Creeping Stroke - Venous thrombosis presenting with pc-stroke.pptx
CELL CYCLE,.pptx
1. CELL CYCLE,
APPLICATIONS,
DNA DAMAGE AND
REPAIR
By Dr. Shounak J. Kamat
1st year Resident
DNB Radiation Oncology
HCG Cancer Centre- Borivali-
Mumbai
Guide:- Dr. Trinanjan Basu
2. Overview
Cell Cycle
Regulation of Cell Cycle
Effect of Radiation on Cell Cycle
DNA Damage
DNA Repair Mechanisms
3. Cell Cycle
A cell cycle is a precisely programmed series of
events which enables a cell to duplicate its
contents and divide into 2 daughter cells
It has the following phases:-
1] G1 phase
2] S phase Interphase
3] G2 phase
4] M phase or the Mitosis phase
5] G0 or the resting state of cells that
have withdrawn from active cell cycle
4. Interphase
It is the phase in which cells spend most of their lives preparing for mitosis.
It is divided into 3 stages:-
1] G1 phase – phase of cell growth and protein synthesis
2] S phase – phase where DNA and centrosomes are replicated
3] G2 phase – phase where energy replenishment, mitotic specific protein
synthesis, cytoskeleton dismantling and additional growth take
place.
7. Prophase
In this phase the chromosomes which
were invisible microscopically during
interphase begin to condense and
become visible
Also centrosomes begin to assemble at
the poles of cells
8. Metaphase
In this phase the chromosomes align
along a plane that bisects the cell and
become attached to microtubule fibres
of mitotic spindle
Also the nuclear membrane
disappears at this time
9. Anaphase
During this phase the chromatids are
pulled apart by the mitotic spindle to the
opposite poles of the cell.
10. Telophase and Cytokinesis
The chromatids cluster into two sets ,
they de-condense and a new nuclear
membrane forms around each set of
chromatids
During this time the cytoplasm of the cell
also divides into two thus yielding two
daughter cells
12. Cyclin Dependent Kinases
They are serine/threonine kinases that sequentially regulate progression of cell
through the cycle via phosphorylation.
They do this via 4 mechanisms
Association with cyclins
Assosiation with CDK inhibitors
Addition of phosphate groups to activate CDK activity
Deletion of phosphate groups to inhibit CDK activity
14. Association with Inhibitors
2 Families of inhibitors are involved in regulating cyclin–cdk activity:
- p16 INK 4a family
- p21 Cip/Kip family
INK Protein binds to cdk 4/6 and interferes with its binding to cyclin D
Cip/Kip family of inhibitors interact with both cyclins and their associated cdks
(mainly with cdk2 and cyclin E) and disable kinase activity
ubiquitin-mediated degradation of inhibitors ensures that the inhibitors are
present during a specific period of time during the cell cycle.
15. Regulation by Phosphorylation
This involves both activation and inhibition
Two steps are required for cdks to become active:
1] Dephosphorylation of the inhibitory phosphate groups by cdc25 phosphatases
2] Phosphorylation of a central threonine residue- Thr161 by cdk-activating kinase
(CAK).
16. Cell Cycle Checkpoints
Signaling pathways that sense and
induce a cellular response to DNA
damage.
The components are DNA damage
sensors, signal transducers, or
effectors.
Disruption of checkpoint function
leads to genomic and chromosomal
instability leading to mutations that
can induce carcinogenesis
17.
18.
19. Role of p53 Gene
p53 is a tumor suppressor gene that is critical in the pathway that arrests the G1
checkpoint.
In response to DNA damage, ataxia telengectasia mutated (ATM)
autophosphorylates and releases active monomer which phosphorylates p53 and
activates it.
Activated p53 enhances p21 gene expression which results in sustained inhibition
of G1 cyclin and Cdks.
This in turn inhibits Rb phosphorylation and progression from G1 to S.
Mutation in this gene compromises this checkpoint function and results in
damaged dna replication leading to carcinogenesis.
21. In Chinese hamster cells
Most sensitive cells to Radiation are the ones
in M and G2 [ steep curve, no shoulder]
Most radioresistent cells are in late S phase
[less steep curve but very broad shoulder]
Other phases G1 and Early S are intermediate
between the two extremes.
22. In HeLa Cells
Similar to the hamster cells in most
aspects
Major difference is length of G1 phase in
HeLa cells is appreciably long and at the
beginning of G1 there is a peak of
radioresistance followed by a trough of
radiosensitivity towards end of G1.
23.
24. CELL CYCLE SPECIFIC DRUGS
During M Phase:-
Taxanes and Vinca Alkaloids cause a synergistic action by acting on DNA and causing its damage.
During S Phase:-
Drugs such as Capecitabine, Gemcitabine and 5-fluorouracil inhibit nucleotid formation and DNA replication.
Since this phase is radioresistant, they cause a synergistic action with radiation.
During G2-M phase:-
Drugs such as Topoisomerase Inhibitors arrest the cells in G2M phase which is a radiosensitive phase thus
causing increase in the effect caused by radiation.
25. EFFECT OF OXYGEN ON CELL CYCLE
OXYGEN ENHANCEMENT RATIO (OER)
Ratio of doses administered under hypoxic to aerated conditions needed to
achieve same biologic effect.
OER for Xrays and Gamma rays has a value between 2.5 to 3.5.
