells.

3. Stages of the cell cycle
To divide, a cell must complete several important tasks:
it must grow,
copy its genetic material (DNA),
and physically split into two daughter cells.
Cells perform these tasks in an organized, predictable series of steps that make up the cell cycle.

4. In eukaryotic cells, or cells with a nucleus, the stages of the cell cycle are
divided into two major phases: interphase and the mitotic (M) phase.
● During interphase, the cell grows and makes a copy of its DNA.
● During the mitotic (M) phase, the cell separates its DNA into two sets
and divides its cytoplasm, forming two new cells.

5. Schematic of the cell cycle. Outer ring:
I = Interphase, M = Mitosis; inner ring:
M = Mitosis, G1
= Gap 1, G2
= Gap 2,
S = Synthesis; not in ring: G0
= Gap 0/Resting

6. G1 ,S and G2 phases together are known as interphase.
The prefix inter- means between, reflecting that interphase
takes place between one mitotic (M) phase and the next.

7. Gi Phase-first gap phase,
❖ the cell grows physically larger,
❖ copies organelles,
❖ and makes the molecular building blocks it will need in
later steps
In the great majority of cases, cells do indeed grow before
division.
However, in certain situations during development, cells may
intentionally split themselves up into smaller and smaller
pieces over successive rounds of cell division.

8. G1
phase (First growth phase or Post mitotic gap phase)
The first phase within interphase, from the end of the previous M
phase until the beginning of DNA synthesis, is called G1
.
It is also called the growth phase.
During this phase, the biosynthetic activities of the cell, which are
considerably slowed down during M phase, resume at a high rate.
In this phase, the cell increases its supply of proteins, increases the
number of organelles (such as mitochondria, ribosomes), and grows in
size. In G1
phase, a cell has three options.

9. S phase (DNA replication)
The ensuing S phase starts when DNA synthesis commences; when it is
complete, all of the chromosomes have been replicated, i.e., each chromosome
consists of two sister chromatids.
Thus, during this phase, the amount of DNA in the cell has doubled, though the
ploidy and number of chromosomes are unchanged.
Rates of RNA transcription and protein synthesis are very low during this phase.
An exception to this is histone production, most of which occurs during the S
phase.

10. In S phase, the cell synthesizes a complete copy of the DNA in its
nucleus.
It also duplicates a microtubule-organizing structure called the
centrosome.
The centrosomes help separate DNA during M phase.

11. G2 Phase -the cell grows more, makes proteins and organelles,
and begins to reorganize its contents in preparation for mitosis
G2
phase occurs after DNA replication and is a period of protein synthesis and rapid cell growth
to prepare the cell for mitosis.
During this phase microtubules begin to reorganize to form a spindle (preprophase).
Before proceeding to mitotic phase, cells must be checked at the G2
checkpoint for any DNA
damage within the chromosomes.
The G2
checkpoint is mainly regulated by the tumor protein p53. If the DNA is damaged, p53 will
either repair the DNA or trigger the apoptosis of the cell.
If p53 is dysfunctional or mutated, cells with damaged DNA may continue through the cell cycle,
leading to the development of cancer.

14. M phase
During the mitotic (M) phase, the cell divides its copied DNA and
cytoplasm to make two new cells. M phase involves two distinct
division-related processes: mitosis and cytokinesis.

15. Mitotic phase (chromosome separation)
The relatively brief M phase consists of nuclear division (karyokinesis).
It is a relatively short period of the cell cycle.
M phase is complex and highly regulated.
In mitosis, the nuclear DNA of the cell condenses into visible
chromosomes and is pulled apart by the mitotic spindle, a
specialized structure made out of microtubules.

16. The sequence of events is divided into phases, corresponding to the
completion of one set of activities and the start of the next. These
phases are sequentially known as:
● prophase
● prometaphase
● metaphase
● Anaphase
● Telophase

17. Condensation of chromosomes[edit]
DNA that was replicated in interphase is condensed from DNA strands with
lengths reaching 0.7 μm down to 0.2-0.3 μm.[3]
This process employs the
condensin complex.[11]
Condensed chromosomes consist of two sister chromatids
joined at the centromere.[12]
Movement of centrosomes[edit]
During prophase in animal cells, centrosomes move far enough apart to be
resolved using a light microscope.
Microtubule activity in each centrosome is increased due to recruitment of
γ-tubulin. Replicated centrosomes from interphase move apart towards opposite
poles of the cell, powered by centrosome associated motor proteins.
Interdigitated interpolar microtubules from each centrosome interact with each
other, helping to move the centrosomes to opposite poles

