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Frequency 8 7 6 5 4 Frequency 3 2 1 0 46 54 60 64 68 71 75 82 1 RNA VIRUS REPLICATION STRATEGIES 2 Objectives • Classes of RNA genomes Baltimore Classification • Different strategies used by RNA viruses • RNA replication and mRNA synthesis • Initiation, elongation and termination of replication • Examples 3 4 5 6 RNA VIRUS STRATEGIES RNA -> RNA RNA-dependent RNA polymerase RNA -> DNA RNA-dependent DNA polymerase - reverse transcriptase Host cell DNA -> RNA DNA-dependent RNA polymerase 7 ALL ANIMAL RNA VIRUSES CODE FOR A POLYMERASE 8 Need to make mRNA PLUS (POSITIVE) SENSE RNA GENOMES (+ve) sense mRNA AAA 9 Need to make mRNA MINUS (NEGATIVE) SENSE RNA GENOMES (+ve) sense mRNA AAA (-ve) sense genomic RNA 10 Need to make mRNA MINUS (NEGATIVE) SENSE RNA GENOMES RNA polymerase must be packaged in virion. (+ve) sense mRNA If used, RNA modifying enzymes are packaged in virion. AAA (-ve) sense genomic RNA 11 Need to make mRNA DOUBLE-STRANDED RNA GENOMES RNA polymerase must be packaged in virion. (+ve) sense mRNA If used, RNA modifying enzymes are packaged in virion. AAA Double-stranded genomic RNA 12 Need to make mRNA RETROVIRUSES Reverse transcriptase must be packaged in virion. DS DNA DS DNA +VE RNA 13 RNA viruses that do NOT have a DNA phase Genome RNA-dependent Infectivity of RNA RNA polymerase (=transcriptase) IN VIRION Initial event in cell Plus-stranded RNA No Infectious Translation Negativestranded RNA Yes Non-infectious Transcription Doublestranded RNA Yes Non-infectious Transcription 14 RETROVIRUSES Genome Plus-stranded RNA RNAdependent DNA polymerase (=reverse transcriptase) in virion Infectivity of RNA Initial event in cell Yes Non-infectious Reverse transcription 15 16 GENOME SIZE • • • • • POLYMERASE ATTACHMENT PROTEIN CAPSID PROTEIN RELATIVELY FEW OTHER PROTEINS VIRION SIZE MAY BE QUITE LARGE BUT LIMITED NUMBER OF PROTEINS 17 The monocistronic mRNA problem mRNA AAAAAAAA RIBOSOMES PROTEIN AAAAAAAA • Make one monocistronic mRNA per protein • Make a primary transcript and use alternative splicing • Make a large protein and then cut it into smaller proteins • Include special features in the mRNA which enable ribosomes to bind internally 18 19 20 21 22 23 GENOME SIZE • TAUTOMERIZATION • PROOF READING 24