Viroids are small, circular single-stranded RNA molecules. They are the smallest known pathogens, ranging in size from the Coconut cadang-cadang viroid (246 nucleotides) to the Citrus exocortis viroid (375 nucleotides).

Unlike a virus, the extracellular form of a viroid does not have a proteinaceous coat or capsid, it is naked RNA. Even more startling the RNA molecule does not contain code for any protein. The viroid is totally dependent on its host for replication.

Viroids are sometimes considered to be escaped introns and, like self-splicing introns, appear to be remnants of the RNA world.

Biology of Microorganisms. Brock et al. 7th Edition. Prentice Hall International, 1994
Viroids are the smallest of the small in the continuum of self-replicating entities we might call life, the ultimate parasites, taggers on, and lazy ne'er do wells. They're not content with the bulkiness of being a virus, which requires you to actually go to the trouble of coding for a couple of proteins to turn a cell's machinery to your whim, and a few more to make a capsid to get yourself around in. They're certainly not going to go to all the effort of creating a full fledged cell with it's own mechanisms for replication.

Viroids are simple, circular loops of RNA a couple of hundred base pairs long. Now, "proper" RNA in a eukaryotic cell is not circularized, and is often hundreds of thousands of base pairs long. Here is where the viroid finds its niche. The viroid has a promoter telling the cell that it's a piece of RNA that needs to be copied. The cell dutifully begins copying, but not expecting circular RNA, just keeps going around and around and around the viroid. Eventually, it'll decide to stop, creating a really long antisense RNA strand of the viroid. When this strand gets transcribed back, it contains a whole lot of copies of the original viroid, all one right after the other. But the viroid is a ribozyme, each copy capable of cutting itself out of the long chain into unit length, and then looping itself back into a circle.

Viola! The simple viroid has made thousands of copies of itself without ever once being translated into a protein, or in fact coding for anything. Truly a masterwork of subtlety.

The only problem is getting the viroid into the nucleus to be replicated in the first place. Most of them take the whole trick one step further and hijack viruses to get themselves about. This flavor of viroid is called a virusoid or satellite virus. Viroids are small enough that they can get picked up in the virus's capsid and are taken along when the virus infects a new cell.

There's just something about a tiny little fleck that manages to infect a virus.

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