18. Formation of the mitotic spindle
Microtubules involved in the interphase scaffolding break down as the replicated centrosomes separate.[3]
The movement of centrosomes to opposite poles is accompanied in animal cells by the organization of
individual radial microtubule arrays (asters) by each centromere.Interpolar microtubules from both
centrosomes interact, joining the sets of microtubules and forming the basic structure of the mitotic spindle.
Plant cells do not have centrosomes and the chromosomes can nucleate microtubule assembly into the
mitotic apparatus.In plant cells, microtubules gather at opposite poles and begin to form the spindle
apparatus at locations called foci.The mitotic spindle is of great importance in the process of mitosis and will
eventually segregate the sister chromatids in metaphase.
Beginning of nucleoli breakdown
The nucleoli begin to break down in prophase, resulting in the discontinuation of ribosome production.[3]
This
indicates a redirection of cellular energy from general cellular metabolism to cellular division The nuclear
envelope stays intact during this process

19. Prophase is the first step of cell division in mitosis. As it occurs after G2 of interphase, DNA has been already replicated when prophase begins

20. Meiotic prophase[edit]
Meiosis involves two rounds of chromosome segregation and thus undergoes prophase twice, resulting in prophase I and prophase II.
[12]
Prophase I is the most
complex phase in all of meiosis because homologous chromosomes must pair and exchange genetic information.
[3]: 98
Prophase II is very similar to mitotic prophase.
[12]
Prophase I[edit]
Prophase I is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. In addition to the events that occur in mitotic prophase, several crucial
events occur within these phases such as pairing of homologous chromosomes and the reciprocal exchange of genetic material between these homologous
chromosomes. Prophase I occurs at different speeds dependent on species and sex. Many species arrest meiosis in diplotene of prophase I until ovulation.
[3]: 98
In
humans, decades can pass as oocytes remain arrested in prophase I only to quickly complete meiosis I prior to ovulation.
[12]
Leptotene[edit]
Main article: Leptotene stage
In the first stage of prophase I, leptotene (from the Greek for "delicate"), chromosomes begin to condense. Each chromosome is in a haploid state and consists of two
sister chromatids; however, the chromatin of the sister chromatids is not yet condensed enough to be resolvable in microscopy.[3]: 98
Homologous regions within
homologous chromosome pairs begin to associate with each other

21. Zygotene[edit]
In the second phase of prophase I, zygotene (from the Greek for "conjugation"), all maternally and paternally derived chromosomes have
found their homologous partner.[3]: 98
The homologous pairs then undergo synapsis, a process by which the synaptonemal complex (a
proteinaceous structure) aligns corresponding regions of genetic information on maternally and paternally derived non-sister chromatids of
homologous chromosome pairs.[3]: 98 [12]
The paired homologous chromosome bound by the synaptonemal complex are referred to as
bivalents or tetrads.[10][3]: 98
Sex (X and Y) chromosomes do not fully synapse because only a small region of the chromosomes are
homologous.[3]: 98
The nucleolus moves from a central to a peripheral position in the nucleus.[14]
Pachytene[edit]
The third phase of prophase I, pachytene (from the Greek for "thick"), begins at the completion of synapsis.[3]: 98
Chromatin has condensed
enough that chromosomes can now be resolved in microscopy.[10]
Structures called recombination nodules form on the synaptonemal
complex of bivalents. These recombination nodules facilitate genetic exchange between the non-sister chromatids of the synaptonemal
complex in an event known as crossing-over or genetic recombination.[3]: 98
Multiple recombination events can occur on each bivalent. In
humans, an average of 2-3 events occur on each chromosome.[13]: 681
Diplotene[edit]
In the fourth phase of prophase I, diplotene (from the Greek for "twofold"), crossing-over is completed.[3]: 99 [10]
Homologous chromosomes
retain a full set of genetic information; however, the homologous chromosomes are now of mixed maternal and paternal descent.[3]: 99
Visible
junctions called chiasmata hold the homologous chromosomes together at locations where recombination occurred as the synaptonemal
complex dissolves.[12][3]: 99
It is at this stage where meiotic arrest occurs in many species.

