Q.Recall the experiments done by Frederick Griffith, Avery, MacLeod and McCarty, where DNA was speculated to be the genetic material. If RNA, instead of DNA was the genetic material, would the heat killed strain of Pneumococcus have transformed the R-strain into virulent strain? Explain.

In the experiments by Griffith, and later by Avery, MacLeod, and McCarty, DNA was identified as the genetic material responsible for transforming the non-virulent R-strain into the virulent S-strain. A key observation was that the transforming principle from the heat-killed S-strain remained functional.

If RNA, instead of DNA, were the genetic material, the heat-killed strain of Pneumococcus would not have transformed the R-strain into a virulent strain. This is because RNA is significantly less stable to heat than DNA. The high temperatures used to kill the S-strain would denature and degrade the RNA, destroying its structural integrity and its ability to carry genetic information. Consequently, the denat …

The journey to identify DNA as the genetic material is a fascinating chapter in biology, marked by pivotal experiments that gradually unveiled its role. Frederick Griffith's work in 1928 provided the first hint of a "transforming principle," and later, Avery, MacLeod, and McCarty's meticulous biochemical analysis in 1944 conclusively demonstrated that DNA was this principle. Understanding these experiments is key to addressing your question.

Griffith worked with Streptococcus pneumoniae (pneumococcus) bacteria, which cause pneumonia. He observed two main strains:

• S-strain (Smooth): These bacteria have a polysaccharide coat, giving them a smooth appearance. They are virulent, meaning they cause disease and death in mice.
• R-strain (Rough): These bacteria lack the polysaccharide coat, appearing rough. They are non-virulent and do not cause disease in mice.

Griffith conducted a series of experiments:

• When R-strain bacteria were injected into mice, the mice lived.
• When S-strain bacteria were injected into mice, the mice died.
• When heat-killed S-strain bacteria were injected into mice, the mice lived. This showed that the virulence factor was destroyed by heat.
• Crucially, when a mixture of heat-killed S-strain and live R-strain bacteria was injected into mice, the mice died. Furthermore, live S-strain bacteria were recovered from the dead mice.

Griffith concluded that some "transforming principle" from the heat-killed S-strain had enabled the live R-strain bacteria to synthesize a smooth polysaccharide coat and become virulent. However, he could not identify the biochemical nature of this principle.

Avery, MacLeod, and McCarty took Griffith's work further. They aimed to identify the chemical nature of this transforming principle. They purified biochemicals (proteins, DNA, RNA, etc.) from the heat-killed S-strain bacteria. They then tested which of these purified components could transform live R-strain bacteria into S-strain bacteria.

Their key experiments involved treating the heat-killed S-strain extract with specific enzymes:

• When the extract was treated with proteases (enzymes that digest proteins), transformation still occurred. This indicated that proteins were not the transforming principle.
• When the extract was treated with RNase (an enzyme that digests RNA), transformation still occurred. This indicated that RNA was not the transforming principle.
• When the extract was treated with DNase (an enzyme that digests DNA), transformation was inhibited. This meant that DNA was essential for transformation.

Their conclusion was profound: DNA was the genetic material responsible for the transformation.

Now, let's consider your hypothetical scenario: If RNA, instead of DNA, was the genetic material, would the heat-killed strain of Pneumococcus have transformed the R-strain into a virulent strain?

The answer is no, it is highly unlikely that transformation would have occurred. The critical factor here is the difference in stability between DNA and RNA, particularly under heat.

• DNA Stability: DNA is a remarkably stable molecule. Its deoxyribose sugar lacks a hydroxyl group at the 2' position, making it less susceptible to hydrolysis. Furthermore, its double-stranded structure provides protection, and the presence of thymine instead of uracil also contributes to its stability. While high heat can denature DNA (separate its strands), it generally does not completely degrade the phosphodiester backbone as readily as it degrades RNA. The "heat-killed" step in Griffith's experiment involved heating the S-strain to a temperature that killed the bacteria but left the DNA largely intact and functional enough to act as the transforming principle. …

Instead of arguing from RNA's chemistry in detail, apply the same stability criterion used to judge any candidate genetic material (Section 5.3, criterion ii: it 'should be stable chemically and structurally'). RNA's 2'-OH group makes it fail this test under heat where DNA passe …