![]() RNA-dependent RNA polymerases and reverse transcriptases are unique to viruses since the host cell does not require the RNA replication or reverse transcription. Thus, all four possible types of polymerases, i.e., RNA-dependent RNA polymerases, RNA-dependent DNA polymerases, DNA-dependent RNA polymerases, and DNA-dependent DNA polymerases, are found in viruses. Hepatitis B virus is a dsDNA virus that replicates through an RNA intermediate using a virally encoded reverse transcriptase. Retroviruses are ssRNA viruses that replicate through a DNA intermediate, thus requiring a virus-encoded RNA-dependent DNA polymerase (reverse transcriptase). ![]() Reverse-transcribing viruses replicate using reverse transcription, a process for making DNA from RNA templates. RNA viruses replicate and transcribe their genomes using RNA-dependent RNA polymerases. Single-stranded RNA genomes can be either plus (+) sense or minus (−) sense. RNA viruses have RNA genomes, which can also be either double-stranded (dsRNA) or single-stranded (ssRNA). DNA viruses replicate their genomes using DNA-dependent DNA polymerases (also called DNA polymerases) and transcribe mRNA using DNA-dependent RNA polymerases (also called RNA polymerases). Note that many eukaryotic dsDNA viruses do not encode a viral polymerase and instead use cellular enzymes in the nucleus for viral genome transcription and replicationĭNA viruses use DNA as their genetic material, which is generally double-stranded (dsDNA) but may also be single-stranded (ssDNA). ![]() Viral plus-sense RNA can serve as an mRNA and does not require modification for transcription. The steps that viral polymerases catalyze are indicated with RNA-dependent RNA polymerase (RdRp), reverse transcriptase (rt), DNA-dependent DNA polymerase (DdDp), and DNA-dependent RNA polymerase (DdRp). Viral genome transcription and replication steps are shown as blue and black arrows, respectively. The seven virus classes are DNA (ssDNA and dsDNA), RNA (plus-sense (+) ssRNA, minus-sense (−) ssRNA, and dsRNA), and reverse-transcribing (ssRNA-reverse transcribing (rt) and dsDNA-rt) viruses. Figure 12.1 summarizes the seven classes of viruses along with their mode of polymerase usage.īaltimore virus classification scheme based on genome type and method of mRNA synthesis ( Baltimore 1971). Based on the chemical nature of the viral genome, the need for mRNA synthesis, and the particular genome replication strategy used, viruses are grouped into seven different classes ( Baltimore 1971). In contrast, some small DNA viruses that spend their entire life cycle in the nucleus do not need to encode their own polymerase, instead relying entirely on host cell machinery for genome replication and mRNA synthesis. Viruses that spend their entire life cycle in the cytoplasm do not have access to host polymerases and thus need to encode their own polymerases for transcription and replication. The requirement for a virus-encoded polymerase for either transcription and/or replication depends on the cellular location of genome replication and the nature of the viral genome, i.e., whether it is DNA or RNA, double-stranded or single-stranded, or whether it is plus- or minus-sense. Viruses must then synthesize identical copies of their genomes for encapsidation into newly assembled virus capsids. Viruses first need to convert their genomes into mRNA that can be translated into viral proteins. ![]() Several steps in the virus life cycle require the activity of a polymerase. ![]()
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