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.
DNA viruses
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
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.
what about plant viruses?
ReplyDeletePlant viruses are also included in each these families, based upon there genome and capsid symmetries. They are as diverse as Animal viruses. In introductory paragraph I set the limitats that I will focus on Only Medically important viruses. These were 20 Families and Sub-Families as a whole.
ReplyDeletea very informative article and a very good effort indeed. it is showing how intellectual the author is n he is really having command over what he read and have good comprehensive abilities as evident by his intellectualism of correlating such things very cleverly which we usually don't consider to be acquainted of.
ReplyDeleteThank you Mehvish... It is a thing of great honor that this article was read by you. Your encouragements are caudally acknowledged by author...
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