Molecular Basis of Inheritance
Introduction
The molecular basis of inheritance explains how genetic information is stored, copied, and passed from one generation to the next. DNA is the primary genetic material in almost all living organisms, while RNA serves as genetic material in some viruses.
Understanding why DNA functions as genetic material is a fundamental topic in genetics, molecular biology, and introductory biology. Classic experiments by Frederick Griffith, Avery, MacLeod and McCarty, and Hershey and Chase provided important evidence that DNA carries hereditary information.
In this guide, we will explain these landmark experiments in a simple, step-by-step manner, including Griffith's transformation experiment, the biochemical nature of the transforming principle, and the Hershey–Chase experiment. These concepts are especially useful for students studying high-school biology, AP Biology, introductory college biology, and genetics.
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Both DNA & RNA act as genetic material, but in most of the living organisms, only DNA acts as genetic material and RNA acts as genetic material in some viruses only.
Examples: TMV, QB bacteriophage.
Criteria of Genetic Material
Genetic material:
It should be able to express and store information.
It should be metabolically most stable chemically in the cell.
It must replicate, because one of its copies may be transferred to the next generation.
It must be able to produce variation, which plays an important role in evolution.
Note
Proteins were considered as genetic material without any experimental evidence.
At present, on the basis of experimental evidence, it has been proved that DNA acts as genetic material in living organisms.
Griffith's Transformation Experiment
There were two strains of bacteria:
R-II: Non-capsulated / non-pathogenic — causes no death of the organism.
S-III: Capsulated / pathogenic — causes death of the organism.
Griffith injected these two strains into mice and divided the experiment into four parts:
Living R-II injected into mice → No death of mice
Living S-III injected into mice → Death of mice
Heat-killed S-III injected into mice → No death
Living R-II + heat-killed S-III → Death
Conclusion of Griffith
Some chemical substance may have been transferred from the heat-killed S-III form to the living R-II form.
The R-II form became pathogenic, causing the death of mice.
Definition of Transformation
Transformation is the conversion of one form of bacteria into another form without any direct contact, by absorbing genetic material from the surrounding medium.
Competence
Competence: The ability of a recipient bacterium to absorb DNA from the surrounding medium is called competence.
Biochemical Nature of Transforming Material
By Avery, MacLeod & McCarty
They purified DNA, RNA and protein from the heat-killed S-III form of bacteria and tested which substance was able to transform the R-II form into the S-III form.
Experiment
It was found that:
Only DNA can convert R-II form into S-III form.
RNA and protein cannot cause transformation.
Experiment with Enzymes
DNA, RNA and protein were separately digested using:
DNase
RNase
Protease
Result:
When DNase was used → transformation was inhibited.
When RNase or Protease was used → transformation was not inhibited.
Conclusion
DNA is the transforming principle.
Therefore, DNA acts as genetic material.
Hershey–Chase Experiment
The Hershey and Chase experiment provided further evidence that DNA is the genetic material.
They used bacteriophages infecting E. coli bacteria.
Bacteriophage Structure
A bacteriophage contains:
Capsid → Protein
DNA → Phosphorus-containing genetic material
Radioactive Labelling
Two separate batches of bacteriophages were prepared:
Protein labelled with ³⁵S
DNA labelled with ³²P
This was because:
Protein contains sulphur (S) but generally no phosphorus as a characteristic label.
DNA contains phosphorus (P) but no sulphur as a characteristic component.
Both bacteriophages were used to infect E. coli bacteria.
Steps
Infection — bacteriophages infect E. coli.
Blending — blender was used to remove phage particles from the bacterial surface.
Centrifugation — bacterial cells and phage particles were separated.
Radioactivity was examined in the different fractions.
Result
The ³⁵S label remained associated mainly with the phage protein coat, whereas ³²P was found with the bacterial cells.
Conclusion
DNA enters the bacterial cell and carries the genetic information.
Therefore, DNA is the genetic material.