With respect to cell cycle the OER in the G2 and M phase and also in the G1
phase is lower than in S phase because of the radiosensitivity of these phases.
Thus maintaining an aerated state helps in increasing the efficacy of the
radiation treatment.
27. Structure of DNA
DNA is made up of 2 strands and
arranged in a helical pattern.
The strands are formed of a Deoxy
ribose sugars and phosphate
molecules.
On those strands are the purine and
pyrimidine molecules which are
bound by Hydrogen bonds.
28. Effect of Radiation on DNA
Direct Effect Indirect Effect
There are 2 effects :-
30. Types of DNA damage
Single strand breaks
Double strand breaks
DNA crosslink formations
Radiation induced chromosome and chromatid aberrations.
31. Single strand breaks
This occurs when one strand of DNA is
damaged or broken.
More common type of defect
Is readily repaired because of availability
of template on intact opposite strand.
Of lesser biologic significance
Approx. 1000 SSBs per cell after 1 -2 Gy
32. Double strand break
This occurs when both the strands of
DNA are damaged.
Less common defect
Has more significance since this takes
time to be repaired and can result in
mutations, carcinogenesis and cell
death
Approx. 40 DSBs after 1-2 Gy
33. Measuring DNA Strand Breaks
Pulsed Field Gel Electrophoresis (PFGE)
Single-cell Gel Electrophoresis (also known as the comet assay).
DNA damage induced nuclear foci assay
34. DNA crosslink formations
This occurs in oxidative stress when O2 free radicals form intermediaries which then
react with DNA nucleotides and form covalent links between the nucleotides.
They can be of the following types
Intrastrand crosslinks- when crosslinking occurs within same strand
Interstand crosslinks- when crosslinking occurs between opposite strands of DNA
DNA Protein crosslink- between DNA and an oxidised protein
35.
36. Radiation Induced Chromosome Aberrations
Occurs when cells are irradiated in the early interphase stage before the chromosome
has been duplicated.
Can be of the following types
1] Dicentric chromosome 2] Ring aberration
37. Radiation Induced Chromatid Aberration
Occurs when the cell is irradiated in the late interphase after the chromosome has
duplicated
Anaphase Bridge
40. Base Excision Repair
Removal of the defective base by a
Glycosylase/DNA lyase
Followed by removal of the sugar
residue by an apurinic endonuclease
1 (APE1)
then replacement with the correct
nucleotide by DNApolymerase
completed by DNA ligase
the complex of replication factor C
(RFC)/proliferating cell nuclear
antigen (PCNA)/DNApolymerase /
performs the repair synthesis,
the overhanging flap structure is
removed by the flap endonuclease 1
(FEN1)
DNA strands are sealed by ligase I
Single Base Defect Multiple Base Defect
41.
42. Nucleotide Excision Repair
Nucleotide excision repair (NER) removes bulky adducts in the DNA such as pyrimidine
dimers.
Subdivided in 2 pathways:-
1] Global genome repair:- to repair DNA that encodes or does not encode for genes.
2] Transcription coupled repair:- to repair DNA strands of actively transcribed genes
Post DNA damage, RNA polymerase blocks access to the site of damage. The TC-NER
prevents this blockade by removing RNA polymerase from the site.
43. The 2 mechanisms differ only in detection
of lesion, remaining pathway is same.
The essential steps in this pathway are:-
(1) damage recognition
(2) DNA incisions that bracket the lesion
usually 24 to 32 nucleotides in length.
(3) removal of the region containing the
adducts
(4) repair synthesis to fill in the gap
region
(5) DNA ligation
44. DNA Double-Strand Break Repair
Homologous Recombination
Repair [HRR]
This requires an undamaged DNA
strand as a participant in repair as a
template
Error free process since repair is
performed by copying information from
undamaged homologoes chromatid.
Primararily occurs in late S/G2 phase
when undamaged sister chromatid is
available to act as template
Non Homologous End Joining
[NHEJ]
This mediates end to end joining
Error prone and accounts for many
premutagenic lesions induced in DNA by
ionizing radiation
Occurs in G1 phase when template
doesn’t exist..
45. NHEJ
NHEJ can be divided into five steps:
(1) end recognition by Ku binding
(2) recruitment of DNA-dependent protein
kinase catalytic subunit (DNA-PKcs)
(3) end processing
(4) fill-in synthesis or end bridging
(5) ligation
46. HRR
In this, two of the protiens
used are encoded by genes
BRCA 1 and 2
Accessory factors such as
Rad54, Rad 54B and Rdh54
help recognize and invade
the homologous region.
After D-loop formation,
DNA polymerase is involved
to elongate the invading
strand.
47. Cross Linking Repair
Steps
Signals for repair is stalling of the DNA replication fork.
The crosslink is removed in a multistep process:-
1] Crosslink is removed from one strand by a NER, resulting in a
strand break and a DNA adduct.
2] The strand break requires HRR for restitution.
3] Finally, the adduct that remains is removed by NER
48. Miss Match Repair
The mismatch repair (MMR) pathway removes
base–base and small insertion mismatches that
occur during replication.
Steps
1] The mismatch must be identified by
sensors that transduce a signal of a
mismatched base repair
2] MMR factors are recruited
3] The newly synthesized strand harboring
the mismatch is identified and the
incorrect/ altered nucleotides are excised
4] Resynthesis and Ligation of the DNA.