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The capsid faces may consist of one or more proteins. For example, the foot-and-mouth disease virus capsid has faces consisting of three proteins named VP1–3. [6] Some viruses are enveloped, meaning that the capsid is coated with a lipid membrane known as the viral envelope.
The nucleocapsid of the virion is made up of the nucleic acid and the capsid. Remember that the genomes of most viruses are very small. Genes code for instructions to make proteins, so small genomes cannot code for many proteins. Therefore, the virion capsid consists of one or only a few proteins that repeat over and over to form the structure.
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. [1] Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. [2] [3] Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity.
Some viruses are simple ribonucleoproteins, containing only one molecule of RNA and a number of identical protein molecules. Others are ribonucleoprotein or deoxyribonucleoprotein complexes containing a number of different proteins, and exceptionally more nucleic acid molecules.
The DNA or RNA of viruses consists of either a single strand or a double helix. [28] Viruses can reproduce rapidly because they have relatively few genes. For example, influenza virus has only eight genes and rotavirus has eleven. In comparison, humans have 20,000–25,000.
Virus classification is the process of naming viruses and placing them into a taxonomic system similar to the classification systems used for cellular organisms. Viruses are classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause.
The capsid/nucleocapsid is round with triangulation number T=16 and exhibits icosahedral symmetry. The capsid is isometric and has a diameter of 100–110 nm. [2] The capsid consists of 162 capsomer proteins with a hexagonal base and a hole running halfway down the long axis. The core consists of a fibrillar spool on which the DNA is wrapped.
The capsomere is a subunit of the capsid, an outer covering of protein that protects the genetic material of a virus. Capsomeres self-assemble to form the capsid. [1] In this diagram of an Adenovirus, the capsid molecules are clearly visible. Subunits called protomers aggregate to form capsomeres. Various arrangements of capsomeres are: 1 ...