Download Online Counseling Resource YCMOU ELearning Drive…

Online Counseling Resource
YCMOU ELearning Drive…
School of Architecture, Science and Technology
Yashwantrao Chavan Maharashtra
Open University, Nashik – 422222, India
OC-SBT044-U01-01
Introduction
Programmes and Courses
 SEP – SBT044 – CP01
 SEP - SBT044_CP101
 SEP - SBT044_CP101
School of Science and Technology, Online Counseling Resource…
Credits
 Academic Inputs by
 Mrs. Rasika Bhore
 M.sc (Microbiology)
 [email protected]
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School of Science and Technology, Online Counseling Resource…
How to Use This Resource

Counselor at each study center should use this presentation to deliver
lecture of 40-60 minutes during Face-To-Face counseling.

Discussion about students difficulties or tutorial with assignments should
follow the lecture for about 40-60 minutes.

Handouts (with 6 slides on each A4 size page) of this presentation should
be provided to each student.

Each student should discuss on the discussion forum all the terms which
could not be understood. This will improve his writing skills and enhance
knowledge level about topics, which shall be immensely useful for end
exam.

Appear several times, for all the Self-Tests, available for this course.

Student can use handouts for last minutes preparation just before end
exam.
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School of Science and Technology, Online Counseling Resource…
Learning Objectives
 After studying this module, you should be able to:
 Discuss various repair mechanisms used by cell
to overcome defects introduced by mutation.
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School of Science and Technology, Online Counseling Resource…
Introduction
 It is a challenging for a cell to carry out all
functions normally & overcome the
mutations.
 For the survival, cell must have:
 The enzymatic machinery that replicates
DNA must be inherently accurate.
 The cell must repair accidental damage to
DNA that would destroy its function.
 Cell possess various repairing mechanisms
which maintain its stability.
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School of Science and Technology, Online Counseling Resource…
Formation Of Dimer Photoproduct
 UV irradiation promotes the formation of covalent bonds
between adjacent pyrimidine residues creating Cyclobutane
containing thymine dimer & mutagenic photoproduct.
 As the COC bonds in this ring are shorter than the normal
0.34-nm base stacking in B-DNA, the DNA is distorted at this
spot and is no longer a proper template for either replication
or transcription.
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School of Science and Technology, Online Counseling Resource…
DNA Repair Enzyme- Photolyase
 Affected DNA can be repaired directly by
some enzymes that simply reverse the
chemical change.
 Photolyase, a flavin and pterin dependent
enzyme, binds at the dimer and uses the
energy of visible light to break the
cyclobutane ring, restoring the pyrimidines
to their original form. This process called
Photoreactivation.
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School of Science and Technology, Online Counseling Resource…
O6-Methyl-guanine-methyltransferase
 Damage by alkalyting chemicals like methylnitrosoguanidine causes methylation of base
guanine at the oxygen of carbon atom 6.
 The product o6-methylguanine often mispairs with
thymine, changing GC to AT during replication.
 O6-methyl-guanine methyltransferase, encoded
by ada gene, recognizes o6-methylguanine in the
DNA duplex & removes the methyl group by
transferring it to an amino acid of the enzyme.
 This enzyme also removes possibly disruptive
methyl groups from phosphates of the DNA
backbone.
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School of Science and Technology, Online Counseling Resource…
Excision Repair System
 Most important repair mechanism.
 Damaged DNA is recognized, removed
either as free bases or as nucleotides,
and the gap is filled by synthesis of new
DNA using the complementary strand as
a template.
 Types of excision repair:
1) Base excision repair
2) Nucleotide excision repair
3) Mismatch repair
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School of Science and Technology, Online Counseling Resource…
Base Excision Repair
 Base excision repair acts on single bases
that have been damaged through oxidation
or other chemical modification during
normal cellular processes.
 The damaged base is removed by DNA
glycosylase, which cleaves the glycosidic
bond, creating an AP (apurinic site or
apyrimidinic site) site, where the sugar is
with no base attached.
 AP endonuclease that cleaves adjacent to
AP site.
 An exonuclease removes the deoxyriboseP and a number of additional residues.
 The single base gap is filled by DNA
polymerase and ligase.
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School of Science and Technology, Online Counseling Resource…
Nucleotide Excision Repair
 Nucleotide excision repair removes a whole
oligonucleotide that contains the damage.
 Thymine-thymine dimer residues are examples of
damage caused by UV radiation.
 In E. coli three genes involved (uvrA, B and C). The
protein UvrA recognizes the damage, recruits UvrB
and UvrC which cleave at 3' and 5' site of damage,
respectively, producing an oligonucleotide of 12 or
13 bases.
 Helicase necessary to remove oligonucleotide,
DNA polymerase fills gap, ligase seals.
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School of Science and Technology, Online Counseling Resource…
Genetic Defect
 In mammalian cells, nucleotide excision repair is
the main pathway for removal of carcinogenic
lesions caused by sunlight or other mutagenic
agents.
 Such lesions are recognized by XPA protein whose
defect leads to human skin disease xeroderma
pigmentosum, an inherited human syndrome
occurs when exposed to UV component of sunlight.
 At sites recognized by XPA, a multiprotein
endonuclease is assembled and the damaged
strand is cleaved and repaired.
© 2007, YCMOU. All Rights Reserved.
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School of Science and Technology, Online Counseling Resource…
Mismatch Repair System
 Recognizes mismatches resulting from replication.
 Scans newly replicated DNA, identifies mismatch, excises the
mismatched base specifically from new strand so error can be
repaired.
 The old strand of DNA be distinguished from the new strand
after replication in E. coli because new strand not yet
methylated at GATC sequences.
 In E. coli mismatch repair is initiated by the protein MutS,
which recognizes mismatch, and forms complex with MutL
and MutH. Then MutH (an endonuclease) cleaves the
unmethylated DNA strand at a GATC sequence.
 Eukaryotes have a similar mismatch repair system, but the
mechanism by which they identify the newly replicated DNA
strand is not known.
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School of Science and Technology, Online Counseling Resource…
Recombinational Repair
 Recombinational repair make use of corresponding
DNA segment from a separate but identical DNA
molecule.
 If both bases of a complementary pair are altered
by mutagen, neither can act as a template for the
other, which is overcome by this repair system.
 The key to recombinational repair is an enzyme
that anneals the sequences on either side of a
lesion.
 In E.coli, the RecA protein carries out this vital
function.
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School of Science and Technology, Online Counseling Resource…
SOS Repair
 SOS genes are induced by severe damage which
can even stop DNA synthesis, rather than merely
changing the pairing properties of bases.
 Example of an SOS- inducing lesion is pyrimidine
dimer, which can not base pair at all.
 When a replication fork meets a dimer, it stops &
reinitiates some distance away, leaving a gap in the
DNA to which the recombination RecA protein
binds, induces SOS genes.
 Some SOS genes are uvrA, uvrB, uvrC, & RecA
itself all are induced after damage of DNA.
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School of Science and Technology, Online Counseling Resource…
Error Prone Repair
 Many times cell can continue DNA synthesis despite
the apparently complete lack of template
information at the site of a lesion.
 A site which a apurinic or apyrimidinic i.e. lack the
base is also repaired but usually makes a mutation
in doing so, because there is no information to
choose correct base to repair DNA molecule.
 This is called ‘Error prone repair’.
 E.coli genes umuC & umuD involves is error prone
repair.
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School of Science and Technology, Online Counseling Resource…
What We Learn………….







