Date: September 2004
We been studying Griffith's experiment at school for more
than a week now. I can't seem to understand exactly what happened no
matter how many times I ask my biology teacher to explain it to me. I was
hoping you could help me by showing me the experiment with the mice/rat
in full details.
Griffith was actually studying an outbreak of pneumonia. There seemed to be
2 different strains of bacteria that were causing the illness. When he
looked at the bacteria on the different agar plates he noticed that one of the
strains had a gooey, glistening look to it, which he named "smooth". This
bacteria actually had a structure called a capsule, which is a covering that
makes it difficult for the immune system to kill it. The other bacteria did not
have this capsule so appeared more dry, so he called this strain "rough".
When he injected mice with the rough strain, they got sick, but were able to
This is because the mouse's white blood cells could attack and kill
the bacteria. So rough did NOT kill the mice. When he did the same with the
smooth bacteria they died. Again, this is because it was difficult for the
mouse's white blood cells to attack the bacteria. So, the smooth bacteria DID
kill the mice. Next, he boiled some of the smooth bacteria, which killed
them. When he injected these into the mice, of course they lived. Then, he
made a mixture of the DEAD smooth and the LIVE rough (separately which had been
shown NOT to kill the mice.) The mice DIED. He was of course confused by
this and took samples from the mice. The bacteria that grew from the
cultures were SMOOTH. So he hypothesized that something from the dead smooth
bacteria was changing the rough bacteria into smooth bacteria. He said that the
rough bacteria had been TRANSFORMED by something that came from the smooth. He
called this the "transforming factor". In the 1940's Avery showed that this
factor was DNA.
We now know that bacteria can salvage genes from their
environment from bacteria that have died. The rough bacteria were taking the
"smooth gene" from dead smooth bacteria and exchanging it for the rough gene.
This obviously makes them better able to survive in the environment. Go to
google.com and select images.
Then type in Griffith, transformation and you
will get plenty of diagrams to choose from.
When Griffith separately injected mice with heat killed smoothe bacteria having a
polysaccharide coat or with live avirulent rough bacteria without a polysaccharide
coat, the mice survived, but when he injected the above two inocula together, the
mice died of pneumonia and live smoothe bacteria could be isolated from the lungs
of the dead mice. Were the heat killed smoothe bacteria somehow brought back to life?
No! What was going on was DNA from the heat killed smoothe bacteria (encoding the
genes for synthesizing the polysaccharide coat) was taken up by the live rough
bacteria and incorporated into their "chromosome" thus conferring upon them the
property of virulence (synthesis of the polysaccharide coat). When he purified
the active ingredient (transforming principle) from the smoothe bacteria, he found
that this transforming principle was not inactivated by proteases (enzymes that
degrade protein) but that it was destroyed by DNAase (an enzyme that degrades DNA).
The logical conclusion then was that the transforming principle (or genetic material)
was DNA. Also, the purified transforming principle exhibited all the chemical
properties of DNA.
Regards, Ron Baker, Ph.D.
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Update: June 2012