Video Transcript
The diagram provided shows a basic
outline of Griffith’s experiment into bacterial transformation. He determined there were two
strains of the bacteria that cause 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. What will happen to the mouse in
experiment 4? (A) It will survive or (B) it will
die.
This question asks us about the
famous experiment of Frederick Griffith, who was a British bacteriologist. His experiment in 1928 was the
first to reveal the existence of a transforming principle. This result later led to the
discovery that DNA acts as the carrier of genetic information. Let’s take a closer look at this
experiment to work out the correct answer to this question.
Griffith utilized two strains of
the bacteria Streptococcus pneumoniae. One strain, whose colonies looked
smooth and was therefore called the smooth strain, caused pneumonia. As it can cause the disease, it can
also be known as the virulent strain. The other strain appeared rough and
was therefore called the rough strain but did not cause the disease. As it does not cause pneumonia, it
can be known as the nonvirulent strain.
The different effects of the
virulent, smooth strain and nonvirulent, rough strain are shown by experiment 1 and
experiment 2, as outlined in the diagram. We can see that the mouse that was
injected with the rough strain survives, while the mouse injected with the smooth
strain dies of pneumonia.
Now, if we look at experiment 3, we
can see that the smooth, virulent strain of the bacterium was heat-killed. Griffith knew from the previous
work of French microbiologist Pasteur that heat can kill germs. So, unsurprisingly, when Griffith
injected this heat-killed, smooth strain to the mouse, the mouse survived.
In experiment 4, Griffith prepared
a solution containing both the rough, nonvirulent strain and the heat-killed, smooth
strain. Heat destroys the proteins and the
structure of the bacteria. Thus, the DNA that was in the
smooth, virulent bacteria was released into its environment, which in this case is
also the environment of the rough bacteria. Then, this genetic information from
the smooth, virulent strain was taken up by the live bacteria of the nonvirulent,
rough strain. This capacity that bacteria have to
integrate the genetic material that they find in their environment is called
transformation. And the genetic material itself
which is transferred between cells is the transforming principle.
Here, the transformed rough strain
bacteria that were previously nonvirulent became virulent, because they had acquired
the genetic information to cause pneumonia. Therefore, when injected into a
live mouse, the mouse will die from pneumonia.
Now that we have discussed some of
the biological underpinnings of bacterial transformation and Griffith’s experiments,
we can determine what will happen to the mouse in experiment 4 when it’s injected
with the rough strain and heat-killed, smooth strain. The correct answer is (B); it will
die.
