DNA and Its Role in Heredity
DNA: The Genetic Material
• 1900-1910 – Embryologist & geneticists had associated traits
(genes) with chromosomes
 EB Wilson & Nettie Stevens – sex & chromosome make-up
 TH Morgan – sex linked traits (genes)
DNA: The Genetic Material
• 1941 – George Beadle & Edward Tatum demonstrated that a
single gene corresponded to a single enzyme
DNA: The Genetic Material
• 1920s - Frederick Griffith’s transformation experiment
 Virulent vs avirulent pneumococcal bacterial strains
• 1940-44 - Oswald Avery and colleagues identified the transforming
substance of Griffith’s pneumococcal strain
Oswald Avery's Isolation of the Transforming Substance
DNA: The Genetic Material
• 1952 - Alfred Hershey & Martha Chase confirmed DNA is
genetic material of viruses
 Bacteriophage - a virus infecting bacteria
T2 a DNA phage of E. coli
 A DNA core packed in a protein coat

DNA: The Genetic Material
• Hershey-Chase experiment
determined whether viral protein or
DNA entered the bacterium &
directed the synthesis of further viral
particles
Figure 11.3 The Hershey–Chase Experiment
Questions Remaining About of DNA
• How does DNA cause the synthesis of specific proteins?
• How is DNA replicated between nuclear divisions?
• Structure of DNA ultimately provided insight to the answers
The Chemical Constituents of DNA
• 1859 – Friedrich Meidscher discovered and named nucleic
acids (DNA)
• By 1940s known DNA was a polymer of nucleotides.
• DNA was assumed to be non-varying, repeating sequence of
nucleotides unique to individual species
• 1950 - Erwin Chargaff carefully determined that individual
percentages of A & T as well as G & C are equal and the
A:T / G:C ratio varies among organisms
Nitrogenous Bases
Ribose Sugars
RNA
Ribonucleic
Acid
DNA
Deoxyribonucleic
Acid
DNA Structure Determination
• 1953
• James Watson & Francis Crick
• Rosalind Franklin & R Gosling
• Maurice Wilkins
• Jerry Donohue
• Linus Pauling
Figure 11.4 X-Ray Crystallography Revealed the Basic Helical Structure of the DNA Molecule
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Figure 11.6 (b) DNA Is a Double Helix
Models of A & B DNA
A-DNA
B-DNA
Figure 11.7 Base Pairing in DNA Is Complementary
In 1953 DNA Structure Suggests Function
• Complementary Base Pairing
 Replication mechanisms
• Sequence of nucleotides
 Code corresponding to proteins in some way
Determining the DNA Replication Mechanism
• 1957 - Matthew Meselson and Franklin Stahl demonstrated
DNA replication is semiconservative
Figure 11.9 The Meselson–Stahl Experiment
Theoretical Predictions
Experimental Observation
Determining the DNA Replication Mechanism
• 1958 - Arthur Kornberg purified DNA polymerase & used it to
replicate DNA in vitro
 Polymerization only worked with nicked, ds DNA templates
 Found that DNA polymerase
requires a priming 3’- OH
from which to initiate synthesis
The Mechanisms Overview of DNA Replication
• H-bonds between strands are broken, making each strand
available to base pair with new nucleotide
• Sequence of new strand is directed by the sequence of the
template strand – complementary base pairing
• Nucleotides are attached to the 3’ end of each growing
strand
Figure 11.10 Each New DNA Strand Grows from its 5 End to its 3 End
DNA Synthesis is Bidirectional
Two nascent, labeled strands
at each fork means both
parent strands serve as
templates
Implication of Bidirectional Synthesis
•
RULE: Polymerization can only
happen in 5'3' direction
•
Starting at 1 spot
 only 1 strand can serve as
template
 but both strands do
 therefore, one strand
synthesized continuously
 leading strand
 the other strand made
discontinuously
 lagging stand
Okazaki Fragments
• If model is correct,
should be able to find
lagging strand fragments
• Discovered by Reiji &
Tuneko Okazaki
• 1000-2000 nt long DNA
fragments
• Begin with ~12
nucleotides of RNA
Priming
Figure 11.15 Many Proteins Collaborate at the Replication Fork
Mechanics of DNA Synthesis
The Molecular Mechanisms of DNA Replication
• Enzymology
 DNA polymerase
 Primase
 Helicase
 Topisomerase
• replication complex recognizes an origin of replication on
a chromosome.
The Molecular Mechanisms of DNA Replication
• Replication occurs from many origins simultaneously
• Large chromosomes can have hundreds of origins of
replication
• The region replicated from a single origin is called a replicon
• The complex of enzymes is the replisome
Figure 11.17 The Lagging Strand Story (Part 2)
Figure 11.18 Telomeres and Telomerase
DNA Proofreading and Repair
• Proofreading by DNA polymerase minimizes errors
• Mutation rate of most eukaryotic DNA polymerases ~ 10-8
 1 error every 1x108 bp
• Mutation rate in prokaryotic cells is higher ~10-6 – 10-7
• DNA damage
 UV, free radicals, etc..
 Repair mechanisms frequently excise damaged sequences
and resynthesize DNA to repair damage
Practical Applications of DNA Replication
• The technique of DNA sequencing hinges on the use of
modified nucleosides called dideoxynucleotides (ddNTPs).
• ddNTPs lack both 2’ and 3 hydroxyl groups
• 3’ OH is site of addition of next nucleotide
• Like dNTPs, ddNTPs are picked up by DNA polymerase and
added to a growing DNA chain
• But once added, chain elongation is terminated
Figure 11.21 Sequencing DNA
DNA Sequencing
Primer annealed
DNA Sequencing
Primer extended
Practical Applications of DNA Replication
• The polymerase chain reaction (PCR) technique for making
multiple copies of a DNA sequence.
• PCR cycles through three steps:
 Double-stranded fragments of DNA are heated to denature
them into single strands.
 A short primer is annealed
 DNA polymerase catalyzes the production of new DNA
strands
Figure 11.20 The Polymerase Chain Reaction
Cycle 1
Cycle 2
Cycle 3