UV radiations are responsible for formation of cyclobutyl ring of thymine
dimer, which can overcome by the enzyme photolyase.
O6-Methyl-guanine-methyltransferase removes methyl group from DNA
introduced by chemical mutagens.
Base excision repair involves removal of damaged base creates a AP site
which is filled by DNA polymerase & ligase.
uvrA, B and C genes acts in recognizing & removing whole damaged
nucleotide.
In mismatch repair, proteins scans newly replicated DNA, identifies
mismatch, excises the mismatched base specifically from new strand so
error can be repaired.
Recombinational repair make use of corresponding DNA segment from a
separate but identical DNA molecule.
SOS genes induced on severe damage to DNA, also involved in error
prone repair.
School of Science and Technology, Online Counseling Resource…
Critical Thinking Questions
 While proofreading activity, DNA polymerase leaves
about one mistake per 1010-11 replicated base pairs.
If a wrong nucleotide is introduced by the enzyme
then it replicates during lifetime & intolerable
mutation can occur. Which repair system can be
useful to overcome this problem?
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School of Science and Technology, Online Counseling Resource…
Tips For Critical Thinking Questions
 Mismatch excision repair recognizes &
removes wrong nucleotides.
© 2007, YCMOU. All Rights Reserved.
School of Science and Technology, Online Counseling Resource…
Study Tips
 Book
 Book

Title: DNA in detail

Author: J. D. Watson

Title: DNA repair
mechanisms

Author: Altman, Albert J.
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School of Science and Technology, Online Counseling Resource…
Study Tips
1. www.web.virginia.edu
DNA repair
2. Howard-Flanders, P.1981. “ Inducible Repair Of
DNA”.sci.Amer. 245:72-103
© 2007, YCMOU. All Rights Reserved.
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End of the Presentation
Thank You !