Wednesday, October 29, 2014

Viroids: a look at the smallest of the small and the origins of life

(shamefully stolen, but can’t find the original source.)

Last week in class we discussed 4 possible sources for viruses:
     1.  Viruses were present in primordial soup
           2. Viruses emanate from space (Fred Hoyle)
       3. Viruses assimilated from parts of other cells
       4.     Viruses are degenerate cells

The review“Viroids: Survivors from the RNA world?” from Annual Review of Microbiology, elaborates on the first possibility of sources for (plant) viruses and the rest of life. The article looks at the evidence from viroids and their close relatives to better understand their origins and the origins of life.  In addition to viroids, the review also discusses plant viroid-like satellite RNAs, retroviroids, and hepatitis delta virus (HDV), which have the smallest genome of animal viruses.

 According to this article, the consensus is that RNA emerged before DNA and proteins. DNA and proteins eventually took over life on earth, and relegated RNA to intermediate functions.  But are there survivors from this first RNA world?  

(more shameful stealing,
Primordial Soup – better known as the RNA World? 

Theories and thoughts on origins of viroids: 
       1.     Viroids are escaped introns. However, viroids do not seem to be simplified version of viruses or transposons because of their distinct reaction mechanisms. Additionally, viroids, such as PSTVd, weren’t found in the cellular DNA of normal plants.
       2.     Viroids are surviving relics of a precelllar evolution in an RNA World.

Most of the article’s evidence affirming viroids as the survivors of precellular ancestors relies on demonstrating that they could fit in an RNA world. 

Evidence for viroids as relics of an RNA World:
·        Viroids have small genomes and are nonprotein-coding RNAs.
  The underlying idea is the simplest is the oldest.
·        High G+C content and circularity
  The underlying idea is that this increases replication fidelity helps the genome replicate   completely, meaning greater stability.  The RNA world is possible!
·          A number of viroids have structural periodicity.
   The underlying idea is that in addition to being associated with protein binding, structural      periodicity may have allowed larger genomes to evolve. Modular evolution would have allowed for    high mutation rates, which would have shortened the evolutionary time required to make a complex    structure, and possibly allow for complex structures that may have not been directly selected.    Complex life could have come from an RNA world!
·         Some viroids have hammerhead ribozymes, the “finger print of the RNA world.”
   The underlying idea is that the small size and relatively low complexity of hammerhead ribozymes    make viroid replication in a pre-protein world possible.


Is such a thing what actually happened? The jury is still out.

Pithy quotes from the review:
“Thus viroids are essentially transcriptional parasites, whereas viruses are essentially translational parasites. “

By Olivia

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