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8/14/2019 02 DNA Structure and Replication
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Vasudevan 1
DNA Structu reAndRepl icati on
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Vasudevan 2
Purines : Adenine, GuaninePyrimidines : Thymine, Cytosine
Bases- Purines
Pyrimidines
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Vasudevan 3
O
HOH
OH
P O
O
O
O
H
OH
P OO
O
O
H
OH
P
OH
O O
O-H2C
_
O-H2C
_
O-H2C
_5' phosphate end
3' OH end
Adenine
Cystosine
Thymine
OH
3
H
Base + Deoxy ribose = Nucleoside
2
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Vasudevan 4
O
HOH
OH
P O
O
O
O
H
OH
P OO
O
O
H
OH
P
OH
O O
O-H2C
_
O-H2C
_
O-H2C
_5' phosphate end
3' OH end
Adenine
Cystosine
Thymine
OH
3
Base + Deoxy ribose + Phosphoric acid
= Nucleotide
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Vasudevan 5
O
HOH
OH
P O
O
O
O
H
OH
P OO
O
O
H
OH
P
OH
O O
O-H2C
_
O-H2C
_
O-H2C
_5' phosphate end
3' OH end
Adenine
Cystosine
Thymine
OH
3
Phospho
Diester
linkage
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Vasudevan 6
O
HOH
OH
P O
O
O
O
H
OH
P OO
O
O
H
OH
P
OH
O O
O-H2C
_
O-H2C
_
O-H2C
_5' phosphate end
3' OH end
Adenine
Cystosine
Thymine
5-T-C-A-3
Polarity
Phospho
Diester
linkage
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Vasudevan 7
5 end 3 end
Phosphate
bonds
Bases
jutting
inside
Double
Helix
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Vasudevan 8
Base pairing rule
A to T
G to C
Nucl eoti d
Hydrogenbondsbetweenbases
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Vasudevan 9
Watson
1962
Crick
1962
Wilkins
1962
Edwin
Chargaff
1905- 2002
Rosalind
Franklin
1921-1958
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Vasudevan 10
Watson-Crick Model of DNA
1. Right handed Double Helix
2. Base pairing rule, A toT; G to C
by Hydrogen bonding
3. Antiparallel 5 33 5
4. 10 base pairs form one spiral
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Vasudevan 11
DNA wraps twice aroundhistone octamer to form
one nucleosome
DNA strand
Cor e histones as octamer
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Vasudevan 12
1 DNA double helix
2 FormsNucleosomes
3 Forms Supercoil
4 Condensed tochromatin
5 Finally condensed as chromosomes
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Vasudevan 13
1 DNA double helix
2 FormsNucleosomes
3 Forms Supercoil
4 Condensed tochromatin
5 Finally condensed as chromosomes
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Vasudevan 14
1 DNA double helix
2 FormsNucleosomes
3 Forms Supercoil
4 Condensed tochromatin
5 Finally condensed as chromosomes
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Vasudevan 15
1 DNA double helix
2 FormsNucleosomes
3 Forms Supercoil
4 Condensed tochromatin
5 Finally condensed as chromosomes
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Vasudevan 16
1 DNA double helix
2 FormsNucleosomes
3 Forms Supercoil
4 Condensed tochromatin
5 Finally condensed as chromosomes
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Vasudevan 17
Inactiv ation of DNADuringDif ferenti ationAll human cells are derived from a
single cell, the zygote. Therefore,
all cells contain the same geneticinformation.
In a cell, about 90% DNA arepermanently inactive.
Diff erentiat ion .
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Vasudevan 18
Intr ons, Exons, C istr onsOnly about 10% of the human DNAcontains genes; the rest silent
areas.
The segments of the gene codingfor proteins are called exons (expressed regions).
They are interspaced in the DNAwith stretches of silent areas,
called introns (intervening areas).
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Vasudevan 19
The primary transcripts contain
intron sequences;
which are later removed to
produce mature mRNA.
Introns are not translated.
