Video Transcript
The diagram provided shows a basic
outline of Griffith’s experiment on bacterial transformation. He determined there were two
strains of the bacteria that caused pneumonia: a smooth virulent strain and a rough
nonvirulent strain. He injected samples of mice with
different forms of these strains, as outlined in the diagram. Which of the following was not
determined by Griffith’s experiment? Option (A) genetic material could
be passed from the cells of the heat-killed smooth strain to the cells of the rough
strain. Option (B) the heat-killed smooth
strain of bacteria would not kill a mouse injected with it. Option (C) the material being
passed between strains of bacteria was DNA. Or option (D) the cells of the
rough strain of bacteria could be altered to become virulent.
This question outlines the
experiments performed by Frederick Griffith, demonstrating a process called
bacterial transformation. We’re being asked to figure out
which of the answer choices was not one of his discoveries. To answer this question, let’s
understand the experiments Griffith performed. As mentioned in our question,
Griffith used two strains of a bacterium called Streptococcus pneumoniae. The first was the rough strain or
the R Strain, which is nonvirulent. This means the bacteria belonging
to this strain cannot cause a harmful infection. The second strain was the smooth
strain or the S strain, which is virulent. This means that bacteria belonging
to this strain can cause a harmful infection in another organism. They are called smooth because they
have a polysaccharide coating surrounding their outermost layer.
Griffith proved that the R strain
was nonvirulent by injecting mice with R strain bacteria. As you can see in the experiment
labeled one in the diagram, these mice remained healthy. He also proved that the S strain
was virulent. Mice injected with these bacteria
quickly developed pneumonia and died, as you can see in experiment two.
Next, let’s look at experiment
three. Griffith used heat to kill the
virulent bacteria belonging to the S strain. When he injected these heat-killed
S strain bacteria into mice, they survived. This showed that the heat-killed S
strain bacteria had lost their virulence. If we look back at our answer
choices, we can see that option (B) states this finding. Since we’re being asked which
option was not one of Griffith’s findings, we can rule this answer choice out.
Finally, let’s analyze experiment
four. Here, Griffith made a mixture of
heat-killed S strain bacteria and live R strain. From his previous experiments,
neither of these bacteria should’ve been able to kill the mice. In experiment one, the mice
infected with the rough strain bacteria survived. And in experiment three, the mice
infected with the heat-killed smooth strain also survived. However, when he injected this
mixture into a new group of mice, they all died. What’s more, he found living
bacteria that appeared just like the S strain bacteria in the blood of these dead
mice. Griffith concluded that although
the S bacteria had been heat-killed, a molecule carrying genetic material had
somehow moved from these dead S bacteria and into the live R bacteria. This caused the R bacteria to
transform into S bacteria. These transformed bacteria were
then able to cause pneumonia in the mice, killing them. Options (A) and (D) in our question
state these two conclusions of Griffith’s experiments. So we can rule out these answer
choices too.
Let’s take another look at option
(C). It says that the material being
passed between the strains of bacteria was DNA. However, although Griffith did
demonstrate bacterial transformation, he never figured out exactly what this
molecule was. He simply called it the
transforming principle. The answer to our question is
therefore (C). The material being passed between
strains of bacteria was DNA.
