DNA Replication & Repair.

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
Adeninemust pair with Thymine
Guaninemust 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
Thetwo strands of the parental molecule
separate, and each acts as a templatefor 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 Abinds to this
site causing double strands to separate.
As the 2 DNA strands open at the origin,
Replication Bubblesform
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 Helicaseunwinds
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
Beforenew DNA strands can form,
there must be RNA primerspresent to
start the addition of new nucleotides
Primaseis the enzyme that synthesizes
the RNA Primer
DNA polymerase 3can then add the
new nucleotides and forms new strand.

12. 12
DNA Replication
DNA polymerasecan only add
nucleotides to the 3’ endof the DNA
This causes the NEWstrand 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 strandsynthesized 5’to 3’in the
direction of the replication fork movement.
It is continuous
Requires a single RNA primer
Lagging strandsynthesized 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 1removes 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-glycosylaseenzyme 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……..