By ¹Obiri Darko Stella, ²Sackey Lyanne, and ³Kwakye Sylvester.
Viruses are obligate, intracellular parasites that live in infected cells and produce virions to disseminate their genes. They cannot reproduce by themselves and must commandeer a host cell’s machinery to synthesize viral proteins and, in some cases, to replicate the viral genome.
The definitive properties of viruses can be summarized as:
• A virus is an infectious, obligate intracellular parasite.
• The viral genome comprises DNA or RNA.
• The viral genome directs the synthesis of viral components by cellular systems within an appropriate host cell.
• Infectious progeny virus particles, called virions, are formed by de novo self-assembly from newly synthesized components.
Note: A progeny virion assembled during the infectious cycle is the vehicle for transmission of the viral genome to the next host cell or organism, where its disassembly initiates the next infectious cycle.
RNA viruses, which usually replicate in the host-cell cytoplasm, have an RNA genome, and DNA viruses, which commonly replicate in the host-cell nucleus, have a DNA genome. Viral genomes may be single- or double-stranded, depending on the specific type of virus. The entire infectious virus particle, called a virion, consists of nucleic acid and an outer shell of protein that both protects the viral nucleic acid and functions in the process of host-cell infection.
NB: The simplest viruses contain only enough RNA or DNA to encode four proteins; the most complex can encode some two hundred proteins.
Classification of Viruses
Four characteristics used in the classification of all viruses:
1. Nature of the nucleic acid in the virion (DNA/RNA)
2. Symmetry of the protein shell (capsid)
3. Presence or absence of a lipid membrane (envelope)
4. Dimensions of the virion and capsid
The most commonly-used system of virus classification was developed by David Baltimore in 1971 which bases on classifying viruses according to their genome and replicating strategy. The Baltimore Classification System initially included six(6) classes of viruses. Nevertheless, a seventh(7th) class was added to lodge the gapped DNA genome of Hepadnaviridae.
The Baltimore Classification System Of Viruses
Class I: Double stranded DNA (dsDNA) viruses
A double stranded DNA virus enters the host nucleus before it begins to replicate. It makes use of the host polymerases to replicate its genome, and is therefore highly dependent on the host cell cycle. The cell must therefore be in replication for the virus to replicate. Examples of Class I viruses are Herpesviridae, Iridoviridae, Asfarviridae, Poxviridae, Polyomaviridae, and Adenoviridae
Class II: Single stranded DNA (ssDNA) viruses
Most ssDNA viruses have circular genomes and replicate mostly within the nucleus by a rolling circle mechanism. Examples of Class II viruses are Allenoviridae, Circoviridae, and Parvoviridae.
Class III: Double stranded RNA (dsRNA) viruses
Double stranded RNA viruses replicate in the core capsid in the host cell cytoplasm and do depend as heavily on host polymerases as DNA viruses. The genomes of Class III viruses may be segmented, and unlike viruses with more complex translation, each gene codes for only one protein. Examples of Class III viruses include Reoviridae and Birnaviridae.
Class IV: Positive-sense single stranded RNA ((+)ssRNA) viruses
Class IV ssRNA viruses have positive-sense RNA genomes, meaning they can be directly read by ribosomes to translate into proteins. They are further divided into viruses with polycistronic mRNA and those with complex transcription. Polycistronic mRNA is translated into a polyprotein that is subsequently cleaved to form separate proteins. Viruses with complex transcription use ribosomal frameshifting and proteolytic processing to produce multiple proteins from the same gene sequences. Examples of Class IV viruses are Hepeviridae, Togaviridae, Nodaviridae, Coronaviridae, Picornaviridae, Flaviviridae, Arteriviridae, and Astroviridae.
Class V: Negative-sense single stranded RNA ((-)ssRNA) viruses
Class V viruses have a negative-sense RNA genome, meaning they must be transcribed by a viral polymerase to produce a readable strand of mRNA. The genomes of Class V viruses may be segmented or non-segmented. Examples of Class V viruses are Bornaviridae, Bunyaviridae, Filoviridae, Rhabodviridae, Arenaviridae, Orthomyxoviridae, and Paramyxoviridae.
Class VI: Positive-sense ssRNA reverse transcriptase viruses
Group VI viruses have a positive sense, single-stranded RNA genome, but replicate through a DNA intermediate. The RNA is converted to DNA by reverse transcriptase and then the DNA is spliced into the host genome for subsequent transcription and translation using the enzyme integrase. Examples are Retroviridae.
Class VII: Double stranded DNA (dsDNA) reverse transcriptase viruses
Class VII viruses have a double-stranded DNA genome, but unlike Class I viruses, they replicate via a ssRNA intermediate. The dsDNA genome is gapped, and subsequently filled in to form a closed circle serving as a template for production of viral mRNA. To reproduce the genome, RNA is reverse transcribed back to DNA. Examples are Hepadnaviridae.
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