22. Diakinesis[edit]
In the fifth and final phase of prophase I, diakinesis (from the Greek for "double movement"), full chromatin condensation has occurred and all four sister chromatids can
be seen in bivalents with microscopy. The rest of the phase resemble the early stages of mitotic prometaphase, as the meiotic prophase ends with the spindle apparatus
beginning to form, and the nuclear membrane beginning to break down.
[10][3]: 99
Prophase II[edit]
Prophase II of meiosis is very similar to prophase of mitosis. The most noticeable difference is that prophase II occurs with a haploid number of chromosomes as
opposed to the diploid number in mitotic prophase.[12][10]
In both animal and plant cells chromosomes may de-condense during telophase I requiring them to
re-condense in prophase II.[3]: 100 [10]
If chromosomes do not need to re-condense, prophase II often proceeds very quickly as is seen in the model organism
Arabidopsis.
[10]
Prophase I arrest[edit]
Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of meiosis.[15]
In
humans, as an example, oocytes are formed between three and four months of gestation within the fetus and are therefor present at birth. During this prophase I
arrested stage (dictyate), which may last for decades, four copies of the genome are present in the oocytes. The adaptive significance of prophase I arrest is still not fully
understood. However, it has been proposed that the arrest of ooctyes at the four genome copy stage may provide the informational redundancy needed to repair damage
in the DNA of the germline.
[15]
The repair process used appears to be homologous recombinational repair
[15][16]
Prophase arrested oocytes have a high capability for
efficient repair of DNA damages.
[16]
DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility.
[16]

23. Prometaphase is the phase of mitosis following prophase and preceding metaphase, in eukaryotic somatic cells. In prometaphase, the nuclear
membrane breaks apart into numerous "membrane vesicles", and the chromosomes inside form protein structures called kinetochores.[1]
Kinetochore microtubules emerging from the centrosomes at the poles (ends) of the spindle reach the chromosomes and attach to the
kinetochores,[1]
throwing the chromosomes into agitated motion.[2]
Other spindle microtubules make contact with microtubules coming from the
opposite pole. Forces exerted by protein "motors" associated with spindle microtubules move the chromosomes toward the centre of the cell.
Prometaphase is not always presented as a distinct part of mitosis. In sources that do not use the term, the events described here are instead
assigned to late prophase and early metaphase.

24. Types of microtubules
The microtubules are composed of two types, kinetochore microtubules and
non-kinetochore microtubules.
● Kinetochore microtubules begin searching for kinetochores to attach to.
● A number of non-kinetochore microtubules or polar microtubules find and interact
with corresponding nonkinetochore microtubules from the opposite centrosome to
form the mitotic spindle.

25. Transition from prometaphase to metaphase
The role of prometaphase is completed when all of the kinetochore microtubules have
attached to their kinetochores, upon which metaphase begins.
An unattached kinetochore, and thus a non-aligned chromosome, even when most of
the other chromosomes have lined up, will trigger the spindle checkpoint signal.
This prevents premature progression into anaphase by inhibiting the
anaphase-promoting complex until all kinetochores are attached and all the
chromosomes aligned.
Early events of metaphase can coincide with the later events of prometaphase, as
chromosomes with connected kinetochores will start the events of metaphase
individually before other chromosomes with unconnected kinetochores that are still
lingering in the events of prometaphase.

26. Stages of early mitosis in a vertebrate cell with micrographs of chromatids

27. Metaphase (from the Greek μετά, "adjacent" and φάσις, "stage") is a stage of mitosis in
the eukaryotic cell cycle in which chromosomes are at their second-most condensed and
coiled stage (they are at their most condensed in anaphase).
These chromosomes, carrying genetic information, align in the equator of the cell before
being separated into each of the two daughter cells. Metaphase accounts for
approximately 4% of the cell cycle's duration.
Preceded by events in prometaphase and followed by anaphase, microtubules formed in
prophase have already found and attached themselves to kinetochores in metaphase.

28. The analysis of metaphase chromosomes is one of the main tools of classical
cytogenetics and cancer studies.
Chromosomes are condensed (thickened) and highly coiled in metaphase,
which makes them most suitable for visual analysis.
Metaphase chromosomes make the classical picture of chromosomes
(karyotype). For classical cytogenetic analyses, cells are grown in short term
culture and arrested in metaphase using mitotic inhibitor.
Further they are used for slide preparation and banding (staining) of
chromosomes to be visualised under microscope to study structure and
number of chromosomes (karyotype).
Staining of the slides, often with Giemsa (G banding) or Quinacrine,
produces a pattern of in total up to several hundred bands.

