Summary
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.
Glossary
acellular
lacking cells
capsid
protein coating of the viral core
capsomere
protein subunit that makes up the capsid
envelope
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
virion
individual virus particle outside a host cell
virus core
contains the virus genome