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Summarizing Viral Evolution, Morphology, and Classification


Viruses are tiny, acellular entities that can usually only be seen with an electron microscope. Their genomes contain either DNA or RNA—never both—and they replicate using the replication proteins of a host cell. Viruses are diverse, infecting archaea, bacteria, fungi, plants, and animals. Viruses consist of a nucleic acid core surrounded by a protein capsid with or without an outer lipid envelope. The capsid shape, presence of an envelope, and core composition dictate some elements of the classification of viruses. The most commonly used classification method, the Baltimore classification, categorizes viruses based on how they produce their mRNA.



lacking cells


protein coating of the viral core


protein subunit that makes up the capsid


lipid bilayer that envelopes some viruses

group I virus

virus with a dsDNA genome

group II virus

virus with a ssDNA genome

group III virus

virus with a dsRNA genome

group IV virus

virus with a ssRNA genome with positive polarity

group V virus

virus with a ssRNA genome with negative polarity

group VI virus

virus with a ssRNA genomes converted into dsDNA by reverse transcriptase

group VII virus

virus with a single-stranded mRNA converted into dsDNA for genome replication

matrix protein

envelope protein that stabilizes the envelope and often plays a role in the assembly of progeny virions

negative polarity

ssRNA viruses with genomes complimentary to their mRNA

positive polarity

ssRNA virus with a genome that contains the same base sequences and codons found in their mRNA

replicative intermediate

dsRNA intermediate made in the process of copying genomic RNA

reverse transcriptase

enzyme found in Baltimore groups VI and VII that converts single-stranded RNA into double-stranded DNA

viral receptor

glycoprotein used to attach a virus to host cells via molecules on the cell


individual virus particle outside a host cell

virus core

contains the virus genome

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