Question

A is an infectious agent composed solely of \(\mathrm{RNA}\). Whereas a is an infectious agent composed solely of protein. a. retrovirus, bacteriophage d. retrovirus, prion b. viroid, virus e. viroid, prion c. prion, virus

Short answer

The correct options are 'd. retrovirus, prion' and 'e. viroid, prion'.

Step-by-step solution

Identify Infectious Agent based on Composition

Let's start with the agent that is solely composed of RNA. From the options, we have two possible agents that are made solely from RNA: retroviruses and viroids.

Identify Protein-based Infectious Agent

Now, we need to identify the infectious agent that is composed solely of protein. The only option for this is prions, as the other options have either RNA or DNA in their structure.

Match the Criteria with Options

Match the identified infectious agents with the choices. The option that has an RNA-based agent and a protein-based agent is the one we're looking for. In this case, it's the option with 'retrovirus' and 'prion' or 'viroid' and 'prion'.

Key concepts

RNA Infectious Agent

Understanding the role of RNA in infectious agents is crucial for deciphering how some pathogens invade and hijack host cells. RNA infectious agents include viruses and viroids. While viruses possess a protein coat known as a capsid that encases the RNA, viroids are simpler, consisting only of a short strand of circular RNA with no protective protein coat.

Viruses, including the infamous influenza, rely on their RNA genome to replicate inside host cells, commandeering the host's machinery to produce viral components. In contrast, viroids are mainly known to attack plants, causing various diseases. They do not encode proteins but can interfere with the host cell's regulatory systems at the genetic level. The simplicity of viroids makes them a fascinating subject for studying the evolution of pathogens and the minimal requirements for an infectious agent.

Protein-Based Infectious Agent

A protein-based infectious agent is an unconventional pathogen known as a prion. Prions are misfolded proteins able to induce other normally folded proteins within the host organism to also misfold, creating a domino effect. This process is associated with a group of rare, progressive neurodegenerative diseases in humans and animals, such as Creutzfeldt-Jakob disease and bovine spongiform encephalopathy, commonly called mad cow disease.

Unlike other infectious agents, prions contain no genetic material, neither RNA nor DNA, making them unique in the realm of infectious diseases. Their ability to propagate without nucleic acids challenges previous understandings of what is required for an infectious agent to multiply. Prions' resistance to standard sterilization procedures puts a significant emphasis on understanding their transmission to prevent iatrogenic spread of diseases.

Prions

Prions represent a class of proteins with the unique ability to transmit their misfolded shape onto normal variants of the same protein. It's this transmissible nature that enables prions to be infectious. The pathology of prion diseases, or transmissible spongiform encephalopathies (TSEs), involves the accumulation of these misfolded proteins, which eventually leads to cell death and tissue damage in the brain. Prions challenge the conventional definition of an infectious agent, as their mode of infection and propagation doesn't involve any nucleic acids, a characteristic ordinarily deemed essential for the replication of infectious agents.

Prions are resistant to many forms of decontamination and can persist in the environment, which makes them particularly hazardous. The study of prions has provided valuable insights into protein folding and misfolding, and their possible implications in other neurodegenerative disorders, like Alzheimer's and Parkinson's diseases.

Viroids

Vuroids are among the smallest infectious agents known, consisting solely of a short strand of circular RNA without any protein coating. They infect plant cells and can result in severe economic losses in agriculture due to crop damage. Viroids do not encode proteins; their pathogenicity is linked to their ability to interact with and disrupt the normal function of the host's RNA.

Through mechanisms still not fully understood, viroids can silence genes, interfere with RNA regulation, and evade plant defense systems. Understanding viroids is essential not only for plant pathology but also for insights into RNA interference, a process with significant implications in gene regulation and biotechnology.

Retroviruses

Retroviruses are RNA viruses that have a unique replication process. They carry an enzyme called reverse transcriptase, which converts their RNA into DNA once inside a host cell. This DNA is then integrated into the host's genome, allowing the virus to be replicated whenever the host cell's DNA replicates. HIV, the virus responsible for AIDS, is the most well-known retrovirus.

The lifecycle of retroviruses poses a challenge for treatment because the viral DNA becomes a permanent part of the host's genetic material. This integration is also the reason why retroviruses can cause cancer and why they have been used in gene therapy to introduce new genes into host organisms. The study of retroviruses has tremendously advanced our understanding of viral infections, the immune response, and the complex viral-host relationship at the molecular level.