It is a short and concise slide about DNA replication and Repair. It is prepared keeping in mind for Undergraduates level but PG also might find it handy.

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……..