Context: Scientists at Stanford University have reported another extremely simple and unusual form of life using next-generation sequencing. When analysing genetic material from bacteria present in the human gut, the scientists identified a new form of life lying between viruses and viroids on the scale of simplicity. They called them ‘obelisks’.
Viruses:
- Viruses are the organisms at the boundary between the living and the non-living. Their obligate host dependence, parasitism, and small genome sizes collectively made sure they are not classified as ‘life’ per se.
- Each virus is composed of a nucleic acid (DNA or RNA) core that serves as the genetic material, surrounded by a protein coat, and, in some cases, a lipid (fat) layer outside that coat.
- Viruses’ life cycle is simple: they infect a host cell, use the cell’s machinery to make more copies of themselves, then infect a new cell to repeat the cycle.
Viroids:
- In 1971, Theodor Diener, a plant pathologist in the U.S. Department of Agriculture’s Research Center in Maryland tried to isolate the pathogen that caused potato spindle tuber disease. He discovered Viroids.
- Viroids do not contain the lipid layer or the protein coat found in viruses. It appears to be just plain, naked RNA. This viroid would enter a cell as RNA, force the cell to make more copies of itself, and the new RNAs would then infect other cells. These life-forms are called 'viroids’ since they resembled viruses.
- However, viroids have one important distinction from Viruses. Usually, genetic material contains a code that tells cells how to make various proteins. This is true of all known organisms, including viruses. But the RNA of viroids do not code for any protein. For the most part, they were just small pieces of RNA that served no function apart from propagating themselves.
- Viroid RNA was tiny (250-400 base pairs versus a few thousand in RNA viruses). They primarily infect plants, causing a variety of diseases characterised by stunted growth, leaf distortion, and other symptoms. They are transmitted through various means, including mechanical transmission via contaminated tools, seed transmission, pollen, and insect vectors. Examples of viroids include the Potato spindle tuber viroid (PSTVd), the Citrus exocortis viroid (CEVd), and the Coconut cadang-cadang viroid (CCCVd).
Obelisks:
- Obelisks and viroids, both have circular RNA for genomes. However, the obelisk RNA was much longer — around a thousand base pairs — and appeared to code for two proteins, neither of which bore any similarity to any known protein from any other life form.
- Further research is being continued to answer some questions regarding obelisks. For example, how do they make copies of their genome? How do they transmit? Are they pathogenic to bacteria? How did they evolve? Do they have roles to play in human health and disease?