CISTRONS
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Vasudevan 20
REPLICATION
Synthesis of new DNA strandon the basis of template strand
Semiconservativereplication
A new complementary strand issynthesised over the old template
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Vasudevan 21
Parent cell
DNA
First
generation
Second
generation
Meselson (Left)
and Stahl (Right)
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Vasudevan 22
Conservative
replication(theoretical;
but actually
not taking place)
Semi -conservati verepl icati on(actual )
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Vasudevan 23
Conservative
replication(theoretical;
but actually
not taking place)
Semi -conservati verepl icati on(actual )
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Vasudevan 24
Old strands
Parent strands
Partially
unwound
New strands madeBased on
Base Pairing rule
Old New Old New
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Vasudevan 25
Salient features of Replication
1.Each strand serves as a TEMPLATE
over which new COMPLEMENTARY
strand is synthesised
2. Base parining rule, A with T; G to C
3. Polymerisation of the newstrand is taking place from 5 to 3
direction
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Vasudevan 26
5 3
OH
DNAPolymerase (DNAP)
5 in number; alpha is main enzyme
Arthur Kornberg
NP 1959
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Vasudevan 27
5 3
C
OH
DNAPolymerase action
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Vasudevan 28
5 3
OH
T
DNAPolymerase action
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Vasudevan 29
5 3
DNAPolymerase action
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Vasudevan 30
RNA primer is needed for DNA synthesis
primaseRNA primer
Old DNA strand
DNA polymerase
New DNA strandAnother DNAP
Completed new DNA strand
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Vasudevan 31
Old DNA strand
DNA polymerase
New DNA strandAnother DNAP
Completed new DNA strand
RNA primer
RNA primer primase
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Vasudevan 32
Old DNA strand
DNA polymerase
New DNA strandAnother DNAP
Completed new DNA strand
RNA primer
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Vasudevan 33
RNA primer is needed for DNA synthesis
Old DNA strand
DNA polymerase
New DNA strandAnother DNAP
Completed new DNA strand
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Vasudevan 34
DNA uncoils RNA primer
New DNA
Replication bubble (Replication fork)
5
5
3
3
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Vasudevan 35
3
5
3
Di recti onof forkmovem ent
5
3
Leadi ng s trandShort RNA pr imerOkazaki fragment
Poi nt of joini ngaf terremovi ng RNA prmerLaggi ng st rand
3
Old strand
5
Lagging strand and Okazaki pieces
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Vasudevan 36
3
5
3
Di recti onof forkmovem ent
5
3
Leadi ng s trandShort RNA pr imerOkazaki fragment
Poi nt of joini ngaf terremovi ng RNA prmerLaggi ng st rand
3
Old strand
5
Lagging strand and Okazaki pieces
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Vasudevan 37
3
5
3
Di recti onof forkmovem ent
5
3
Leadi ng s trandShort RNA pr imerOkazaki fragment
Poi nt of joini ngaf terremovi ng RNA prmerLaggi ng st rand
3
Old strand
5
3
5
Lagging strand and Okazaki pieces
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Vasudevan 38
3
5
3
Di recti onof forkmovem ent
5
3
Leadi ng s trandShort RNA pr imerOkazaki fragmentOrOkazaki piecesoi nt of joini ngaf terremovi ng RNA prmer
Laggi ng st rand3
Old strand
53
5
Lagging strand and Okazaki pieces
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Vasudevan 39
3
5
3
Di recti onof forkmovem ent
5
3
Leadi ng s trandShort RNA pr imerOkazaki fragment
Poi nt of joini ngaf terremovi ng RNA pri merLaggi ng st rand
3
Old strand
5
5
Lagging strand and Okazaki pieces
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Vasudevan 40
3
5
3
Di recti onof forkmovem ent
5
3
Leadi ng s trandShort RNA pr imerOkazaki fragment
Poi nt of joini ngaf terremovi ng RNA prmerLaggi ng st rand
3
Old strand
5
5
Lagging strand and Okazaki pieces
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Vasudevan 41
Summary of DNA replication
1. Unwinding of parental DNA toform a replication fork.2. RNA primer complementary to
the DNA template is synthesised
by RNA primase.3. DNA synthesis is continuous inleading strand (towards replica- tion
fork) by DNA polymerase.4. DNA synthesis is discontinuous inlagging strand (away from the
fork), as Okazaki fragments.
S f
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Vasudevan 42
5. In both strands, the synthesis isfrom 5' to 3' direction.
6. Then the RNA pieces are removed;the gaps filled by deoxynucleo-
tides and the pieces are ligated by
DNA ligase.
