Transcript DNA: Genetic Material

DNA: Genetic
Material
Advanced Biology
Learning Targets

I can describe the two experiments
that led to the discovery of DNA as
the genetic material.
Griffith’s Experiment

Fredrick Griffith worked with two
strains of S. pneumoniae (tried to
develop a vaccine against it)
One strain had a capsule and would
appear smooth when grown in a Petri
plate (S)
 One strain did not have a capsule and
appeared rough when grown in a Petri
plate (R)

Griffith’s Experiment

Injected two groups of mice with
different strains
Mice with the S bacteria became sick
and died
 Mice with the R bacteria remained
healthy

Griffith’s Experiment

Prepared a vaccine by raising the
temperature and killed the bacteria so
they could not reproduce


Left capsule intact to see if it was the
capsule that caused the virulence (ability to
kill)
Injected the mice with heat killed S
bacteria


Mice lived
Capsule did not kill the mice
Griffith’s Experiment

Then mice injected with heat killed S
bacteria were given living R bacteria
Mice died
 R bacteria were able to transform into
virulent S bacteria and acquired the
capsules from heat killed S bacteria

Hershey-Chase Experiment

Scientists knew that viruses infect
bacteria (host) and use them to make
more viruses

Didn’t know how the bacterial cell was
reprogrammed
• DNA or protein
Hershey-Chase Experiment

Used bacteriophage T2 (virus that
infects a bacterium) and E. coli


DNA has phosphorus (phosphate group) and
protein has sulfur
Exposed one batch of T2 phage and E. coli
to radioactive sulfur
• Sulfur (tracer) would attach to the T2 phage’s
protein coat

Exposed another batch of T2 and E. coli to
radioactive phosphorus
• Phosphorus (tracer) would attach to the phage’s
DNA
Hershey-Chase Experiment
Both sulfur and phosphorus tagged
phages were allowed to infect two
separate batches of E. coli
 Batches were then put in a centrifuge
(agitator) to separate the phage
(virus) and the bacteria


Virus is lighter and was on the upper
half of the test tube, while E. coli’s
DNA is heavier and was located on
the bottom half of the test tube
Hershey-Chase Experiment

Able to conclude that the DNA was
the substance that was passed along,
not the protein

DNA is the heredity material