Preparation of notes during examinations preparation is one of the most tedious tasks, but key of my happiness is that it always give me some daunting clues about facts which are not provided in books. While I was preparing for my mid-terms at university. I came to know a great correlation in infection causing viruses size with other components. It is non-statistical analysis, performed on very limited number of viruses. But damn interesting.
Sizes of medically important viruses
These viruses are well-known and volumes of literature is available on all of them. Their sizes vary from 10nm to 380nm. But important thing to consider is that most of viruses concentrate from 50-100nm sizes. So viruses at other both extremes are very very less in number, but rare means something unique in all. So, I will be discussing them one by one.
These viruses are in range of 50-100nm diameter, with Herpes virus and Pox virus as exceptions which are moderately larger than this range. Herpes and Pox viruses are different from other viruses in a sense that their sizes are very much variable as compared to other DNA viruses. Most of DNA viruses have size variation of about 10nm e.g Adenoviruses (70-80nm), Papovaviridae (45-55nm), while Herpes virus and Pox viruses are ranging from 120-150nm and 170-260nm respectively. Exceptions like Hepadna-viruses are discrete 42nm particles with partially double stranded genomes and associated reverse transcriptase enzyme.
RNA viruses are categorized into (i) ssRNA and (ii) dsRNA viruses. RNA viruses which are single stranded can be of +ve polarity (sense) or -ve polarity, based upon their ability to translate directly or possession of RNA trancriptase association to RNA genome.
When we check RNA viruses, we find them in all ranges of sizes. They are as small as 20nm diameter (Parvoviruses) and highly large viruses such as Rhabdoviridae. It was a thing of interest that +ve sense RNA viruses were relatively smaller in size as compared to -ve sense RNA viruses. For example, viruses of Flaviviridae are largest to all in +ve sense viruses, which vary in size from 40-70nm (although coronavirus is also larger than this but its exception in all). While viruses of dsRNA are also present at this upper limit, Reoviridae members vary in size from 60-80nm, almost same size as of Adenoviruses of ds-DNA viurses. As, size among RNA viruses start increasing from 80nm, virus type completely shifts to -ve sense, size variability begin to increase which become highest at Rhabdoviridae. Rhabdoviridae members are ranging from 130-380nm in size in one dimension. Another exception at this extreme of size is that genomes of Arenaviridae members can be linear or circular and these are the only viruses which have ribosomes in side nucleocapsid.
Size: A direct impact on viral symmetry
Most remarkable finding which was observed is that as sizes increase viruses start to be asymmetrical. Smallest viruses are very fine with ico-sahedral symmetry. And this thing was found common in all viruses including DNA and RNA. The largest viruses which were observed with icosahedon were Adenoviruses among DNA viruses and Reoviridae members of RNA viruses. Then, helical symmetry begins, which was optimistically ended at 180nm. Coronaviridae members are largest and most variable members with helical symmetry. Now, irregular shaped viruses with complex dimension appear. They are two which are Pox virus and Rhabdovirus. Rabdoviruses are bullet shaped having length (130-380nm) and diameter (50-95nm).
Viral envelop is size dependent
This was observed that viruses with smaller sizes were without any kind of envelop and outermost covering was capsid protein. As sizes continue to increase, envelop begins to incorporate in viral structures. Envelop was found to impart asymmetry in structure and viruses were begin to be more variable in shapes as compared to smaller sized viruses. It was also observed that all viruses which have historical importance such as Herpes and Pox Viridae members contain an envelop outside their nucleocapsid, which is usually host derived.