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8.1 Identifying DNA as the Genetic Material
SPONGE 1
Open your biology book to page 225, and
write a summary explaining what makes
this mouse glow.
8.1 Identifying DNA as the Genetic Material
• The mouse’s green glow comes from the green fluorescent
protein GFP. Scientists put a gene from a glowing jellyfish
into a virus that was allowed to infect a mouse egg. The
jellyfish gene became part of the mouse’s genes. As a result
the mouse cells produce the same protein.
8.1 Identifying DNA as the Genetic Material
KEY CONCEPT
DNA was identified as the genetic material through a
series of experiments.
8.1 Identifying DNA as the Genetic Material
• Frederick Griffith (1928)
– Microbiologist studying the bacterium that causes
pneumonia
» He studied two types
o S- smooth
o R- rough
8.1 Identifying DNA as the Genetic Material
•Griffith injected the two types of bacteria into mice
•only the S type killed the mice.
8.1 Identifying DNA as the Genetic Material
What would Griffith expect to find if he combined the live
S bacteria with live R bacteria?
8.1 Identifying DNA as the Genetic Material
•When Griffith heat-killed the S bacteria and injected it into the
mice, they were unaffected.
8.1 Identifying DNA as the Genetic Material
What would Griffith expect to find if he combined the dead
S bacteria with live R bacteria?
8.1 Identifying DNA as the Genetic Material
•When Griffith injected mice with a combination of heatkilled S bacteria and R bacteria the mouse died!!!
8.1 Identifying DNA as the Genetic Material
•When he did a blood test on the dead mouse he found live S bacteria
in the dead mice!
Hypothesize with your table about what happened
when the heat-killed S bacteria was combined with
the R bacteria.
8.1 Identifying DNA as the Genetic Material
• A “transforming material” passed from dead S bacteria to
live R bacteria, making them deadly.
8.1 Identifying DNA as the Genetic Material
• Oswald Avery, a biologist, spent 10 years trying to
figure out what Griffith had discovered (1944)
• He combined R bacteria with an extract made from S
bacteria and observed R bacteria turning into S
bacteria.
8.2 Structure of DNA
SPONGE 2
1. What happened when Griffith injected the S
bacteria into the mice?
2. The R bacteria?
3. The heat-killed S bacteria?
4. What did the mixture of dead S bacteria and
live R bacteria do to the mice?
5. Why did this happen? (#4) What was the
transforming material?
8.2 Structure of DNA
1. What happened when Griffith injected the S
bacteria into the mice? They died.
2. The R bacteria? They lived.
3. The heat-killed S bacteria? They lived.
4. What did the mixture of dead S bacteria and live
R bacteria do to the mice? Killed them.
5. Why did this happen? The live-S bacteria was
found in the dead mouse. DNA from the dead S
bacteria was transferred into the harmless R
bacteria, making it deadly.
8.1 Identifying DNA as the Genetic Material
• Wanted to know if it was DNA or protein that
transformed harmless R bacteria into deadly S bacteria
– tests showed DNA was
present when proteins
were not.
– chemical makeup
matched that of DNA.
8.1 Identifying DNA as the Genetic Material
– When enzymes that were known to break down
proteins were added, R bacteria still transformed
into S bacteria.
– Transformation ONLY stopped when an enzyme
was added that destroyed DNA
8.1 Identifying DNA as the Genetic Material
• He thought DNA was the transforming principle.
8.1 Identifying DNA as the Genetic Material
• Hershey and Chase studied viruses that infect bacteria, or
bacteriophages. (1952)
– They confirmed that DNA was the genetic material
8.1 Identifying DNA as the Genetic Material
–They tagged viral
proteins with radioactive
sulfur
• They tagged viral DNA
with radioactive
phosphorus
• Tagged DNA was found inside the bacteria; tagged
proteins were not. This confirmed that DNA was the
genetic material!