Genetics of Viruses

I. MICROBIAL GENETICS

I. Genetics of Viruses

LEARNING OBJECTIVES FOR THIS SECTION

Genetics 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, the "dependent" part of the name tells what type of nucleic acid is being copied. The "polymerase" part of the name tells what type of nucleic acid is being synthesized, eg, 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 transtated 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 transtated 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 transtated 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 transtated 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 transtated 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 transtated into viral proteins by host cell ribosomes. Retroviruses, such as HIV-1, HIV-2, and HTLV-1 are examples.

QUIZ YOURSELF ON THIS SECTION

RETURN TO UNIT 6 TABLE OF CONTENTS