Model Answer
0 min readIntroduction
For decades, the central dogma of molecular biology – DNA makes RNA, and RNA makes protein – served as the cornerstone of our understanding of inheritance. This model posited that genetic information flows unidirectionally, with proteins acting as the functional molecules and carriers of heritable traits. However, this paradigm was fundamentally challenged in the 1980s with the groundbreaking discovery of prions, infectious proteins capable of self-replication and causing devastating neurodegenerative diseases. This discovery, awarded the Nobel Prize in Physiology or Medicine in 1997 to Stanley Prusiner, revolutionized our understanding of inheritance, demonstrating that information could be transmitted through protein conformation alone, independent of nucleic acids.
The Classical Concept of Inheritance
Traditionally, inheritance was understood to be based on the transmission of genetic information encoded in DNA. DNA provides the blueprint for synthesizing RNA, which in turn directs the production of proteins. Proteins, with their diverse structures and functions, were considered the primary agents of phenotypic expression and the vehicles for heritable traits. Mutations in DNA led to altered protein structures, resulting in changes in phenotype. This model explained Mendelian inheritance and the vast majority of known genetic diseases.
Discovery of Prions and Their Unique Properties
In the early 1980s, Stanley Prusiner investigated the cause of scrapie, a fatal neurodegenerative disease affecting sheep. He hypothesized that an unknown infectious agent was responsible, but unlike viruses or bacteria, it was resistant to conventional sterilization methods like radiation and heat. He termed this agent a “prion,” a portmanteau of “proteinaceous infectious particle.”
- Prion Protein (PrP): Prions are misfolded versions of a normal cellular protein called PrPC (Prion Protein Cellular).
- Conformational Change: The infectious prion, PrPSc (PrP Scrapie), has the same amino acid sequence as PrPC but adopts a different three-dimensional conformation – a beta-sheet rich structure instead of an alpha-helix rich structure.
- Self-Templating: PrPSc acts as a template, inducing the misfolding of normal PrPC proteins into the infectious PrPSc form. This leads to a cascade of misfolding and aggregation, forming amyloid plaques in the brain.
- Lack of Nucleic Acid: Crucially, prions are infectious even after being purified to contain only protein, demonstrating that they do not require nucleic acids (DNA or RNA) for replication.
How Prions Changed the Concept of Inheritance
The discovery of prions fundamentally altered the concept of inheritance in several ways:
- Non-Mendelian Inheritance: Prion diseases do not follow Mendelian patterns of inheritance. Transmission can occur sporadically, genetically (through mutations in the PRNP gene), or through exposure to infectious prions.
- Epigenetic Inheritance: Prion propagation can be considered a form of epigenetic inheritance, where a change in phenotype is not due to an alteration in the DNA sequence but rather to a change in protein conformation.
- Information in Protein Structure: Prions demonstrated that information can be encoded and transmitted not just in the sequence of amino acids (DNA-directed protein synthesis) but also in the three-dimensional structure of a protein.
- Challenge to the Central Dogma: The prion discovery directly challenged the central dogma of molecular biology, showing that information flow isn’t always DNA → RNA → Protein. Protein can directly influence the conformation of other proteins, leading to replication of the infectious phenotype.
Prion Diseases in Humans and Animals
Several devastating diseases are caused by prions:
| Disease | Host | Symptoms |
|---|---|---|
| Creutzfeldt-Jakob Disease (CJD) | Humans | Rapidly progressive dementia, myoclonus, ataxia |
| Variant Creutzfeldt-Jakob Disease (vCJD) | Humans | Linked to bovine spongiform encephalopathy (BSE) consumption; psychiatric symptoms, dementia |
| Bovine Spongiform Encephalopathy (BSE) | Cattle | Neurological symptoms, behavioral changes ("mad cow disease") |
| Scrapie | Sheep & Goats | Neurological degeneration, intense itching |
| Kuru | Fore people of Papua New Guinea | Ataxia, tremors, dementia (transmitted through ritualistic cannibalism) |
Implications for Evolution
While the role of prions in evolution is still debated, some scientists suggest that prion-like mechanisms may have played a role in the evolution of protein function and the development of new traits. The ability of proteins to self-template and propagate conformational changes could have provided a mechanism for rapid adaptation and innovation.
Conclusion
The discovery of prions was a paradigm shift in our understanding of inheritance. It demonstrated that genetic information isn’t solely encoded in nucleic acids, but can also be transmitted through protein conformation. This challenged the central dogma of molecular biology and opened new avenues for research into non-Mendelian inheritance, epigenetic mechanisms, and the evolution of protein function. While prion diseases are rare, their impact on our understanding of biology has been profound, highlighting the complexity and adaptability of life at the molecular level.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.