virus classification

baltimore classification

• what does it divide according to

• divides all viruses into seven groups according to their mechanism of replication and the type of nucleic acid present in the virion

what is described as the positive sense

the RNA strand (messenger sense)

what must occur before the translation of virus proteins

the genome of negative sense RNA viruses must be copied to make positive sense RNA

what do all RNA viruses replicate via

• a double stranded RNA intermediate

• but retroviruses are an exception. they use an rna:dna intermediate in their replicaiton instead f dsRNA.

what is the difference in group 3-5 in baltimore

which rna replicative intermediate is present in the virus particle

group 1 replication

• the host DNA dependent RNA polymerase can directly bind to the viral dsDNA template to transcribe mRNA

• virus uses host DNA dependendent DNA polymerase to make exact copies

group 2 replication

• host enzyme converts ss to a dsDNA intermediate

• then replication as 1

replication in group 3

• virus has RdRp inside which transcribes each genome segment separately

• these are used to to synthesise viral proteins and serve as templates for new dsRNA

replication in group 4

• acts directly as mRNA

• used by host ribosomes to translate viral proteins including rdrp

• the newly made rdrp copies the original positive strand to create a complementary negative strand creating a dsDNA intermediate

• the negative strand serves as a template to synthesise more positive

replication in group 5

• viral RdRp transcribes the negative sense genomic RNA into positive sense mRNA

group 6 replication

• virus carries reverse transcriptase whcih synthesises single strand of DNA complementary to the viral RNA

• the enzymes ribonuclease activity breaks down the original RNA strand

• reverse transcriptase synthesises a second complemenary DNA strand forming dsDNA

replication in group 7 viruses

• initial dna genome has a gap in one strand

• host enzymes repair te gap to form ciruclar dna

• this is transcribed into mRNA