16.1: What are Viruses?
A virus is a microscopic infectious particle that consists of an RNA or DNA genome enclosed in a protein shell. It is not able to reproduce on its own: it can only make more viruses by entering a cell and using its cellular machinery. When a virus infects a host cell, it removes its protein coat and directs the host’s machinery to transcribe and translate its genetic material. The hijacked cell assembles the replicated components into thousands of viral progeny, which can rupture and kill the host cell. The new viruses then go on to infect more host cells.
Why Study Viruses?
Viruses can infect different types of cells: bacteria, plants, and animals. Viruses that target bacteria, called bacteriophages (or phages), are very abundant. Current research focuses on phage therapy to treat multidrug-resistant bacterial infections in humans. Viruses that infect cultivated plants are also highly studied since epidemics lead to huge crop and economic losses.
Viruses were first discovered in the 19th century when an economically-important crop, the tobacco plant, was plagued by a mysterious disease—later identified as Tobacco mosaic virus. Animal viruses are of great importance both in veterinary research and in medical research. Moreover, viruses underlie many human diseases, ranging from the common cold, chickenpox, and herpes, to more dangerous infections like yellow fever, hepatitis, and smallpox.
The Structure of a Virus
Viruses come in a variety of shapes that are specialized in attacking their target cell. The two major components of all viruses are the viral genome and its protective protein coat, known as the capsid. The viral genome is made up of single or double-stranded RNA or DNA, and it encodes the proteins that make up the capsid. Together, the viral genome and the capsid are known as the nucleocapsid.
A unique feature of many eukaryotic viruses is the presence of a phospholipid membrane, known as the envelope that surrounds the capsid. This envelope typically originates from the membranes of previously infected host cells, but can also include viral proteins (called envelope proteins) attached to it. Finally, some animal viruses have a cluster of virus-encoded proteins, the viral tegument, in the space between the envelope and capsid.
The viral life cycle can be broken into the following five steps: attachment, entry, replication, assembly, and release. The proteins on the surface of the virus help it recognize specific host cells. Some viruses use these surface proteins to bind host cell receptors and initiate internalization by endocytosis, while envelope-coated viruses can directly fuse with the host cell membrane.
Some bacteriophages do not enter the cell; they inject their genome (and viral enzymes) into the host cell. Once inside the cell, the virus is uncoated and directs the machinery of the host cell to transcribe and translate its genome. The host cell packages the new copies of the viral genome into viral particles to make progeny. The progeny viruses may be stored in the host cell before release or continually extruded from the cell by budding off from the cell membrane. The viral infection cycle is classified as lytic or lysogenic. In the lytic cycle, the new viruses burst out of the host cell thus killing it. In the lysogenic cycle, the viral DNA is incorporated into the host genome where it lays dormant and is copied each time the host cell replicates.