Can you name the chromosomal component responsible for carrying the genetic information? The answer to this question was still unclear, even though many researchers discovered that the factors responsible for the inheritance of traits come from within the organisms. They all failed to identify the DNA as Genetic Material.

Although Mendel proposed the principles of inheritance in \(1865\) and Meischer discovered nuclein simultaneously, it took a long time to prove that DNA acts as the genetic material. The hereditary substance in humans and almost all animals is DNA or deoxyribonucleic acid. The DNA of nearly every cell in a person’s body is identical.

What is DNA?

DNA stands for “Deoxyribonucleic Acid”, which acts as a genetic material found in all the prokaryotic and eukaryotic cells and in many viruses also that codes the genetic information for the transmission of inherited traits.
DNA is an organic macromolecule that is composed of two polynucleotide chains that coil around each other to form a double helical structure, which carries the genetic instructions for the development, functioning, growth and reproduction of all organisms and many viruses.

Properties of Genetic Material

1. It should be capable of storing genetic information.
2. It should be capable of replicating with high efficiency in successive generations.
3. It should form the basis for the transmission of hereditary characters that it controls.
4. It should be involved in gene action, which results in the ultimate expression of the characteristics within the organism.
5. It should be able to undergo a mutation that creates variation among organisms.
6. It must show a very wide diversity in form.

DNA as the Genetic Material

The evidence for DNA acting as the genetic material for most of the organisms is mainly divided into two types:
1. Direct Evidence
2. Indirect Evidence

1. Direct Evidence

The direct evidence includes the following experiments which prove the DNA as the genetic material:

Experiments to prove DNA as the Genetic Material

The confirmation of DNA being the genetic material and not the proteins was confirmed by the following experiments:

Griffith’s Experiment on Bacterial Transformation

1. Frederich Griffith, a British bacteriologist, conducted a series of experiments in \(1928\) with the bacteria Diplococcus pneumoniae (or Streptococcus pneumoniae), which causes pneumonia in mammals.
2. He grew the bacteria in the culture plates and found that they produce two types of colonies, i.e.,
i. S-strain (capsulated type): It forms smooth shiny colonies as these bacteria have a mucous (polysaccharide) coat on their surface. These are virulent and cause pneumonia.
ii. R-strain (non-capsulated type): It forms rough colonies as they do not have a mucous (polysaccharide) coating on their surface. These are non-virulent and do not cause pneumonia.
3. He conducted a series of experiments as given below:
i. He injected S-strain into the mice, the mice developed pneumonia and died.
ii. He injected R-strain into the mice, the mice did not get the disease and survived.
iii. He injected heat-killed S-strain into the mice, the mice did not get the disease and remained alive. He killed the bacteria by heating them so they became ineffective and could not kill the mice.
iv. He injected a mixture of heat-killed S-strain along with the live R-strain, the mice suffered pneumonia and died. He recovered live S-strain bacteria from the blood sample of the dead mice.

Fig: Griffith’s Experiment on Mice Explaining the Bacterial Transformation

In the last experiment, Griffith injected heat-killed S-strain with live R-strain. Then, how did these living S-strain appear in the blood and cause the death? Griffith concluded that there was some factor called the ‘transforming principle’ (an early name for the genetic material DNA) in heat-killed S-strain bacteria that was transferred to the R-strain, which transformed into live S-strain. It enabled the R-strain to synthesise smooth polysaccharide coats and become virulent. The R-strain that was transformed into S-strain continued thereafter, producing S-strain only.
However, the biochemical nature of the genetic material was not defined.

Biochemical Characterisation of Transformation Principle

1. The principle behind these transformations was not known until \(1944\) when Avery, McCarty and Macleod studied the isolated components of heat-killed S-strain bacteria in detail. They did a series of experiments and got the following results:
i. Removed the polysaccharide capsule from the heat-killed S-cells + R-cells = Mice died.
ii. Removed the protein fraction from the heat-killed S-cells + R-cells = Mice died.
iii. Added DNAase enzyme into heat-killed S-cells + R-cells = Mice survived.
2. In experiments (i) and (ii), the DNA of heat-killed S-cells was intact, and so it transformed live R-cells into S-type, but in the experiment (iii), the enzyme disintegrates the DNA, and so R-cells were not transformed.
3. They also discovered that protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNAases) did not affect the transformation. So, the transforming substance was not a protein or RNA but the DNA of the heat-killed S-strain bacteria.
4. This experiment demonstrated that DNA can be transferred from one organism to another and continues to express its genetic qualities.
5. It also indicated that DNA is the genetic material that is not destroyed by killing the cells by heat treatment, whereas all other materials like lipids, polysaccharides, proteins got destroyed by the heat treatment.

