Video Transcript
The diagram provided shows a modified version of the experiment conducted by Avery
and his colleagues. The virulent S cells were killed by high temperatures and divided into three
samples. A different enzyme was added to each sample, and then the solution was mixed with
live, but nonvirulent, R cells. Assume that mice injected with R cells that have successfully undergone bacterial
transformation will die. In which of these three experiments will the mice die?
This question asks us about an important experiment conducted by Avery and his
colleagues in the 1940s. While it may seem like common knowledge now, scientists back then were unsure whether
proteins, RNA, or DNA carried genetic information that could be passed along to
other cells.
To look at this question, Avery and colleagues made use of a phenomenon known as
bacterial transformation. This is where bacterial cells can take up and incorporate DNA from their
environment. They looked at different strains of Streptococcus pneumoniae, a bacterium that
could cause pneumonia and death in mice.
The S strain, or smooth strain, has a smooth appearance under the microscope. And the R strain, or rough strain, has a rough appearance. These strains are genetic variants of Streptococcus pneumoniae that differ in
their physical appearance as well as their ability to cause disease in mice. When a mouse is injected with the rough strain, the mouse doesn’t develop pneumonia
and survives. However, when the mouse is injected with the smooth strain, it develops pneumonia and
dies.
When the S strain is killed by exposing it to heat and then injecting it into mice,
the mice survive. And interestingly, when heat-killed S-strain bacteria are mixed with R-strain
bacteria, the mice die. This happens because when the S-strain bacteria are killed, they leave behind some
DNA. This DNA can be taken up by transformation of the R strain. And this gives the R-strain bacteria the ability to cause pneumonia and death in
mice.
So, if we now look at the provided diagram, we can see that each sample contains
heat-killed S-strain bacteria. Therefore, each of these samples contains DNA in the sample that’s able to transform
R-strain bacteria to become virulent and cause death in mice. So the mice will die unless we add something that will cause the DNA to degrade.
In each experiment, a different type of enzyme is added. In the first example, the protease enzyme will only break down proteins in the sample
and not the S-strain DNA. Therefore, this DNA persists and will transform the R-strain bacteria to kill the
mice. In the second experiment, RNase is added, which only degrades RNA and not DNA. So, in this example, the DNA remains intact and the mice go on to die. In the last experiment, DNase is added, which degrades DNA. So, in this experiment, the S-strain DNA is degraded and can’t transform the R-strain
bacteria, so these mice survive.
So, to answer our question, the experiments that will cause the mice to die are 1 and
2.
