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Griffith’s Mouse Experiment

Fredrick Griffith

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http://www.wwnorton.com/college/biology/discoverbio3/full/content/index/animations.a

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This experiment demonstrates

the power of genetic material.2

In his first experiment, Griffith wished to

determine the pathogenicity (disease –causing

capability) of his rough strain (Type IIR) of

Streptococcus bacteria 3

To begin, Griffith injected cultures of

his rough strain into mice.

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Two weeks after injection, Griffith found that the

mice survived the introduction of the rough strain

into their systems. 5

How would you interpret these results?

Select from the choices above.

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Smooth

In the second experiment, Griffith wished to determine

the pathogenicity (disease-causing capability) of his

smooth (Type IIS) strain of Streptococcus bacteria. 8

To begin, Griffith injected living cultures of his

smooth strain into mice.

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Two weeks after injection, Griffith found that the mice

were killed as a result of the introduction of the

smooth bacteria into their systems. 10

How would you interpret the results? Select from

the choices above. 11 12

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In his third experiment, Griffith wished to determine

whether the viability of the smooth strain was required for

pathogenicity (disease-causing capability). To do this he first

needed to kill these bacteria by boiling them for a short

period of time. 13

Now that the smooth bacteria were dead, Griffith

could test whether or not they could cause disease in

that state. 14

Griffith injected the heat killed bacteria into the

mouse.

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Two weeks after injection, Griffith found that the

mice survived the introduction of the heat-killed,

smooth bacteria. 16

How would you interpret these results. Select

from the choices above.

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In his final experiment, Griffith wished to determine

whether the factor present in the living smooth bacteria

that causes disease could be transferred to nonpathogenic

material. 19

Griffith first boiled pathogenic, smooth

bacteria, heat killing them .

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In the next step, Griffith needed to mix his heat-killed,

smooth bacteria, with living, rough bacteria.

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After mixing the two strains, Griffith injected a mouse

with the mixture. Neither the rough bacteria nor the

heat-killed smooth bacteria were capable of causing

disease on their own. 22

The mixture was injected into mice, and the mice

were incubated for two weeks.

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After two weeks, Griffith found that the mice

died.

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How would you interpret these results? Select

from the choices above.

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The evidence for Griffith’s hypothesis came from an

investigation of the dead mice.

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When bacteria were recovered from the dead mice,

Griffith cultured them and found living, smooth

bacteria. Griffith reasoned that the only way for this

to have occurred was if living bacteria (rough, in this

case) were instructed to become smooth. Griffith

proposed the following explanation. 28

Griffith proposed that when the smooth

bacterial culture was heat killed…

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…components present inside the smooth bacteria that

caused the bacteria to be pathogenic might have been

released into the media after death of the bacteria.30

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Therefore, when nonpathogenic, rough bacteria

were introduced into the culture.

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…the cellular components from the killed, smooth

bacteria were able to enter the living, rough bacterial

cells. 32

Once inside, the cellular components then transformed

the living rough bacteria cells into living, smooth cells.

Griffith therefore determined the cellular components

transforming factors. At that time, the exact molecule,

(or molecules) that make up the transforming factor

were not known. 33

Bacteriophage with phosphorus-32 in DNA

Phage infectsbacterium

Radioactivity inside bacterium

Bacteriophage with sulfur-35 in protein coat

Phage infectsbacterium

No radioactivity inside bacterium

Hershey-Chase ExperimentSection 12-1

Bacteriophage with phosphorus-32 in DNA

Phage infectsbacterium

Radioactivity inside bacterium

Bacteriophage with sulfur-35 in protein coat

Phage infectsbacterium

No radioactivity inside bacterium

Hershey-Chase ExperimentSection 12-1

Bacteriophage with phosphorus-32 in DNA

Phage infectsbacterium

Radioactivity inside bacterium

Bacteriophage with sulfur-35 in protein coat

Phage infectsbacterium

No radioactivity inside bacterium

Hershey-Chase ExperimentSection 12-1

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Fig. 2.2: Frederick Griffith’s Transformation Experiment - 1928

“transforming principle” demonstrated with Streptococcus

pneumoniae

Griffith hypothesized that the transforming agent was a “IIIS” protein.

But this was only a guess, and Griffith turned out to be wrong.37

SUMMARY

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39Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

Oswald T. Avery’s Transformation Experiment - 1944

Determined that “IIIS” DNA was the genetic material

responsible for Griffith’s results (not RNA).

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Bacteriophage = Virus that attacks

bacteria and replicates by invading a living cell and using the cell’s molecular machinery.

Structure of T2 phage

Bacteriophages are composed of

DNA & protein

Hershey-Chase Bacteriophage Experiment - 1953

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Life cycle of virulent T2 phage:

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Experiment

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1. T2 bacteriophage is composed

of DNA and proteins:

2. Set-up two replicates:

• Label DNA with 32P• Label Protein with 35S

3. Infected E. coli bacteria with two types of labeled T2

4. 32P is discovered within the bacteria and progeny phages,

whereas 35S is not found within the bacteria but released with phage ghosts.

Hershey-Chase Bacteriophage Experiment - 1953

Conclusions about these early experiments:

Griffith 1928 & Avery 1944:

DNA (not RNA) is transforming agent.

Hershey-Chase 1953:

DNA (not protein) is the genetic material.

Gierer & Schramm 1956/Fraenkel-Conrat & Singer 1957:

RNA (not protein) is genetic material of some viruses, but no known prokaryotes or eukaryotes use RNA as their genetic material.

Alfred HersheyNobel Prize in Physiology or Medicine1969

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Structure of DNA

James D. Watson/Francis H. Crick 1953 proposed the Double Helix Model based on two sources of information:

1. Base composition studies of Erwin Chargaff

• indicated double-stranded DNA consists of ~50% purines

(A,G) and ~50% pyrimidines (T, C)

• amount of A = amount of T and amount of G = amount of C

(Chargraff’s rules)

• %GC content varies from organism to organism

Examples: %A %T %G %C %GC

Homo sapiens 31.0 31.5 19.1 18.4 37.5Zea mays 25.6 25.3 24.5 24.6 49.1Drosophila 27.3 27.6 22.5 22.5 45.0

Aythya americana 25.8 25.8 24.2 24.2 48.4

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Structure of DNA

James D. Watson/Francis H. Crick 1953 proposed the Double Helix Model based on two sources of information:

2. X-ray diffraction studies by Rosalind Franklin & Maurice Wilkins

Conclusion-DNA is a helical structure with

distinctive regularities, 0.34 nm & 3.4 nm.

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