Fig: Avery, McCarty and Macleod Experiment

Hershey and Chase Experiment on Bacteriophages

1. The unmistakable proof that DNA is the genetic material was confirmed by the transduction experiments (Bacteriophage Experiments) of Hershey and Chase \(\left( {1952} \right).\)
2. They used bacteriophage \({{\rm{T}}_2}\) for their experiments, which infects Escherichia coli (the bacterium present as commensal in the human intestine).
3. E. coli can also be grown over a culture medium. \({{\rm{T}}_2}\) bacteriophage is made up of DNA and protein coat (capsid). Thus, it is the most suitable material to determine whether DNA or protein acts as the genetic material.
4. They grew two cultures of E. coli; one culture was supplied with radioactive sulphur, \({{\rm{S}}^{35}}\) and the other culture was supplied with radioactive phosphorus, \({{\rm{P}}^{32}}.\)
5. Radioactive sulphur gets incorporated into sulphur-containing amino acids (cysteine and methionine) and thus becomes a part of bacterial proteins.
6. Radioactive phosphorus gets incorporated into the nucleotides, which form nucleic acids, mostly DNA.
7. After the labelling of bacteria of both the cultures, three steps were followed:
i. Infection: Both types of labelled bacteriophages were allowed to infect normally cultured bacteria in separate experiments. DNA was introduced, and the protein coat was left out.
ii. Blending: As the infection proceeded, the bacterial cells were agitated in a blender by spinning them in a centrifuge.
iii. Centrifugation: The virus particles were separated from the bacteria by spinning them in a centrifuge. The heavier bacteria settled down in the form of pellets, and the supernatant contained the lighter viral coats which do not enter the bacterial cells. Both the pellet and supernatant were analysed.
It was found that the phage with labelled protein did not make the bacteria labelled. Instead, the radioactivity was restricted to the supernatant, which was found to contain only empty phage capsids (protein coats).
In the second culture where the bacteriophage labelled with radioactive DNA was introduced, it was found that shaking did not produce any radioactivity in the supernatant having empty capsids. Instead, the bacteria became labelled, proving that only the DNA of the phage enters the bacteria.
8. The progeny of the two types of bacteria was again tested for radioactivity. It was observed that radioactivity was absent in the viruses derived from parents having labelled protein but the viruses derived from parents having labelled DNA possessed radioactivity. This shows that the genetic material is the DNA and not the protein.
9. The Hershey and Chase experiment is also called the Blender Experiment.

Fig: Hershey and Chase Experiment

2. Indirect Evidence

Following is the indirect evidence which led to the conclusion that DNA acts as the genetic material.
1. Localisation: The genetic material should have a fixed location within the cell; otherwise, the genes will not function properly. DNA is always located primarily within the chromosome in the nucleus of the eukaryotic cell.
2. Stability: DNA is a metabolically stable macromolecule because it does not undergo the anabolic or catabolic processes within the cell-like various other macromolecules.
3. Sensitivity to mutagens: The genetic material should undergo mutations and should be sensitive to the mutagens (like UV-rays, X-rays, etc.) and a variety of other chemical compounds. When the cells of an organism are exposed to mutagens, they can cause a change in the gene structure which are DNA segments only.
4. DNA content: There should be a proper correlation between the DNA content and the number of chromosomes. It has been observed that the diploid cells contain twice as much DNA as the haploid cells of the same species have.

Requirement for DNA to Act as the Genetic Material

Following are the requirements for DNA to act as the genetic material:
1. Replication: It should be able to generate its replica.
2. Stability: It should be chemically and structurally stable.
3. Mutation: It should provide scope for the mutation that is required for the process of evolution.
4. Expression: It should be able to express itself in the form of ‘Mendelian Characters’.
5. Repairing Mechanism: It should be able to repair itself and thermally stable.

Why does DNA Act as a Better Genetic Material than RNA?

1. To be able to replicate: DNA has the ability to replicate itself as well as RNA also has an amazing capability of making RNA copies of itself. But all other molecules in the living system, such as proteins, fail to fulfil this criterion itself.
2. Chemical and structural stability: Chemically, DNA is less reactive and structurally more stable when compared to RNA. Thus, among the two nucleic acids, DNA is the better genetic material.
3. To be able to mutate: Both DNA and RNA have the capability to mutate. In fact, RNA being unstable can mutate at a faster rate. This is the reason why most viruses have RNA as the genetic material.
4. To be able to express itself as Mendelian characters: DNA and RNA both can express characters, but DNA is dependent on RNA for protein synthesis, while RNA can synthesise directly.
5. Thermal stability and repairing mechanism: Both DNA and RNA have the ability to repair themselves, and DNA has high thermal stability compared to RNA.

Thus, from the above points, it is clear that both DNA and RNA can function as genetic material, but DNA being more stable is preferred for the storage of genetic information. However, RNA is better for the transmission of genetic information.

Summary

Thus, from the above discussion, it is concluded that both DNA and RNA can function as genetic material, but due to more stability of DNA, it is considered for the storage of genetic information. The confirmation of DNA is the genetic material that was given by the Hershey and Chase experiment. Till then, there was confusion about whether DNA or the proteins act as the genetic material though it was known for a long time that chromosomes carry the genetic information. It took a long time to prove that DNA acts as the genetic material. The requirements to be a better genetic material includes the ability to replicate, stability, can do mutation and able to express itself as Mendelian characters.

FAQs

Q.1. Why is DNA called the genetic material?
Ans: DNA is called the genetic material because of the following reasons:
i. DNA has the ability to replicate.
ii. DNA is chemically and structurally more stable.
iii. DNA has the capability to mutate.
iv. DNA has the ability to express itself as the Mendelian Character.

Q.2. What confirmed DNA as the genetic material?
Ans: Hershey and Chase’s experiment confirmed that DNA is the genetic material.

Q.3. Is DNA the only genetic material?
Ans: No, DNA and RNA are both considered to be the genetic material but due to more stability of DNA, it is preferred for the storage of genetic information. RNA acts as the genetic material in some viruses.

Q.4. Which organisms have no DNA?
Ans: All the organisms have DNA except some viruses that contain RNA as the genetic material.

Q.5. Is life possible without DNA?
Ans: No, life is not possible without DNA for all the self-reproducing cellular organisms except some RNA viruses.