Text of Dna replication and enzymes involved in dna replication
1. DNA REPLICATIONDNA REPLICATION Guided By- Dr.Cletus DSouza
Dept. Of Biochemistry Presented By- Narayan Prahlad V VIth Semester
Dept.of Molecular Biology
2. ContentsContents Introduction Chemistry of DNA Synthesis
Extension of 3-OH of Primer Enzymes involved in replication
Mechanism of Replication Proof Reading Conclusion Reference
Acknowledgement
3. INTRODUCTIONINTRODUCTION
4. Chemistry of DNA SynthesisChemistry of DNA Synthesis
Requires 4 Deoxynucleoside Triphosphates (dGTP,dCTP,dATP,dTTP).
Have three Phosphate groups-attached via 5-OH & 2 deoxyribose.
Second essential substrate- Primer:Template Junction Primer- Should
have exposed 3-OH.
5. Extension of 3-OH of PrimerExtension of 3-OH of Primer New
chain grows-extension of 3 end of primer-Phosphodiester bond is
formed by SN2 mechanism.
6. EnzymesEnzymes DNA Polymerase- Mainly catalyses DNA
synthesis. Multimolecular forms of this enzyme- DNA Pol-I, DNA
Pol-II, DNA Pol-III DNA Pol-I: Polymerase Activity. 5 to 3
Exonuclease Activity-Used to remove primer and replace with DNA.
DNA Pol-III: Core replication enzyme. Holoenzyme Has proof reading
capabilities(3 to 5 exonuclease activity).
8. DNA Pol-III The fingers and the thumb are composed-helices.
The incoming dNTP is in red (for the base and the deoxyribose) and
yellow (for the triphosphate moiety). The template strand of the
DNA is shown in dark gray, and the primer strand is shown in light
gray.
9. Continued..Continued.. Helicase- Hexameric assembly of 6
identical subunits. Uses ATP (Hydrolysis)-unwind DNA. Sliding
Clamp- Part of DNA Pol-III (-Subunit). Encircles and slides along
DNA-tethering it to template.
10. Continued..Continued.. Single Strand DNA Binding Protein
(SSB)- Tertamer of identical subunits. Binds to ssDNA-preventing it
from recoiling. DNA Primase- Synthesis of
Oligoribonucleotide(Primer) -extended by DNA Pol-III. Ligase- Seals
the gaps between Okazaki fragments-forming a continuous
strand.
11. Mechanism of ReplicationMechanism of Replication STEP-1 -
UnwindingUnwinding of DNA by Helicaseof DNA by Helicase
12. Step-2- Binding of SSBs to ssDNABinding of SSBs to
ssDNA
13. Step-3 Binding of TopoisomeraseBinding of
Topoisomerase
14. Step-4 Action of DNA Pol-IIIAction of DNA Pol-III
15. Sliding ClampSliding Clamp
16. Thrombone ModelThrombone Model In order to explain
synthesis on lagging strand- Thrombone model was proposed.
17. Synthesis of DNA on Lagging StrandSynthesis of DNA on
Lagging Strand
18. Action of DNA Pol-I & LigaseAction of DNA Pol-I &
Ligase DNA Pol-I replaces RNA fragments with DNA and fills the
gaps. Ligase joins the fragments Phosphodiester bonds and forms a
continuous strand.
19. Proof ReadingProof Reading DNA Pol-3. When a mismatched
nucleotide comes-rate of addition of nucleotide by polymerase slows
down. Exonuclease catalytic site in DNA Pol-3. Error rate- 1 in
1010 bp added, occasionally- 1 in 105 .
20. ConclusionConclusion DNA replication is a very important
phase in cell cycle. Replication is a must so as to ensure the
equal distribution of genetic material to daughter cells. Thus all
enzymes have to coordinate well in order for replication to occur
successfully.
21. ReferencesReferences Molecular Biology of Gene by
Watson;7th edition; Cold Spring Harbour Laboratory Press, Cold
Spring Harbour New York. Biochemistry 5th edition by Lubert
Stryer;W.H Freeman Company. Molecular Cell Biology by Lodish;6th
edition;W.H Freeman and Company;New York.
http://sites.fas.harvard.edu/~biotext/animations/replication1.swf
22. AcknowledgementAcknowledgement I would like thank my guide
Dr.Cletus DSouza for his valuable guidance. I would also like to
thank our course coordinator Dr.N.S Devaki for providing me this
opportunity to present this seminar. Thank You all for your patient
listening.