IV. VIRUSES
D. CLASSIFICATION OF VIRUSES
Fundamental Statements for this Learning Object:
1. Viruses can store their genetic information in six different types of nucleic acid which are named based on how that nucleic acid eventually becomes transcribed to the viral mRNA.
2. (+) and (-) strands of nucleic acid are complementary. Copying a (+) stand gives a (-) strand; copying a (-) stand gives a (+) strand.
3. Only (+) strands of viral RNA can be translated into viral protein.
4. Regarding the enzymes involved in nucleic acid replication, the "dependent" part of the name refers what type of nucleic acid is being copied. The "polymerase" part of the name refers what type of nucleic acid is being synthesized.
LEARNING OBJECTIVES FOR THIS SECTION
Viruses are infectious agents with both living and nonliving characteristics.
1. Living characteristics of viruses
a. They reproduce at a fantastic rate, but only in living host cells.
b. They can mutate.
2. Nonliving characteristics of viruses
a. They are acellular, that is, they contain no cytoplasm or cellular organelles.
b. They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery. In other words, viruses don't grow and divide. Instead, new viral components are synthesized and assembled within the infected host cell.
c. The vast majority of viruses possess either DNA or RNA but not both.
Classification of Viruses
Viruses can store their genetic information in six different types of nucleic acid which are named based on how that nucleic acid eventually becomes transcribed to the viral mRNA (see Fig. 1) capable of binding to host cell ribosomes and being translated into viral proteins.
In the diagrams below, (+) and (-) represent complementary strands of nucleic acid. Copying of a (+) strand by complementary base pairing forms a (-) strand. Only a (+) viral mRNA strand can be translated into viral protein. Regarding the enzymes involved in nucleic acid replication, the "dependent" part of the name refers what type of nucleic acid is being copied. The "polymerase" part of the name refers what type of nucleic acid is being synthesized, e.g., DNA-dependent RNA-polymerase would synthesize a strand of RNA complementary (def) to a strand of DNA. These six forms of viral nucleic acid are:
a. (+/-) double-stranded DNA (see Fig. 2) . To replicate the viral genome, DNA-dependent DNA polymerase enzymes copy both the (+) and (-) DNA strands producing dsDNA viral genomes. To produce viral mRNA molecules, DNA-dependent RNA polymerase enzymes copy the (-) DNA strand into (+) viral mRNA. The (+) viral mRNA can then be translated into viral proteins by host cell ribosomes. Examples include most bacteriophages, Papovaviruses, Adenoviruses, and Herpesviruses.
b. (+) single-stranded DNA (see Fig. 3). To replicate the viral genome, DNA-dependent DNA polymerase enzymes copy the (+) DNA strand of the genome producing a dsDNA intermediate. DNA-dependent DNA polymerase enzymes then copy the (-) DNA strand into ss (+) DNA genomes. To produce viral mRNA molecules, DNA-dependent RNA polymerase enzymes copy the (-) DNA strand into (+) viral mRNA. The (+) viral mRNA can then be translated into viral proteins by host cell ribosomes. Examples include Phage M13 and Parvoviruses.
c. (+/-) double-stranded RNA (see Fig. 4) . To replicate the viral genome, RNA-dependent RNA polymerase enzymes copy both the (+) RNA and (-) RNA strands of the genome producing a dsRNA genomes. To produce viral mRNA molecules, RNA-dependent RNA polymerase enzymes copy the (-) RNA strand into (+) viral mRNA. The (+) viral mRNA can then be translated into viral proteins by host cell ribosomes. Reoviruses are an example.
d. (-) RNA (see Fig. 5). To replicate the viral genome, RNA-dependent RNA polymerase enzymes copy the (-) RNA genome producing ss (+) RNA. RNA-dependent RNA polymerase enzymes then copy the (+) RNA strands producing ss (-) RNA viral genome. The (+) mRNA strands also function as viral mRNA and can then be translated into viral proteins by host cell ribosomes. Examples include Orthomyxoviruses, Paramyxoviruses, Rhabdoviruses.
e. (+) RNA (see Fig. 6). To replicate the viral genome, RNA-dependent RNA polymerase enzymes copy the (+) RNA genome producing ss (-) RNA. RNA-dependent RNA polymerase enzymes then copy the (-) RNA strands producing ss (+) RNA viral genome. To produce viral mRNA molecules. RNA-dependent RNA polymerase enzymes copy the (-) RNA strand into (+) viral mRNA. The (+) viral mRNA can then be translated into viral proteins by host cell ribosomes. Examples include Picornaviruses, Togaviruses, and Coronaviruses.
f. (+) RNA Retroviruses (see Fig. 7). To replicate the viral genome, reverse transcriptase enzymes (RNA-dependent DNA polymerases) copy the (+) RNA genome producing ss (-) DNA strands. DNA-dependent DNA polymerase enzymes then copy the (-) DNA strands to produce a dsDNA intermediate. DNA-dependent RNA polymerase enzymes then copy the (-) DNA strands to produce ss (+) RNA genomes. To produce viral mRNA molecules, DNA-dependent RNA polymerase enzymes copy the (-) DNA strand into (+) viral mRNA. The (+) viral mRNA can then be translated into viral proteins by host cell ribosomes. Retroviruses, such as HIV-1, HIV-2, and HTLV-1 are examples.
Table of Selected Viral Families, Viruses, and Species Affected
© Kristine Edwards, Anna Rovid-Spickler and Glenda Dvorak, authors, Licensed for use, ASM MicrobeLibrary.
The Table below describes some of the medically important viruses.
1. single-stranded
DNA; naked; polyhedral capsid
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Last updated: Feb., 2020
Please send comments and inquiries to Dr.
Gary Kaiser