29. Normal metaphase spreads are used in methods like FISH and as a hybridization matrix for
comparative genomic hybridization (CGH) experiments.
Malignant cells from solid tumors or leukemia samples can also be used for cytogenetic analysis to
generate metaphase preparations. Inspection of the stained metaphase chromosomes allows the
determination of numerical and structural changes in the tumor cell genome, for example, losses of
chromosomal segments or translocations, which may lead to chimeric oncogenes, such as bcr-abl in
chronic myelogenous leukemia.

30. Anaphase (from the Greek ἀνά, "up" and φάσις, "stage"), is the stage of mitosis after the process of metaphase, when
replicated chromosomes are split and the newly-copied chromosomes (daughter chromatids) are moved to opposite poles of
the cell. Chromosomes also reach their overall maximum condensation in late anaphase, to help chromosome segregation
and the re-formation of the nucleus.[1]
Anaphase starts when the anaphase promoting complex marks an inhibitory chaperone called securin for destruction by
ubiquinylating it. Securin is a protein which inhibits a protease known as separase. The destruction of securin unleashes
separase which then breaks down cohesin, a protein responsible for holding sister chromatids together.

31. At this point, three subclasses of microtubule unique to mitosis are involved in creating the forces
necessary to separate the chromatids: kinetochore microtubules, interpolar microtubules, and astral
microtubules.
The centromeres are split, and the sister chromatids are pulled toward the poles by kinetochore
microtubules. They take on a V-shape or Y-shape as they are pulled to either pole.
While the chromosomes are drawn to each side of the cell, interpolar microtubules and astral
microtubules generate forces that stretch the cell into an oval.[3]
Once anaphase is complete, the cell moves into telophase

32. Phases
Anaphase is characterized by two distinct motions. The first of these, anaphase A, moves
chromosomes to either pole of a dividing cell (marked by centrosomes, from which mitotic
microtubules are generated and organised). The movement for this is primarily generated by the
action of kinetochores, and a subclass of microtubule called kinetochore microtubules.
The second motion, anaphase B, involves the separation of these poles from each other. The
movement for this is primarily generated by the action of interpolar microtubules and astral
microtubules.
Anaphase A
A combination of different forces have been observed acting on chromatids in anaphase A, but the
primary force is exerted centrally. Microtubules attach to the midpoint of chromosomes (the
centromere) via protein complexes (kinetochores). The attached microtubules depolymerise and
shorten, which together with motor proteins creates movement that pulls chromosomes towards
centrosomes located at each pole of the cell.

33. In this diagram of a duplicated chromosome, (2) identifies the
centromere—the region that joins the two sister chromatids, or each
half of the chromosome. In prophase of mitosis, specialized regions on
centromeres called kinetochores attach chromosomes to spindle
fibers.
Image of a human cell showing microtubules in green, chromosomes (DNA)
in blue, and kinetochores in pink

34. Anaphase B
The second part of anaphase is driven by its own distinct mechanisms.
Force is generated by several actions. Interpolar microtubules begin at each centrosome and join at the
equator of the dividing cell.
They push against one another, causing each centrosome to move further apart. Meanwhile, astral
microtubules begin at each centrosome and join with the cell membrane.
This allows them to pull each centrosome closer to the cell membrane.
Movement created by these microtubules is generated by a combination of microtubule growth or shrinking,
and by motor proteins such as dyneins or kinesins.

35. Stages of late M phase in a vertebrate cell

36. Telophase is the final stage in both meiosis and mitosis in a eukaryotic cell.
During telophase, the effects of prophase and prometaphase (the nucleolus and
nuclear membrane disintegrating) are reversed.
As chromosomes reach the cell poles, a nuclear envelope is re-assembled around
each set of chromatids, the nucleoli reappear, and chromosomes begin to
decondense back into the expanded chromatin that is present during interphase.
The mitotic spindle is disassembled and remaining spindle microtubules are
depolymerized.
Cytokinesis typically begins before late telophase and, when complete, segregates the
two daughter nuclei between a pair of separate daughter cells.
Telophase is primarily driven by the dephosphorylation of mitotic cyclin-dependent
kinase (Cdk) substrates

38. tate Phase Abbreviation Description
Resting Gap 0 G
0
A phase where the cell has left the cycle and has stopped dividing.
Interphase Gap 1 G
1
Cells increase in size in Gap 1. The G
1
checkpoint control mechanism ensures that everything is
ready for DNA synthesis.
Synthesis S DNA replication occurs during this phase.
Gap 2 G2
During the gap between DNA synthesis and mitosis, the cell will continue to grow. The G2
checkpoint control mechanism ensures that everything is ready to enter the M (mitosis) phase
and divide.
Cell division Mitosis M Cell growth stops at this stage and cellular energy is focused on the orderly division into two
daughter cells. A checkpoint in the middle of mitosis (Metaphase Checkpoint) ensures that the
cell is ready to complete cell division.