7.Proof reading is done by the DNApolymerase.
8. Finally organised into chromatin.
Summary of DNA replication
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Vasudevan 43
Old template strand
containsmethylated
bases; so wrong
base could beidentified
NUCLEOTIDE EXCISION
REPAIR (NER) MECHANISM
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Vasudevan 44
A portion
around
that area is cut
and removed by
endonuclease
Old template strand
contains methylated
bases; so wrong base
could be identified
NUCLEOTIDE EXCISION
REPAIR (NER) MECHANISM
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Vasudevan 45
A portion around
that area is cut
and removed by
endonuclease
Gap is filled with
correct base
sequences byDNA polymerase
Old template strand
contains methylated
bases; so wrong base
could be identified
NUCLEOTIDE EXCISION
REPAIR (NER) MECHANISM
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Vasudevan 46
Old template strand
contains methylated
bases; so wrong base
could be identifiedA portion around
that area is cut and
removed by
endonuclease
Gap is filled with
correct base
sequences by DNA
polymerase
Finally DNA
ligase
seals the nicks
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Vasudevan 47
Diseases associated with
DNA repair mechanisms
1. Xeroderma Pigm en tosum :Defective NER mechanism;
sensitivity to UV light; skin cancers
2. Ataxi a Telangectasi a :defective ATM gene; sensitivy to UV
light; lymphoreticular neoplasms
3. Fanconi 's Anemi a: Defectin DNA repair; increased occurrence
of cancer DNAP coul d not r epl icate thi s area
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Vasudevan 48
3'(Tel omer e)
5' Tel omerase
RNA templ ateinsidetel omer ase
New tel omere repeat
RNA produced bytel omer ase acts asri merDNA pol ymerasecompl etes
laggi ng str and DNAP coul d not r epl icate thi s area
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Vasudevan 49
3'5' Tel omerase
RNA templ ateinsidetel omer ase
New tel omere repeat
RNA produced bytel omer ase acts asri merDNA pol ymerasecompl etes
laggi ng str and
(Tel omer e)
DNAP coul d not r epl icate thi s area
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Vasudevan 50
3'5' Tel omerase
RNA templ ateinsidetel omer ase
New tel omere repeat
RNA produced bytel omer ase acts asri merDNA pol ymerasecompl eteslaggi ng str and
(Tel omer e)
Inhibitors of DNA replication
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Vasudevan 51
Inhibitors of DNA replication
1. Anti bacterial agentsa)Ciprofloxacin
Inhibits Bacterial DNA gyrase
b)Nalidixic acid do
c)Novobiocin do
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Vasudevan 52
2. An ticancer agents
a)Etoposide Humantopo-isomerase
b)Adriamycin do
c)Doxorubicin do
d)6-mercaptopurine Human
DNA polymerase
e)5-fluoro uracil do
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Vasudevan 53
What enz ymes are requi red f orDN A repl icati on?DNA pol ymeraseTopo iso mer aseDNA ligase
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Vasudevan 54
Repl icat ion is in whi ch di rect ion?Pol ymeri sat ion of the new strandof DN A is f rom 5 to 3 di recti onWhat is semi -conservativenatur e of repl icati on?In the daughter c ell , one strandis de rived from the mother cel l;whi le the other strand is n ewlysynthesi sed
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Vasudevan 55
What is semi -di sconti nuou snatur e of repl icati on?In the l eadi ng str and, therepl ication is co ntinuous;but in the laggin g strand,repl ication is taki ng placein smal l pi eces.
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Vasudevan 56
What is l aggi ng str and?The stran d i n whi ch rep licati onis dis- conti nuous is cal ledLaggi ng st randWhat are O kaz aki fragments?The saml l DN A m ole culesattached to its own primer RN Ain the laggi ng st rand are call edOkazaki piec es.
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V d 57
Name the drug s wh ich inhi bi tDN A repl icati on in prokaryotesCi prof loxaci ne, Novobi oci n
Name some anti cancer drugswhi ch inhibi t DN A re pl icati on i nammal ian cel ls5- fluorouraci l6-mercaptopurineCytosi ne a rabi nosi de