The document provides a comprehensive overview of DNA replication, detailing the Watson-Crick model, various replication modes (conservative, semi-conservative, dispersive), and the Meselson-Stahl experiment that supports semi-conservative replication. It lists key enzymes involved in the replication process, such as helicase and DNA polymerases, and outlines the steps of replication, including replication fork formation, elongation, and termination. Additionally, it highlights the differences in DNA replication between prokaryotes and eukaryotes.

1. DNA REPLICATION
Munaza
Roll No. CHE2032

2. CONTENTS
 Introduction
 Watson & Crick model
 Modes of Replication
 Meselson- Stahl experiment
 Enzymes involved in replication
 Steps of replication
 Difference in replication between prokaryotes and eukaryotes

3. DNA REPLICATION
Introduction:
 Double stranded DNA copied to two identical DNA molecules
 Occur at the rate of 1000 nucleotide per sec and 50 nucleotides per second in
mammals
 Polymerization occur from 5’ to 3’ direction only
 Replication is semi-conservative
 Vital for cell growth,reproduction and cell repair
 Base pairing rule maintained
 S phase of interphase in cell cycle

4. Watson-Crick model of DNA replication
 James Watson and Francis crick 1953
 DNA is double stranded Helical molecule
 Two sugar-phosphate back bone on outside
 Four types of bases i.e A,T,G,C
 Held together with H-bond A = T , G ≡ C
 Nobel prize in 1962
 Crystallographic evidence of helical structure
 Rosalind Franklin (1920-1958)

6. Modes of replication
 Many hypothesis explain replication
i) Conservative model
ii) Semi-conservative model
iii) Dispersive model

7. Conservative Replication
 Parental double helix remains conserved
 Duplex generate new DNA copy
 Entirely new material molecule
Semi-Conservative Replication
 Double stranded DNA unwinds
 Each acts as template for synthesis of new strand
 Resulting in two daughter DNA molecules
 One original strand and one new strand

8. Dispersive Replication
 Parental DNA is completely dispersed
 New DNA molecule is mixture of parentel and daughter DNA
 Material distributed randomly between two daughter molecules

10. Meselson–Stahl experiment
 Matt Meselson and Franklin Stahl 1958
 Justification of models
 Semi-conservative replication of DNA
 E.Coli bacteria as model system

12. Enzymes Involved in DNA Replication
 Helicase (initiator protein):
Opens up double stranded DNA at replication fork
 Single-strand binding Protein(SSB):
Coat the DNA around replication fork
Prevents rewinding of DNA
 Topoisomerase(Gyrase):
Works at region ahead of replication fork
Prevents super coiling

13. Enzymes Involved in DNA Replication
 Primase:
Synthesis RNA Primers complimentary to DNA strand
 DNA polymerase I:
Removes and replaces RNA primer with DNA

14. Enzymes Involved in DNA Replication
 DNA polymerase III:
Extends RNA primer
Adding on to 3’ end
Makes bulk of new DNA
 DNA Ligase:
Seals the gaps between DNA fragments

16. Steps Of DNA Replication
 Replication Fork formation:
 Recognition of Ori (245 bp long A=T)
 Helicase enzyme unzip the double strand by breaking H-Bonds
 Replication fork and replication bubble formed
 SSB protein coats separated DNA strand to prevent rewinding

18. Steps Of DNA Replication
 Elongation:
 Primase ( kick starter) enzyme attached at first
 DNA polymerase attaches and creates new strand
 5’-3’ polymerization (add nucleotide on 3’ end )
 Polymerase add nucleotide molecules
 Leading strand ( 3’-5’) continuous
 Lagging strand (5’-3’) RNA Primer attached
 DNA polymerase attaches nucleotide and removes RNA primer
 Okazaki fragments formed,discontinous
 +

20. Steps Of DNA Replication
 Termination:
 DNA Ligase joins okazaki fragments
 Forms unified single strand
 expanding continues until no more template left

21. Prokaryotic
 DNA Replication in cytoplasm
 Single origin of replication
 DNA polymerase I,III
 Large okazaki fragment
Eukaryotic
 DNA Replication in nucleus
 Multiple origin
 DNA Polymerase a,B,Y
 Small okazaki fragment