39. In cytokinesis, the cytoplasm of the cell is split in two, making two
new cells.
Cytokinesis usually begins just as mitosis is ending, with a little
overlap.
Importantly, cytokinesis takes place differently in animal and plant
cells.

40. Some types of cells divide rapidly, and in these cases, the daughter
cells may immediately undergo another round of cell division.
For instance, many cell types in an early embryo divide rapidly,
and so do cells in a tumor.

41. Different cells take different lengths of time to complete the cell
cycle.
A typical human cell might take about 24 hours to divide,
but fast-cycling mammalian cells,
like the ones that line the intestine,
can complete a cycle every 9-10 hours when they're grown in
culture

42. Cell cycle is a series of events that take place when a cell
decides to divide.
These events in the cell cycle need regulation , therefore the
progression of cells through cell cycle is controlled by
checkpoints at different stages
Any defect in the cell, such as a damaged DNA can make the
cell enter the interphase until the damage is corrected.

43. Checkpoints and regulators
Cdks, cyclins, and the APC/C are direct regulators of cell cycle transitions, but they aren’t
always in the driver’s seat. Instead, they respond to cues from inside and outside the cell.
These cues influence activity of the core regulators to determine whether the cell moves
forward in the cell cycle. Positive cues, like growth factors, typically increase activity of
Cdks and cyclins, while negative ones, like DNA damage, typically decrease or block
activity.

44. Cell cycle checkpoints and cell cycle regulators
CELL CYCLE CHECKPOINTS A checkpoint can be defined as a stage in the cell cycle where the cell
examines it’sG1 checkpoint, G2 checkpoint and the M checkpoint .
The cells enters the cell cycle where they divide and double in volume.
Every aspect is checked by the cells internal mechanism making sure that the cell is ready
for division.
Upon entering the cell cycle, the checkpoints make sure that the cell has all the necessary
factors to proceed to the next phase of cell division.
MPF , also known as maturation promoting factor plays a significant role in this. Cyclin and
CDK together make the MPF, which help the cell cross each checkpoint.

46. CYCLINS AND CDKs
To understand cell cycle regulation, these factors have to be understood first. Cyclins and
cdks are evolutionary concerned proteins that work together
Cyclins are group of related proteins that are activated by cyclin dependent kinases, and
they together help the cell cross checkpoints. There are four types of cyclins involved in cell
cycle regulation ,cyclin A, cyclin B, cyclin D and cyclin E. The level of each cyclin fluctuates
according to the phase of the cell cycle. Cyclins regulate cell cycle only when attached to
cdks and to be fully active they must be phosphorylated at specific locations.

47. cdks are important regulators of all cell cycle.
They phosphorylate either S or T amino acids thereby regulating the activity of those
proteins.
Animals have a total of 9 CDKs of which four are really critical to the cell cycle.

49. In order to drive the cell cycle forward, a cyclin must activate or inactivate
many target proteins inside of the cell.
Cyclins drive the events of the cell cycle by partnering with a family of
enzymes called the cyclin-dependent kinases (Cdks).
A lone Cdk is inactive, but the binding of a cyclin activates it, making it a
functional enzyme and allowing it to modify target proteins.

51. CDKs require ATP to perform phosphorylation. They have an ATP binding cleft , to
which the binding of ATP is regulated by 2 mechanisms.
1. The Cdks have a flexible T loop which has threonine (T) residue , which
normally blocks the ATP binding cleft, but not when the threonine is
phosphorylated.
2. Or the cyclins bind Cdks which also expose the ATP binding cleft. Therefore a
fully active is one which is both phosphorylated at the Threonine on the T loop
and is bound to a cyclin.

53. CELL CYCLE REGULATION
Cell cycle is regulated in two ways
1. Positive regulation- which involves cyclins and cyclin
dependent kinases.
2. Negative regulation- which involves Retinoblastoma protein
(Rb), p53 and p21.
p53 is a multifunctional protein which is activated during G1
phase of the cell cycle.

