The document discusses DNA replication and repair, emphasizing the flow of genetic information and the structure of DNA. It explains the semi-conservative model of DNA replication, detailing the roles of enzymes like DNA helicase, primase, and polymerases in synthesizing new DNA strands, as well as the necessity of RNA primers. Additionally, it covers various DNA repair mechanisms to correct replication errors and damage from environmental factors.

1. DNA Replication &
Repair
Abhishek Dahal

2.  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.

3. 3
Nitrogenous Bases
 Double ring PURINES
Adenine (A)
Guanine (G)
 Single ring PYRIMIDINES
Thymine (T)
Cytosine (C)
T or C
A or G

4. 4
Chargaff’s Rule
 Adenine must pair with Thymine
 Guanine must pair with Cytosine
 The bases form weak hydrogen bonds
G C
T A

5. 5
Antiparallel Strands
 One strand of DNA
goes from 5’ to 3’
(sugars)
 The other strand is
opposite in direction
going 3’ to 5’ (sugars)

6. 6
DNA Replication
copyright cmassengale

7. 7
Semi conservative Model of
Replication
 Idea presented by Watson & Crick
 The 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

8. 8
DNA Replication
 Begins at site called as "Origins of Replication"
 Specific Protein Called as dna A binds to this
site causing double strands to separate.
 As the 2 DNA strands open at the origin,
Replication Bubbles form
 Prokaryotes (bacteria) have a single bubble
 Eukaryotic chromosomes have MANY bubbles
Bubbles Bubbles

9. 9
DNA Replication
 Two strands open forming Replication Forks (Y-
shaped region)
 New strands grow at the forks
Replication
Fork
Parental DNA Molecule
3’
5’
3’
5’

10. 10
DNA Replication
 Enzyme DNA Helicase unwinds
and separates the 2 DNA
strands by breaking the weak
hydrogen bonds.
 Single-Strand Binding Proteins
(SSBP) attach and keep the 2
DNA strands separated and
untwisted

11. 11
Replication Requirement
 Before new DNA strands can form,
there must be RNA primers present to
start the addition of new nucleotides
 Primase is the enzyme that synthesizes
the RNA Primer
 DNA polymerase 3 can then add the
new nucleotides and forms new strand.

12. 12
DNA Replication
 DNA polymerase can only add
nucleotides to the 3’ end of the DNA
 This causes the NEW strand to be built
in a 5’ to 3’ direction.
 DNA polymerase also checks for
incoming nucleotides and act as proof
reading.

13. 13
Synthesis of the New DNA
Strands
 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.

14. 3
Polymerase III
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
1
RNA primer replaced by polymerase I
& gap is sealed by ligase

15.  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.

16. 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.

17. 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

18. 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

19. 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

20. 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.

21. 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.

22. THE END
 Thank you……..