MICR 271- The Baltimore System of Virus Classification PDF

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MICR 271: The Baltimore System of Virus Classification Q: Why do dsDNA viruses need to go through a different process to generate mRNA than ssRNA viruses? Viral Genome Replication - The type of genome that a virus has dictates its life cycle, including how it replicates. We will consider a few examples of viral replication. Viruses are grouped using the “Baltimore” system – based upon the characteristics of viral genome types The Baltimore System of Virus Classification - the Baltimore system of grouping viruses, developed by Nobel laureate David Baltimore, relies on the viral genome type and results in 7 basic groups - ICTV (International Committee on Taxonomy of Viruses) also classifies viruses differently, and considers additional characteristics - Considering viral replication using the simplified Baltimore system makes sense since the genome replication strategy is dictated by the genome type. - Here are the different classifications: - Class 1, 2, and 7 are DNA-based, while classes 3, 4, 5, and 6 are RNA-based Class 1: dsDNA Genome - mRNA synthesis and genome replication follows the same process as the dsDNA of the host mRNA synthesis: Transcription of dsDNA with host’s machinery and translation of mRNA into proteins Genome replication: Replication of dsDNA genome with host’s machinery Es: T4 bacteriophage

Class 2: ssDNA genome - ssDNA genome can be a (+) or (-) strand. The genome strategy is similar for both types of genomes. First step is producing dsDNA using the ssDNA genome as a template mRNA synthesis: the dsDNA product is transcribed into mRNA with host machinery Genome replication: dsDNA is produced and replicated with host machinery and then the two strands are separated, generating the ssDNA genome, which is (+) or (-) ssDNA Ex: Parvovirus

Class 7: Reverse Transcribing (RT) dsDNA genome - partially dsDNA genome is first filled in and then used to make an RNA template. This RNA template is used in the RT process to produce ssDNA during the replication of this type of DNA virus mRNA synthesis: dsDNA is transcribed with the viral RNA polymerase to produce mRNA transcripts which are then translated into viral proteins (one of the proteins made is a specialized DNA polymerase) Genome replication: (+) ssRNA template is reverse transcribed into ssDNA with the viral reverse transcriptase. The ssDNA strand is then completed to produce new partially dsDNA genomes Ex: Hepatitis B virus

Class 3: dsRNA genome mRNA synthesis: the dsRNA is separated into two single strands of mRNA. The (+) ssRNA can be used as mRNA, which is translated into viral proteins Genome replication: The (+) ssRNA can also be used as a template to produce new dsRNA genomes Ex: Rotavirus

Class 4: (+) ssRNA genome mRNA synthesis: The (+) ssRNA genome can be used as mRNA that can be translated into proteins Genome replication: (+) ssRNA is used to produce (-) ssRNA, which is then used as template to produce new (+) ssRNA genomes Ex: SARS-CoV-2

Class 5: (-) ssRNA genome mRNA synthesis: (-) ssRNA genome is used as template to produce (+) ss mRNA, which is translated into proteins Genome replication: (-) ssRNA is used to produce (+) ssRNA, which is used as template to produce new (-) ssRNA genomes. Ex: Influenza viruses

Class 6: Retroviruses - From the RNA genome, ssDNA is reverse transcribed, and dsDNA is made with DNA polymerase. This dsDNA is then integrated into the host genome mRNA synthesis: the dsDNA is transcribed into mRNA, which is then translated into viral proteins Genome replication: (+) ssRNA genome is transcribed into ssDNA by viral reverse transcriptase. dsDNA is produced from the ssDNA and then integrated into the host genome with the viral integrase. (+) ssRNA is transcribed from the integrated dsDNA to produce new (+) ssRNA genomes Ex: HIV...