55. when damages are detected during the G1 phase p53 recruits repair enzymes. If not it triggers apoptosis.p53 acts by
triggering p21 which also plays a very important role in cell cycle regulation. The function of p21 is to bind to cyclin
CDK complex, thereby halting the cell cycle and preventing the cell from entering the S phase.p53 and p21 are also
known as tumor suppressor genes.

56. Cell cycle controlled by tumor suppressors,
Transcription genes, proteins, enzyme and signaling molecules.
Gene E2F is responsible for regulating the expression of transcription genes and CDK2,
cyclin E and cyclin F.
mentioned earlier, in case of DNA damage p53 stimulates the production on p21 which
then binds to the cyclin CDK complex and leads to the arrest of cell cycle.
The cells are arrested in this phase until the DNA damage is repaired and the p21 levels
drop.
The tumor suppressor pRb inhibits the expression of E2F gene

57. Cyclin D CDK4 complex and cyclin D CDK 6 complex phosphorylate pRb that leads to the inactivation of Rb protein and E2F
is expressed. Expression of E2F gene in turn causes the expression of transcription genes and formation of cyclin Ecdk2
complex which pushes the cell into the S phase.

58. The most important task performed by the cell s in the S phase
is DNA synthesis.
Cyciln A CDK2 complex is needed for DNA synthesis by the cell
during the S phase causing it’s levels to increase during this
phase of the cell cycle.
Whereas cyclin A CDK1 and Cyclin B cdk1 promote the events
involved in the m phase.

59. Towards the end of mitosis, APC causes the Ubiquitination
and destruction of cyclin A CDK1 and Cyclin B CDK1 which
ultimately leads to the termination of M phase.

60. APC/C
APC/C is an enzyme that works by adding protein tags called ubiquitin(Ub) to their
targets.
Once the target is tagged with ubiquitin, the proteasomes attack and destroy it.
In a similar way, APC/C attaches Ub tag to M cyclin, causing them to be chopped by
proteasomes, allowing the daughter cells to enter G1 phase.

61. The role of APC/C in separation of sister chromatids during anaphase
Image credit
● APC/C first adds the ubiquitin tag to securin ,leading to its destruction be proteasome. Securin normally binds to a protein
separase and inactivates it. However, in the absence of securin, separase gets activated.
● The activated separse then chops cohesin that holds sister chromatids together allowing them to seperate

62. Fluorescence imaging of the cell cycle
Atsushi Miyawaki and coworkers developed the fluorescent
ubiquitination-based cell cycle indicator (FUCCI), which enables
fluorescence imaging of the cell cycle.
Originally, a green fluorescent protein, mAG, was fused to
hGem(1/110) and an orange fluorescent protein (mKO2
) was
fused to hCdt1(30/120).
Note, these fusions are fragments that contain a nuclear
localization signal and ubiquitination sites for degradation, but
are not functional proteins.

63. The green fluorescent protein is made during the S, G2
, or M phase
and degraded during the G0
or G1
phase, while the orange
fluorescent protein is made during the G0
or G1
phase and destroyed
during the S, G2
, or M phase.
A far-red and near-infrared FUCCI was developed using a
cyanobacteria-derived fluorescent protein (smURFP) and a
bacteriophytochrome-derived fluorescent protein

64. Fluorescent proteins visualize the cell cycle progression. IFP2.0-hGem(1/110) fluorescence is
shown in green and highlights the S/G2
/M phases. smURFP-hCdtI(30/120) fluorescence is
shown in red and highlights the G0
/G1
phases

65. A disregulation of the cell cycle components may lead to tumor formation
As mentioned above, when some genes like the cell cycle inhibitors, RB, p53
etc. mutate, they may cause the cell to multiply uncontrollably, forming a tumor.
Although the duration of cell cycle in tumor cells is equal to or longer than that
of normal cell cycle, the proportion of cells that are in active cell division (versus
quiescent cells in G0
phase) in tumors is much higher than that in normal tissue.
Thus there is a net increase in cell number as the number of cells that die by
apoptosis or senescence remains the same.

66. The cells which are actively undergoing cell cycle are targeted in cancer therapy as
the DNA is relatively exposed during cell division and hence susceptible to damage
by drugs or radiation.
This fact is made use of in cancer treatment; by a process known as debulking, a
significant mass of the tumor is removed which pushes a significant number of the
remaining tumor cells from G0
to G1
phase (due to increased availability of nutrients,
oxygen, growth factors etc.).
Radiation or chemotherapy following the debulking procedure kills these cells which
have newly entered the cell cycle