Question Video: Understanding the Results of Avery’s Experiments | Nagwa Question Video: Understanding the Results of Avery’s Experiments | Nagwa

Question Video: Understanding the Results of Avery’s Experiments Biology • Third Year of Secondary School

The diagram 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?

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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.

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