NEET High-Weightage Alert: The chapter Molecular Basis of Inheritance is one of the most critical sections in NEET Biology. Questions are frequently asked from DNA structure, replication, transcription, translation, operon concept, genetic code, and the Human Genome Project. Ensure you master every experiment below!
Introduction to Genetic Material
Inheritance occurs through genetic material called DNA (Deoxyribonucleic Acid). DNA is the molecule responsible for storing, transmitting, and expressing genetic information across generations.
DNA is the genetic material in most living organisms.
The Search for Genetic Material: Historical Experiments
The discovery that DNA is the primary vehicle of inheritance was proven through a series of milestone microbiological experiments:
1. Griffith's Transforming Principle Experiment (1928)
| Organism: | Streptococcus pneumoniae (Pneumonia-causing bacteria) |
Key Observations:
- R strain: Non-virulent (Mice survive)
- S strain: Virulent (Mice die)
- Heat-killed S strain + Live R strain: Mouse died
Conclusion: Some "transforming principle" was transferred from the heat-killed S strain to the live R strain, enabling it to synthesize a smooth polysaccharide coat and become virulent.
2. Avery, MacLeod, and McCarty Experiment
They chemically purified biochemicals (proteins, RNA, DNA) from heat-killed S cells to see which could transform live R cells. Conclusion: DNA alone is the transforming genetic material, as digestion with DNase inhibited transformation.
3. Hershey and Chase Experiment (Unequivocal Proof)
| Organism: | Bacteriophage (Virus infecting bacteria) |
By labeling viral proteins with Radioactive Sulfur ($^{35}\text{S}$) and viral DNA with Radioactive Phosphorus ($^{32}\text{P}$), they demonstrated that only DNA enters bacterial cells and acts as the genetic material. Conclusion: DNA is definitively the genetic material.
Structure of DNA (Deoxyribonucleic Acid)
The structural framework of DNA was proposed by James Watson and Francis Crick, based heavily on the X-ray diffraction studies conducted by Rosalind Franklin and Maurice Wilkins.
Salient Features of DNA Double Helix:
- It is composed of a double helix structure with two antiparallel strands ($5' \rightarrow 3'$ and $3' \rightarrow 5'$).
- The structural skeleton is composed of a alternating sugar-phosphate backbone, while the nitrogenous bases project inwards.
- Nitrogenous Bases & Base Pairing: Adenine pairs specifically with Thymine ($\text{A}=\text{T}$ via two hydrogen bonds), and Guanine pairs with Cytosine ($\text{G}\equiv\text{C}$ via three hydrogen bonds).
| Rule Definition | Mathematical Ratio |
|---|---|
| Chargaff's Rule | $\text{A} = \text{T}$ | $\text{G} = \text{C}$ | $\frac{\text{A}+\text{G}}{\text{T}+\text{C}} = 1$ |
DNA Packaging
Due to the vast length of the DNA polymer relative to cellular dimensions (approx. 2.2 meters long in mammalian cells), specialized folding mechanisms are required. In eukaryotes, long negatively charged DNA strands wrap around positively charged basic histone octamers to form repeating structural units known as nucleosomes, which visually present as a "beads-on-a-string" chromatin fibers configuration under microscopic screening.