A virus is a protein shell carrying genetic material that infects a host cell to make more viruses.
All viruses have genetic material made of nucleic acid. The capsid, or protein envelope, of a virus, is made up of many protein molecules which encloses the genomic material. Capsid proteins are always encoded by the virus genome (which is made of RNA and DNA), meaning that it’s the virus (not the host cell) that provides instructions for making them. Capsid proteins can come in many different shapes. In addition to the capsid, some viruses also have an external lipid membrane known as an envelope, which surrounds the entire capsid.
Viruses with envelopes do not provide instructions for the envelope lipids. Instead, they “borrow” a patch from the host membranes on their way out of the cell. Envelopes do, however, contain proteins that are specified by the virus, which often help viral particles bind to host cells. Although envelopes are common, especially among animal viruses, they are not found in every virus (i.e., are not a universal virus feature).
Practice Questions
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Key Points
• A virus is an infectious particle that reproduces by taking over a host cell and using its machinery to make more viruses.
• A virus is made up of a DNA, or RNA genome inside a protein shell called a capsid. Some viruses have an external membrane envelope.
• Viruses are very diverse. They come in different shapes and structures, have different kinds of genomes, and infect different hosts.
Key Terms
capsid: protein shell of a virus that contains the genetic material
envelope: an external lipid membrane surrounding the capsid that some viruses have
protein envelope: viruses have viral envelopes as their outer layer typically derived from portions of the host cell membranes
genome: the whole of its genetic information
lipid membrane: a group of compounds which form the double-layered surface of all cells