Question

Present an overview of two classical experiments that demonstrated that DNA is the genetic material. Can RNA be the genetic material? Explain.

Short answer

Short Answer: Two classical experiments that demonstrated that DNA is the genetic material are Griffith's experiment of 1928 and the Avery, MacLeod, and McCarty experiment of 1944. Griffith's experiment showed the transformation of non-virulent bacteria into virulent forms, indicating the transfer of genetic material, later identified as DNA. The Avery, MacLeod, and McCarty experiment provided evidence that DNA was responsible for transformation. RNA can also be considered genetic material, particularly in RNA viruses like the influenza virus and HIV, where it holds the genetic code and passes it onto new generations.

Step-by-step solution

Experiment 1: Griffith's Experiment (1928)

Frederick Griffith conducted an experiment that provided indirect evidence that DNA is the genetic material. He studied two forms of pneumonia-causing bacteria: smooth (S) strain (virulent) with a capsule and rough (R) strain (non-virulent) without a capsule. He mixed heat-killed S strain with living R strain and injected it into mice. The result showed that the living R strain had transformed into the virulent S strain, leading to mouse death. The conclusion was that the heat-killed S strain had transferred its genetic material (the "transforming principle") to the living R strain. Later, it was discovered that the transforming principle was DNA.

Experiment 2: Avery, MacLeod, and McCarty Experiment (1944)

To determine what substance was responsible for the transformation in Griffith's experiment, Avery, MacLeod, and McCarty conducted an experiment using purified substances from the heat-killed S strain bacteria. They treated these substances with various enzymes that degraded proteins, RNA, and DNA. The result showed that only when the substance containing DNA was added to the R strain, it transformed into the S strain. The conclusion of this experiment was that DNA is the genetic material responsible for the transformation.

Can RNA be the genetic material?

RNA can also be the genetic material in some cases, particularly in viruses known as RNA viruses, like the influenza virus and HIV. These viruses contain RNA, not DNA, as their genetic material. RNA performs a similar role to DNA, holding the genetic code and passing it onto new generations, although it is less stable and undergoes more rapid changes than DNA. This flexibility allows RNA viruses to adapt quickly to new environments and hosts. So, RNA can also be considered as genetic material, particularly in RNA viruses.

Key concepts

Griffith's Experiment

In 1928, Frederick Griffith conducted a groundbreaking experiment that provided early evidence that DNA is the genetic material. He studied two strains of pneumonia-causing bacteria: the smooth (S) strain, which had a protective capsule and was virulent, and the rough (R) strain, which lacked the capsule and was non-virulent. Griffith discovered that when he mixed heat-killed S strain bacteria with living R strain bacteria and injected the mixture into mice, the mice became sick and died.

He concluded that the R strain bacteria had somehow been 'transformed' into the virulent S strain. The key to this transformation was something within the heat-killed S strain cells—what he called the 'transforming principle'. This hinted that a molecule within the S strain bacteria could carry genetic information to the R strain, though Griffith himself did not identify DNA as this molecule.

Avery, MacLeod, and McCarty Experiment

Building upon Griffith's findings, Avery, MacLeod, and McCarty set out in 1944 to identify the mysterious 'transforming principle'. They performed meticulous experiments using purified substances from the heat-killed S strain of bacteria. By treating these substances with different enzymes that would degrade proteins, RNA, and DNA separately, they were able to pinpoint the transforming principle.

Strikingly, it was found that only the enzyme that degraded DNA prevented the transformation of R strain into S strain. This provided strong evidence that DNA—and not proteins or RNA—was the substance responsible for carrying genetic information and capable of inducing transformation. This was a pivotal moment in genetics, as it firmly established DNA as the molecular basis for heredity.

RNA as Genetic Material

While DNA is the primary genetic material in most organisms, RNA also plays a crucial role in the flow of genetic information—especially in some viruses, like influenza viruses and HIV, where RNA serves as the sole genetic material. These are known as RNA viruses.

In RNA viruses, the RNA does not simply relay information from DNA; it holds the genetic code to produce new viruses. This is carried out through a process called reverse transcription, where RNA is reverse-transcribed into DNA in a host cell. One key aspect of RNA as genetic material is its higher mutation rate compared to DNA. This accounts for the RNA viruses' rapid adaptability to new environments and contributes to difficulties in creating effective and long-lasting vaccines.

Transformation Principle

The concept of the 'transformation principle' comes from the experiments by Griffith and was further elucidated by Avery, MacLeod, and McCarty. It refers to the substance that carries genetic information from one bacterial cell to another, allowing for a change in traits—or 'transformation'.

Initially, the nature of this principle was unknown, but subsequent research proved that DNA is the transformation principle. This discovery was crucial as it cemented the understanding that traits could be transferred through a chemical substance—DNA—between cells, and paved the way for the molecular approach to genetics that would culminate in the discovery of the structure of DNA.