DNA & RNA
• How do genes work?
• What are they made of?
• How do they determine
characteristics of organisms?
• In the middle of the 1900s, questions
like these were on the minds of
Griffith and Transformation
• 1928: British scientist Frederick
Griffith was trying to figure out how
bacteria make people sick.
• Griffith wanted to learn how certain
types of bacteria produce a serious
lung infection known as
What did he do?
• isolated two slightly different strains of
pneumonia bacteria from mice
• Both strains grew very well, but only one of
the strains caused pneumonia.
• disease-causing strain of bacteria = smooth
• harmless strain = colonies with rough edges
• injected mice with the S bacteria
pneumonia and died.
• mice injected with R bacteria totally
• Griffith wondered if the disease-causing
bacteria might produce a poison.
• Heat killed S bacteria into mice Healthy
– The mice survived, suggesting that the cause of
pneumonia was not a chemical poison released
by the disease-causing bacteria.
• Heat killed S bacteria + R bacteria in mice
– He found their lungs filled with the deadly S
– Some factor from the dead bacteria had
“transformed” the harmless bacteria into
• process in which one strain of bacteria is
changed by a gene or genes from another
strain of bacteria
• Griffith hypothesized since the ability to cause
disease was inherited by the transformed
bacteria's offspring, the transforming factor
might be a gene (genetic material)
• Although Griffith made a great discovery,
he did not follow through and figure out
what actually transformed the bacteria.
• Avery came along and did that.
Avery and DNA
• 1944, a group of scientists led by Canadian
biologist Oswald Avery at the Rockefeller
Institute in New York decided to repeat
• to determine which molecule was the genetic
material responsible for transformation.
• Oswald Avery
• Made an extract from the heat killed bacteria
• Used enzymes that destroy the lipids,
carbohydrates, proteins, and RNA.
– Transformation still occurred.
• Used DNA destroying enzyme.
– Transformation did NOT occur.
DNA must be the genetic material.
Avery and other scientists discovered that the
nucleic acid DNA stores and transmits the
genetic information from one generation of an
organism to the next. 14
• Scientists are skeptical.
• Scientists don’t always believe things
with proof from only one experiment…
• So other scientists set out to prove
what the genetic/hereditary
information of an organism is.
Hershey & Chase
• Alfred Hershey and Martha
• 1952, American scientists
• Studied viruses (non living
Martha Chase and Alfred Hershey
The Hershey-Chase Experiment
• Used bacteriophages (virus that infects
• composed of a DNA or RNA core and a
• When a bacteriophage attacks a
bacterium, it injects its genetic
information into the bacterium.
• Those genes take over the cell, producing
many new viruses.
Nucleic Acid or protein??
• They wanted to find out which part of the
phage (DNA or protein), produced new
• Grew bacteriophages with radioactive
– phosphorus-32 (32P) only in DNA
– sulfur-35 (35S) only in Protein
• mixed the marked viruses with bacteria
• waited a few minutes for the viruses to
inject their genetic material.
• separated the viruses from the bacteria
and tested the bacteria for radioactivity.
• Nearly all the radioactivity in the bacteria
was from phosphorus (32P), the marker
found in DNA. 25
• Hershey and Chase
concluded that the genetic
material of the
bacteriophage was DNA,
• DNA is a long molecule made up of units
– Nucleotides are made up of three basic
• 5-carbon sugar (deoxyribose)
• a phosphate group
• a nitrogenous base
• 4 kind of nitrogen bases found in DNA
• adenine (A), guanine (G) = double ringed (purines)
• cytosine (C), thymine (T) = single ringed (pyrimidines)
• backbone of a DNA chain = sugar and
phosphate groups of each nucleotide.
• The nitrogenous bases stick out sideways
from the chain.
• The nucleotides can be joined together in
– any sequence of bases is possible.
– With 4 bases – millions of different
combinations are possible
• American biochemist, 1940s
• discovered that the percentages of
guanine [G] and cytosine [C] bases are
almost equal in any sample of DNA
– Later found that [A] = [T]
• Scientists had NO idea what this was…
Franklin & Wilkins
• British scientists, 1952
• used a technique called X-ray diffraction to get
information about the structure of the DNA
• X-shaped pattern shows that the strands in
DNA are twisted around each other like the coils
of a spring
– a shape known as a helix
– the X suggests that there are two strands in the
– Other clues suggest that the nitrogenous bases are
near the center of the molecule
Watson & Crick
• Francis Crick, a British physicist &
James Watson, an American biologist
• trying to understand the structure of DNA
– by building three-dimensional models of the
• 1953, they are shown a picture of Franklin’s
x-ray and immediately knew the structure
• Watson and Crick's model of DNA was a
double helix, in which two strands were
wound around each other.
• Twisted ladder or spiral staircase
• realized that the double helix accounted for
many of the features in Franklin's X-ray
• did not explain what forces held the two
• discovered that hydrogen bonds could form
between certain nitrogenous bases and
provide just enough force to hold the two
strands together 42
• hydrogen bonds can form only between
certain base pairs
• adenine (A) with thymine (T)
• guanine (G) with cytosine (C)