Anatomy of the Virus

In addition to being the smallest forms of life, viruses are also the most efficient, with the minimum amount of substance and genetic material needed to maintain their existence. Because viruses rely on host cells to provide the complex processes that support life, they need to store only a little information in their genetic material. The smallest recognized viruses, the aforementioned viroids and virusoids, are nothing more than a single strand of DNA or RNA, sometimes only 200-300 nucleotides long. A typical virus might contain only about 9000 nucleotides (a few complex viruses can have up to 100,000 or more), whereas human DNA is over 3 billion nucleotides long . Said another way, a typical virus's DNA could be abbreviated in one-letter sequences in the space of a few pages of this web site - a human's would comprise more than 500,000 pages.

DNA is most frequently found as two strands wound together into the familiar double helix shape. However, some viruses can have only a single strand of DNA. In addition, viruses are the only organisms which can use RNA to contain their genetic information - all other classes of organisms use DNA. These viruses are known as the RNA viruses - the consequences of this will be made clear in a subsequent section dedicated to the RNA viruses. The gene functions of viruses basically fall into two categories - those which carry out a particular stage in the virus's life cycle (cell penetration, latency, or replication), and those used to copy its genetic material and repackage it in new viruses.

Almost all viruses (excepting the viroids and virusoids) surround their DNA or RNA with a protective coat, known as a capsid, which is built from simple proteins. The function of the capsid is to shield the sensitive genetic material from attack or alteration, and to help the virus locate and infect a host.

A 3D model of the icosahedral capsid of the rhinovirus - the virus which causes the common cold

Capsomeres attract each other in a certain pattern. The capsomeres of the tobacco mosaic virus form into a spiral.

Capsomeres self-assemble to form the cylindrical shell of the tobacco mosaic virus.

Electron micrograph showing viruses aggregating into a crystalline pattern inside a cell.

The virus's genetic information contains information for building the proteins which comprise the capsid. Individual proteins fit together like the pieces of a puzzle to form a building block called a capsomere.

Capsomeres are designed to attract each other and fit together in a certain way. When enough capsomeres are brought together, they self-assemble into a completed capsid that partially or fully encapsulates the virus's DNA or RNA. The assembled particle is known as a nucleocapsid.

Virus's capsids occur in a variety of shapes and sizes. The tobacco mosaic virus uses capsomeres that stack together in an ascending spiral, forming a hollow cylinder with the genetic material contained inside. Most viruses' capsomeres (including hepatitis C) assemble into icosahedral (twenty-sided) crystalline forms that form a sphere encapsulating the genetic material. These viruses are particularly efficient in their form; the icosahedral shape is the most efficient shape that can be built from the smallest capsomeres, conserving host cell energy for the production of viruses.

More complex viruses like hepatitis C surround their nuclecapsid with an additional envelope of lipid (fatty material), extruding only a few of their viral proteins through the envelope. The completed package is called a virion or viral particle - a mature, infective virus.

Even complex viruses are extremely small - most human viruses are less than 150 nanometers in diameter. Hepatitis C is only about 50 nanometers in diameter. A nanometer is one billionth of a meter - if you could put 200,000 Hepatitis C viruses end to end, they would be only a single centimeter long.