What is the difference between a virus and a viroid?
Question
The other day I came across a paper explaining how viruses and viroids are fundamentally different, especially in structure and replication. Turns out, viruses are complex particles with protein coats and sometimes envelopes, while viroids are tiny, protein-free RNA molecules that only infect plants. I found it really interesting how differently they operate inside their hosts.
Answer ( 1 )
Structural Complexity
Viruses consist of genetic material (DNA or RNA) enclosed in a protein coat called a capsid, and many also have a lipid envelope. This allows them to exist as stable particles outside the host and be transmitted between hosts. In contrast, viroids are much simpler, made of a small, circular, single-stranded RNA molecule without any protein coat or envelope. Their RNA folds into a rod-like shape using internal base pairing. Viroids are the smallest known infectious agents, with sizes ranging from 246 to 401 nucleotides.
Genetic Composition
Viral genomes can be DNA or RNA, single- or double-stranded, and may be linear or circular. They encode proteins necessary for replication, structure, and interaction with host cells. Viroids have only RNA genomes, which are non-coding and entirely dependent on host cell factors for function. These RNAs include specific structures that allow interaction with host enzymes for replication and movement.
Replication Mechanisms
Viruses usually encode at least some replication enzymes and hijack host machinery to make proteins and assemble new virus particles. The typical viral cycle includes attachment, entry, uncoating, synthesis, assembly, and release. Viroids replicate using host RNA polymerase II, despite it normally transcribing DNA. This replication occurs through a rolling-circle mechanism, either in the nucleus (Pospiviroidae) or chloroplasts (Avsunviroidae), and does not involve capsid formation.
Host Range and Tropism
Viruses infect all domains of life, including bacteria, archaea, and eukaryotes, typically with high host specificity. Viroids, however, are plant-specific, primarily infecting angiosperms. No viroid has been confirmed in animals, fungi, or prokaryotes, indicating a unique dependence on plant cellular machinery.
Pathogenicity
Viruses can cause disease via cell lysis, apoptosis, immune responses, or metabolic changes, often through the expression of viral proteins. Viroids, lacking protein-coding genes, affect host plants by interacting directly with host RNA or by triggering RNA silencing pathways. Symptoms like stunting and leaf distortion result from disrupted plant development rather than cell destruction.
Evolutionary Origins
Viruses exhibit vast diversity, possibly originating independently from cellular components or as reduced cellular parasites. Viroids are considered possible relics of the “RNA world,” an ancient period when RNA molecules were the primary genetic material. Their structure resembles certain catalytic RNAs and introns.
Classification
Viruses are classified into numerous families and orders by the International Committee on Taxonomy of Viruses (ICTV), based on genome type, structure, and replication strategies. Viroids fall into only two families:
Understanding how viruses and viroids differ reveals the complexity and diversity of infectious agents. It also sheds light on the blurred line between living and non-living biological entities.
Source: Annual Review of Phytopathology; Viroids and Satellites (book)