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DNA Replication & Repair
Abhishek Dahal
DNA is a molecule that carries Genetic information from generation to next.
Also called as Reserve Bank Of Genetic information.
Central Dogma of life: Flow of information from DNA to RNA to Protein synthesis.
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Nitrogenous Nitrogenous BasesBasesDouble ring Double ring PURINESPURINES
Adenine (A)Adenine (A)Guanine (G)Guanine (G)
Single ring Single ring PYRIMIDINESPYRIMIDINES
Thymine (T)Thymine (T)Cytosine (C)Cytosine (C) T or C
A or G
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Chargaff’s RuleChargaff’s Rule
AdenineAdenine must pair with ThymineThymine GuanineGuanine must pair with
CytosineCytosine The bases form weak hydrogen
bonds
G C
T A
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Antiparallel StrandsAntiparallel Strands
One strand of DNA goes from 5’ to 3’ (sugars)
The other strand is opposite in direction going 3’ to 5’ (sugars)
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DNA DNA ReplicationReplication
copyright cmassengale
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Semi conservative Model of Semi conservative Model of ReplicationReplication
Idea presented by Idea presented by Watson & CrickWatson & Crick TheThe two strands of the parental
molecule separate, and each acts as a template for a new complementary strand
New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA
Parental DNA
DNA Template
New DNA
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DNA ReplicationDNA Replication Begins at site called as "Begins at site called as "Origins of Origins of
ReplicationReplication"" Specific Protein Called as dna A binds to
this site causing double strands to separate.
As the 2 DNA strands open at the origin, As the 2 DNA strands open at the origin, Replication Replication BubblesBubbles form form
Prokaryotes (bacteria) have a single bubble Eukaryotic chromosomes have MANY
bubbles Bubbles Bubbles
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DNA ReplicationDNA Replication Two strands open forming Two strands open forming Replication Replication
Forks (Y-shaped region)Forks (Y-shaped region) New strands grow at the forksNew strands grow at the forks
ReplicationReplicationForkFork
Parental DNA MoleculeParental DNA Molecule
3’
5’
3’
5’
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DNA ReplicationDNA Replication
Enzyme Enzyme DNA HelicaseDNA Helicase unwinds and separates the 2 unwinds and separates the 2 DNA strands by breaking the DNA strands by breaking the weak hydrogen bonds.weak hydrogen bonds.
Single-Strand Binding Single-Strand Binding Proteins (Proteins (SSBPSSBP)) attach and keep the 2 DNA strands separated and untwisted
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Replication Replication RequirementRequirement
BeforeBefore new DNA strands can form, there must be RNA primersRNA primers present to start the addition of new nucleotides
PrimasePrimase is the enzyme that synthesizes the RNA Primer
DNA polymerase 3 can then add the new nucleotides and forms new strand.
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DNA ReplicationDNA Replication
DNA polymeraseDNA polymerase can only add can only add nucleotides to the nucleotides to the 3’ end3’ end of the of the DNA DNA
This causes the This causes the NEWNEW strand to be strand to be built in a built in a 5’ to 3’ direction.5’ to 3’ direction.
DNA polymerase DNA polymerase also checks for also checks for incoming nucleotides and act as incoming nucleotides and act as proof reading.proof reading.
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Synthesis of the New DNA Synthesis of the New DNA StrandsStrands
Leading strand synthesized 5’ to 3’ in the direction of the replication fork movement.
It is continuous Requires a single RNA primer
Lagging strand synthesized 5’ to 3’ in the opposite direction.
Discontinuous (i.e., not continuous) Requires many RNA primers , DNA is
synthesized in short fragments.
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Polymerase III
5’ 3
’
Leading strand
base pairs
5’
5’
3’
3’
Supercoiled DNA relaxed by gyrase & unwound by helicase + proteins:
Helicase +
Initiator Proteins
ATP
SSB Proteins
RNA Primer
primase
2Polymerase III
Lagging strand
Okazaki Fragments
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RNA primer replaced by polymerase I& gap is sealed by ligase
DNA polymerase 1 removes the RNA primer The gap left behind is Sealed by Ligase and
new Daughter DNA is formed. Thus process of replication is ended.
DNA repair
The process of replication is extremely accurate but errors occurs sometime and cells posses capacity to repair these errors.
Damaged DNA must be repaired If the damage is passed on to subsequent
generations, then we use the evolutionary term - mutation.
Damage from where?
Consequences of DNA replication errors Chemical agents acting on the DNA UV light imparting energy into DNA molecule Spontaneous changes to the DNA
a) Base-excision repair
Presence of the Uracil ,hypoxanthine and xanthine in DNA is a great example base-excision.
N-glycosylase enzyme replace just the defective base. snip out the defective base 2cut the DNA strand Add fresh nucleotide via DNA Polymerase. Gap is sealed by LIGASE
b)Nucleotide-excision repair
UV light and Ionization radiation causes modification of bases, strand breaks, cross-linkage, etc.
It recognizes more varieties of damage in DNA . Cutting of the defective piece by Exinuclease and its
removal (Degraded). Resynthesis of the cut part by DNA polymerase and
ligase. Defect in this mechanism leads to Xeroderma
pigmentosa
c) Mismatch repair These are normally caused by mismatched bases
i.e. AG and CT. Special enzymes scan the DNA for bulky alterations
in the DNA double helix. GATC endonuclease cuts the strand and the strand
is digested by Exonuclease. These gaps are excised and the DNA repaired by
ligase and polymerase enzyme respectively. Defect in this mechanism causes Lynch syndrome
i.e. patient are of high risk of developing Colon cancer.
d) Double-strand break repair
High energy radiation and free radicals causes DNA breakage and leads to cell death.
Repair mechanism is of 2 type 1) Non-homologous end joining(Yeast) 2) Homologous end joining (mamals) Defect: Cancer and Immunodeficiency
syndrome.
THE END
Thank you……..