Friends, in a recent VERITAS(VERITAS: Virus, 6 Mar 2003 http://unvarnished-veritas.posterous.com/virus) we had explored the world of viruses. At that time I had thought that viruses were on the borderline between the living and the non-living. I was wrong! I recently read about viroids. These are much much simpler and smaller than viruses.
So today I am of the opinion that viroids are the borderline between the living and the non-living.
How were viroids discovered? In 1950s it was beleived that all infections were caused either by bacteria or by virus. In 1962 Raymer and Brein were looking for a possible virus behind the potato spindle-tuber disease. They took some infectious material from an infected plant and centrifuged it at 10,000 times the force of gravity for 4 hours! But they could not seperate the “virus” from the infectious material. THis could only mean one thing:
this virus is much much smaller/lighter than any virus man has ever known!
Later experiments showed that these “viruses” not only sedement on centrifugation slower than normal viruses but also slower than the DNA or the RNA that these viruses contain! So this means that these “viruses” were nothing more than tiny strands of DNA or RNA.
We know that viruses contain DNA/RNA surrounded by a protein coat. These newly discovered creatures were only tiny strands of RNA.
Nothing else! They were named viroids. How small are they? A typical viroid is about 1/10 the size of the smallest virus. And a typical viroid contains much less genetic material. Scientists found that the genetic material contained by a viroid is much less then the genetic material required to take over the machinery of the host cell and reproduce. We had seen in the earlier VERITAS that the virus just has enough material to take over a cell’s machinery. It uses the host cell’s genetic material to reproduce itself. The viroids dont even have this minimal machinery!!! Then how do viroids reproduce? This question is still a mystery!
It is now known that several plant diseases are caused by viroids.
Most of these diseases are related to a stunted growth in some part of the plant. Nobody knows why viroids cause only this type of a disease. Some scientists believe that viroids cause a imbalance in a the growth harmone.
It is also unknown why viroids cause disease in one kind of plant and in another variety they multiply without showing any symptom.
Viriods come in two variaties. The linear strand and the circular strand. In the circular strand type the viroid RNA’s one end is tied to the other end. This was the first time scientists saw a circular strand of DNA or RNA!
Lets compare the sizes of viroids with the other microorganisms.
The smallest bacteria is about .2 micrometres big. The smallest virus is about .02 micrometres ( 200 nanometres) . A viroid is about 10 nanometres in size!!!
And then there are even smaller disease causing agents, the prions. These are even smaller: about 5 nms. But you cannot really call them living because they are just protein molecules that clog a cell. But they are deadly: they are the cause of the mad cow disease!
So the borderline between living and nonliving is still unclear. Maybe we would need to modify our definition of life itself to understand this better.
Go, wondrous creature! mount where Science guides:
Go, measure earth, weigh air, and state the tides:
Instruct the planets in what orbs to run,
Correct old time and regulate the Sun;