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DNA
Its History, Structure and Function
WHO DISCOVERED DNA?
• Johann Friedrich
Miescher
• Isolated DNA from
leukocytes
• Called it “nuclein”
MIESCHER’S WORK
• Late 1800s
• Disproved that the nucleus is mostly protein
• Isolated the components of DNA: mostly carbon,
hydrogen, oxygen, nitrogen, and phosphorus
• No known proteins had that much phosphorus
(3%)
WAS DNA THE MOLECULE OF HEREDITY?
• Frederick Griffith (1928)
• Worked with bacteria
• “Transformed” noninfectious bacteria into
virulent bacteria
• Called the part that
controlled virulence a
“transforming principle”
GRIFFITH’S EXPERIMENT
AVERY AND MACLEOD (1944)
THEIR EXPERIMENT
CONCLUSIONS
• A chemical is responsible for the virulence of
Pneumococcus bacteria
• The chemical was not a protein, a lipid, a
carbohydrate, or RNA
• The chemical was DNA
HERSHEY AND CHASE (1952)
THEIR EXPERIMENT
CONCLUSIONS
• The active component of the bacteriophage that
transmits the infective characteristic is the DNA.
• There is a clear correlation between DNA and
genetic information.
•
Source: Access Excellence
WHAT IS DNA’S STRUCTURE?
• Erwin Chargaff (1952)
• Maurice Wilkins and Rosalind Franklin (1953)
• J.D. Watson and Francis Crick (1953)
CHARGAFF’S RULE
• In nuclear DNA:
• %A=%T
• %C=%G
• Led to idea of
complementary base
pairing
WILKINS, FRANKLIN, AND PHOTO 51
PHOTO 51
WATSON AND CRICK
WHAT IS THE STRUCTURE OF DNA?
• Double helix
• Two “backbones”: alternating deoxyribose and
phosphate groups
• Twisted “ladder”: the “rungs” are the paired
nitrogenous bases
• Antiparallel
• Strands run in opposite directions
WHAT IS THE STRUCTURE OF DNA?
• Complementary base pairing
• A pairs with T, C with G
• Hydrogen bonding between bases
• Two bonds between A & T; three between C & G
• Easily broken, easily reformed
THE DOUBLE HELIX
NUCLEOTIDE STRUCTURE
FIVE NUCLEOTIDES
STEPS IN REPLICATION
• Helicase unwinds the double helix, forming replication forks
• DNA polymerase III binds free nucleotides to the exposed
complementary bases
• Replication occurs in the 5‘ to 3‘ direction (lead strand and lag
strand); bases can only be added to the 3’ end of the growing
DNA molecule
• Other enzymes correct errors, remove primers, seal “nicks” in
the backbone
• The resulting daughter DNA molecules each consist of an
original strand and a new strand
PROKARYOTE ORIGIN OF REPLICATION
EUKARYOTE ORIGIN OF REPLICATION