An average human being is made up of about 3.2 billion nucleotide pairs of genetic material and an average flu virus is made up of about 14,000 nucleotides. But the viroid, the newly interpreted viral structure, is made up of only 400 nucleotides, making it one-eightieth the size of a virus, according to plant pathologists in Spain.
Nucleotides are the building blocks of genetic material: DNA and RNA. Most living things contain both types of genetic material, double-stranded DNA and single-stranded RNA. Both types are essential to the “central dogma” of biology, which says that DNA encodes information to make RNA. RNA encodes information for protein production, leading to a specific trait.
Viroids do not fall into any previously known categories. According to an evolutionary biologist at Spain’s National Research Council in València, viroids are simply exposed loops of RNA. This makes them very different from viruses that contain genetic material packaged by proteins. Despite this, there are elements of the viroids that make them pathogenic, such as the effects that the known forms can have on crops. In fact, viroids were discovered because of a shriveling potato crop in New Jersey during the 1920s. At this point, more than 30 viroid forms have been found to affect domestic plants.
Despite this early discovery, discussion of the implications of the viroid has only recently become prevalent. Scientists believe that extremely early forms of life may have contained nothing but RNA. The lack of proteins and DNA may seem like an issue for the central dogma, but at a very simplistic level, RNA could carry out the chemical reactions needed for life — that is, until the new and improved DNA and protein model comes along.
Based on this theory, any organisms that relied solely on RNA would have become obsolete and eventually extinct. However, according to the current issue of Annual Reviews of Microbiology, the viroid may be a leftover specimen of the RNA-ruled age, having existed in their original form for billions of years. Evidence to support this claim includes the way that the viroid spreads from plant to plant through a “wound,” even one inflicted by pruning shears, and gets the plant to copy the viroid genetic material. The new RNA strand then cuts itself free and loops into a new viroid. Experts have hypothesized that interactions would have looked like this during the RNA era, so the viroids are consistent with the theory.
The new questions that arise deal with how the viroid could have remained so unchanged since the days of early life. Many studies linking viroids to crops have been done, but scientists want to branch into wild plants to discover a root for the viroid way of life. One possible area of study would involve finding out how viroids came to rely on plants as a life source. This question, and others, will continue to be addressed, but the recent discussion and discoveries about the viroid will certainly lead to new debates on the structure of primitive life on Earth.