Nucleic Acids and Protein
Synthesis
Chapter 7
7-1 DNA
• The Genetic Code
▫ Various Scientists contributed to the idea that
DNA carries the genetic code.
▫ The genetic code is the way in which cells store the
information needed to pass onto the next
generation.
Science History
• 1928 – Frederick Griffiths - Transformation
▫ Bacterial causes for disease
Used mice to see affect
Smooth colonies causes diseases
Rough colonies don’t
▫ Used heat to kill the bacteria then injected them
No disease
Mixed live and heat-killed bacteria
1944 – Rockefeller Institute
• Avery, McCarty and Macleod
▫ Used enzymes to destroy lipids, proteins and
carbohydrates
▫ Transformation still occurred
▫ Repeated the experiment with enzyme that
destroys RNA
▫ Transformation still occurred
▫ Repeated with enzyme that destroyed DNA
▫ No transformation
1952 – Hershey and Chase
• Used viruses called bacteriophages that infect
bacteria
▫ Bacteriophages [bacteria eaters]
Structure – DNA core and a protein coat
Attach themselves to the surface of a bacterium then
inject a material into the cell
Material begins replicating itself until cell is full and
pops
How a bacteriophage works
Radioactive isotopes
• Hershey and Chase used two different
radioactive isotopes to “label” each part
▫ Phosphorous – 32 on DNA
▫ Sulfur – 35 on protein coat
• Mixed labeled viruses with bacteria and
discovered: Phosphorous-32 inside the cell
Structure of DNA
• Polymer formed from nucleotides
▫ Nucleotides are made up of three parts
5-carbon sugar called deoxyribose
A phosphate group
Nitrogenous base
▫ Only four bases found in DNA
Adenine and Guanine form purines
Cytosine and Thymine form pyrimidines
1950’s Rosalind Franklin and Wilkins
• Shot X-rays at long strands of DNA
• Showed that DNA fibers are twisted
• Spaced out in long regular intervals
• Not enough to determine the structure but
important clues!
1950’s Watson and Crick
• Tried building 3-D models
• Visited Franklin and observed her X-ray
photograph
• Began twisting their models to try and form
similar pattern to Franklins photograph
Chargaff
• Observed that the number of adenine molecules
were equal to the thymine molecules
• Same was true of cytosine and guanine
• This attraction is known as “base pairing”
Replication
• Process of DNA copying itself before the cell can
divide
• Only adenine attaches to thymine
• Only cytosine attaches to guanine
• DNA “unzips” so the exact same molecule is
produced when a new DNA strand is made
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