Mendel’s experiments with garden peas were a setting stone in genetics. He knew that some “factor” was responsible for the characteristics observed in organisms, which were passed down from each parent to the offspring. At that time, he wasn’t aware of what this factor was.
However, one thing was certain; the genetic material was expressible and inheritable. After continued speculation regarding the molecular nature of the genetic material, certain groundbreaking experiments, including those by Fredrick Griffith (1928), Averty, MacLeod, and McCarty (1944), and Hershey and Chase (1952) laid conclusive evidence that helped establish that it is certainly the DNA, and not the RNA or proteins, which is the molecular basis of inheritance.
Genetic material should not only be inheritable, but it must also be able to express all the information that’s coded in it. This is the defining characteristic of genetic material.
The genetic material must be replicable so that it can be transferred to the progeny
The genetic material must be ubiquitous across all life forms
The genetic material must be chemically and physically stable
The expression of the genetic material controls all the activities of the cell and also the duplication of the cells.
Genetic material is prone to mutation and hence, accounts for the variations among organisms
In 1928, Fredrick Griffith performed some transformation experiments in an attempt to determine the nature of genetic material in organisms.
The experiment involved two strains of the bacterium Streptococcus pneumoniae, one being the virulent strain (serotype IIIS), which produced smooth colonies in an agar plate, and the other, the avirulent one (serotype IIR), which produced rough colonies on an agar plate. The virulent strain has a polysaccharide capsule that acts as the virulence factor. The avirulent strains lack this capsule and are easily phagocytosed.
Griffith injected these two different strains of bacteria into mice. The virulent strain, as expected, killed the mice, while the avirulent strain didn’t.
In another experiment, he subjected the virulent strain to heat, killing the bacterial cells. This heat-killed strain was injected into a mouse. Griffith observed that the mouse survived and didn't develop pneumonia.
The third experiment yielded a very interesting observation. He combined the heat-killed virulent strain with the live avirulent strain and injected the mixture into a mouse. Now, the two strains, when injected individually, would not kill the mouse. However, the mixture of the two strains led the mouse to develop pneumonia and die.
The tissue of the dead mouse contained live bacteria, which on culturing, produced smooth colonies, just like the III-S strain. This strain continued to infect and kill other mice for several generations.
From these observations, Griffith concluded that the heat-killed IIIS strain of the bacteria was somehow able to “convert” the avirulent IIR strain into virulent strains that killed the mouse. He called this the transformation and referred to the genetic information that was passed on from the dead IIIS cells to the live IIR cells as the transforming principle.
At that time, Griffith postulated that the transforming principle was probably a polysaccharide or some compound involved in the synthesis of the capsule.
However, this transforming principle was later confirmed to be the Deoxyribonucleic Acid (DNA), as established by the Hershey-Chase experiment in 1952.
Deoxyribonucleic Acid (DNA) is a self-replicating, double-stranded, helical molecule that is the basis for the continuity of life. Each DNA molecule is a polymer of single units known as nucleotides. Each nucleotide monomer contains
A nitrogenous base, which may be a derivative of a purine (adenine or guanine) or a pyrimidine (thymine or cytosine)
A deoxyribose sugar
A phosphate group
DNA contains genetic instructions that command the biological development of all forms of life and even the majority of viruses.
The information is in the form of genes that are inherited by the offspring from its parents. Each gene encodes a specific character for the individual.
Depending upon what variant of a particular gene (i.e., allele) is inherited, the phenotypic traits between individuals vary for that character.
The nucleotide sequence of a gene encodes within it the sequence of amino acids that make up a specific protein.
While DNA is the inheritable genetic material in all living organisms, there are few viruses which carry RNA as their genetic material.
Initially, proteins were considered strong candidates as genetic material, considering their heterogeneity (being made of 20 different amino acids, which form a wide variety of proteins) and functional capabilities in living organisms. However, the one thing proteins are not is the molecular basis of heredity. This is because −
Proteins are not the carriers of genetic information. They are merely translated from nucleic acids. Even the proteins that are involved in nucleic acid replication, packaging, transcription, and translation are encoded in the nucleic acids!
Proteins cannot replicate themselves
Proteins themselves don't undergo any mutation. The mutations occur in the nucleotide sequences that encode the protein.
The experiments by Averty, MacLeod, and McCarty (1944) and by Hershey and Chase (1952) ultimately confirmed that it’s the DNA and not the proteins that are the genetic material of living organisms.
Griffith performed experiments which showed that bacteria could change their functions via transformation by picking up a transforming principle from dead bacterial cells.
The molecular nature of the transformation principle was later established as the DNA
The ideal genetic material should not only store and express genetic information but also be inheritable, self-replicating, prone to mutations and variations, and stable. DNA checks all these characteristics
Proteins, though widely heterogeneous and having immense functional capabilities, do not qualify as genetic material because its neither inheritable nor self-replicating
Q1. Why can’t one conclude from Griffith’s experiment that the R-strains simply picked up the virulent capsule of the killed S-strains?
Ans. This conclusion proves to be false since the living S-cells isolated from the blood sample in the first experiment were able to reproduce and form more of the S-cells. This implies that the polysaccharide capsule trait is heritable and permanent and not just "discarded" after infecting the host.
Q2. How is genetic information translated to proteins?
Ans. The genetic information within a gene is “read” in the form of a triplet code of nucleotides, each triplet being translated into a specific amino acid.
Q3. Does the entire DNA molecule code for proteins?
Ans. No. More than 98% of the eukaryotic genome is non-coding. Only ~1.5% of the genome encodes proteins.
Q4. What is the RNA World hypothesis?
Ans. The RNA world hypothesis proposes that RNA was the first genetic material and the first life form on earth, supported by its ability to store, transmit and replicate genetic information, along with the ability to act as a catalyst.
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