In this article we will discuss about DNA as a genetic material.
Frederick Griffith in 1928, carried out a series of experiments with Streptococcus pneumoniae (a bacterium that cause pneumonia). He observed that when these bacteria (Streptococcus pneumonia) are grown on a culture plate, some of them produce smooth, shiny colonies (S-type), whereas, the others produce rough colonies (R-type). This difference in character (smooth/rough) is due to a mucous (polysaccharide) coat present in the S-strain bacteria, which is not present in the R-strain.
In his experiments, he first infected two separate groups of mice. The mice that were infected with the S-strain die from pneumonia.
‘S’ strains are the virulent strains causing pneumonia.
The mice that were infected with the R-strain do not develop pneumonia and they live.
S-strain (virulent strain) → Inject into mice → Mice die
R-strain (non-virulent strain) → Inject into mice → Mice live
In the next set of experiments, Griffith killed the bacteria by heating them. The mice that were injected heat-killed S-strain bacteria did not die and lived, whereas the mice that were injected a mixture of heat-killed S-strain and live R-strain bacteria, died due to unexpected symptoms of pneumonia.
S-strain (heat killed) → Inject into mice → Mice live
S-strain (heat killed) + R-strain (live) → Inject into mice → Mice die
Griffith concluded that the live R-strain bacteria, were transformed by the heat-killed S-strain bacteria.
He proved that there was some ‘transforming principle’ that was transferred from the heat-killed S-strain, which helped the R-strain bacteria to synthesise a smooth polysaccharide coat and thus, become virulent. That was due to the transfer of the genetic material.
However, he was not able to define the biochemical nature of genetic material from his experiments.
Biochemical Characterisation of Transforming Principle:
Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44) worked to determine the biochemical nature of ‘transforming principle’ in Griffith’s experiment in an in vitro system.
From the heat-killed S-cells, they purified biochemicals (proteins, DNA, RNA, etc.) to observe, that which biochemicals could transform live R-cells into S-cells.
Therefore, they discovered that DNA alone from heat-killed S-type bacteria caused the transformation of non-virulent R-type bacteria into S-type virulent bacteria.
Protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNases) did not cause this transformation. This proved that the ‘transforming substance’ was neither the protein no RNA.
DNA-digesting enzyme (DNase) caused inhibition of transformation, which suggests that the DNA caused the transformation. Thus, these scientists came to the conclusion that DNA is the hereditary material.
Hershey and Chase Experiment:
The proof for DNA as a genetic material came from the experiment. Alfred Hershey and Martha Chase (1952) carried out some experiments with the viruses that infect bacteria. These viruses are called bacteriophages.
The genetic material of bacteriophage enters the bacterial cell after the bacteriophage gets attached to the bacteria. The bacterial cell treats the genetic material of the virus (bacteriophage) like its own genetic material and then produces more virus particles. Hershey and Chase experimented to find out whether it was protein or DNA from the virus that had entered into the bacteria.
For this, they took two separate media for growing these bacteriophages:
(i) Out of two, one medium contained radioactive phosphorus and the other medium contained radioactive sulphur. Viruses (bacteriophage) were then grown on each medium.
(a) The viruses grown in the presence of radioactive phosphorus (32P) contained radioactive DNA (but not radioactive protein). This is because DNA contains phosphorus not protein.
(b) In the same way, the viruses grown in the medium containing radioactive sulphur (35S) now contained radioactive protein (not radioactive DNA). This is because DNA does not contain sulphur.
(ii) These radioactive viruses (bacteriophages) were then allowed to attach to bacteria (E. colt). As the process of infection with virus continued, the bacteria were agitated in a blender and the viral coats of the bacteria were removed.
(iii) When they were spinned in a centrifuge, the virus particles were separated from the bacteria.
(iv)They observed that the bacteria that were infected with virus containing radioactive DNA were radioactive, whereas the bacteria that were infected with radioactive proteins were not radioactive.
(v) This indicates that only DNA not the protein coat entered the bacterial cell.
(vi) Thus, the genetic material that is passed from virus to bacteria is DNA.
Properties of Genetic Material:
From the Hershey and Chase experiment, the fact was established that DNA acts as a genetic material. But later, studies revealed that in some viruses (e.g., Tobacco Mosaic Viruses, QB bacteriophage, etc.) RNA is the genetic material.
Following are the criteria that a molecule must fulfil to act as a genetic material:
(i) It should be able to replicate.
(ii) It should be chemically and structurally stable.
(iii) It should provide the scope for slow changes
(iv) It should be able to express itself in the form of ‘Mendelian characters’. According to these criteria, both DNA and RNA have the ability to direct their duplications (because of the rule of base pairing and complementarity). Both the nucleic acids (DNA and RNA) have the ability to direct their duplications, whereas the other molecules in the living system, fail to duplicate, e.g., protein.