This document discusses DNA replication and describes evidence that supports the semi-conservative model of replication proposed by Watson and Crick. It introduces the three possible models of replication - conservative, dispersive, and semi-conservative - and describes an experiment by Meselson and Stahl that provided evidence the semi-conservative model is correct. Their experiment showed that after replication, the original "heavy" DNA strands remained intact while the new "light" strands contained newly synthesized DNA. This supported the semi-conservative model where each new double helix contains one original strand and one new strand.

1. F: Genetic Control
1 Introduction:
1.1 Structure of DNA and RNA
1.2 Base pairing
1.3 DNA is the genetic material
2 DNA Replication:
3 Transcription: DNA to RNA:
4 Translation: mRNA to protein:
4.1 Introduction
4.2 The Genetic Code
4.3 tRNAs
4.4 Ribosomes
4.5 Mechanism of translation
5 Mutations:
6 Conclusions and Overviews:
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Xavier DANIEL, Ph.D.
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2. F: Genetic Control
DNA Replication is semi-conservative
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Xavier DANIEL, Ph.D.
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Goals of this lesson
Watson and Crick’s notice
Three ways to replicate DNA ?
Evidence for a semi-conservative DNA replication
Summary

3. 1
Xavier DANIEL, Ph.D.
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Goals of this lesson
The student can
 Describe the semi-conservative DNA replication
 Describe the evidence for the semi-conservative DNA replication

4. DNA Replication: Reminder !
Cell division requires replication of genetic material
Before (almost) each cell division, DNA is replicated
DNA replication occurs during the S phase
of the cell cycle
Each strand of the double-stranded DNA is used as a template
One double helix….. Two double helices
Each daughter cell owns
- the same genetic material
- the same genetic material as their parent cell
Xavier DANIEL, Ph.D.
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5. F: Genetic Control
DNA Replication is semi-conservative
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Xavier DANIEL, Ph.D.
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Goals of this lesson
Watson and Crick’s notice
Three ways to replicate DNA ?
Evidence for a semi-conservative DNA replication
Summary

6. Crick and Watson (1953)
Nobel Prize Medicine1962
Xavier DANIEL, Ph.D.
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Watson and Crick’s notice
A / T
G / C

7. “It has not escaped our notice that the specific
pairing we have postulated immediately suggests a
possible copying mechanism for the genetic
material”
Crick and Watson (1953)
Nobel Prize Medicine1962
Xavier DANIEL, Ph.D.
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Watson and Crick’s notice
The semi-conservative model
After each replication, each double helix is made of
 50% of old DNA
 50% of new DNA

8. F: Genetic Control
DNA Replication is semi-conservative
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Xavier DANIEL, Ph.D.
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Goals of this lesson
Watson and Crick’s notice
Three ways to replicate DNA ?
Evidence for a semi-conservative DNA replication
Summary

9.  Watson and Crick’s model = semiconservative replication
 Two other models: the conservative model and the dispersive model.
Xavier DANIEL, Ph.D.
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Three ways to replicate DNA ?
Old DNA
NewDNA

10. Conservative ?
Dispersive ?
Semi-conservative ?
Xavier DANIEL, Ph.D.
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Three ways to replicate DNA ?
Old DNA
NewDNA

11. Conservative ?
Dispersive ?
Semi-conservative ?
Xavier DANIEL, Ph.D.
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Three ways to replicate DNA ?
Old DNA
NewDNA

12. Conservative ?
Dispersive ?
Semi-conservative ?
Xavier DANIEL, Ph.D.
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Three ways to replicate DNA ?
Old DNA
NewDNA

13. F: Genetic Control
DNA Replication is semi-conservative
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Xavier DANIEL, Ph.D.
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Goals of this lesson
Watson and Crick’s notice
Three ways to replicate DNA ?
Evidence for a semi-conservative DNA replication
Summary

14. Which of the three models is right ?
Experiments in the late 1950s
Matthew Meselson and Franklin Stahl
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Evidence for a semi-conservative DNA replication
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15. All DNA contains…
15N ONLY
centrifugation
Xavier DANIEL, Ph.D.
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Evidence for a semi-conservative DNA replication
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16. Xavier DANIEL, Ph.D.
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Evidence for a semi-conservative DNA replication
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New DNA is lighter than old DNA
Old DNA 15N
NewDNA 14N

17. Xavier DANIEL, Ph.D.
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Evidence for a semi-conservative DNA replication
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New DNA is lighter than old DNA
Old DNA 15N
NewDNA 14N

18. Xavier DANIEL, Ph.D.
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Evidence for a semi-conservative DNA replication
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New DNA is lighter than old DNA
Old DNA 15N
NewDNA 14N

19. Parent molecule
dsDNA
Parent molecule
ssDNA
Parent molecule
ssDNA
Parent molecule
ssDNA
Parent molecule
ssDNA
New nucleotides
Parent molecule
ssDNA
New strand
ssDNA
Parent molecule
ssDNA
New strand
ssDNA
Denaturation
Addition of
Xavier DANIEL, Ph.D.
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Evidence for a semi-conservative DNA replication
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20. F: Genetic Control
DNA Replication is semi-conservative
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Xavier DANIEL, Ph.D.
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Goals of this lesson
Watson and Crick’s notice
Three ways to replicate DNA ?
Evidence for a semi-conservative DNA replication
Summary

21. Original double helix
New strands
After each DNA Replication,
Each double helix contains
One strand from the parent molecule
One newly made strand
“Semi-conservative”: KEEP 50%
Xavier DANIEL, Ph.D.
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Summary