Capsid

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Capsid

For the leaf bug, see Miridae
Miridae

The large and diverse insect family Miridae contains the plant bugs, leaf bugs, and grass bugs, and may also be known as capsid bugs....
.

A capsid is the protein shell of a virus. It consists of several oligomer

Oligomer

In chemistry, an oligomer consists of a limited number of monomer units , in contrast to a polymer which, at least in principle, consists of an unbounded number of monomers....
ic structural subunits made of protein

Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
, called protomer

Protomer

In structural biology, a protomer consists of one single protein subunit or several different subunits, that assemble in a defined stoichiometry to form an oligomer....
s; at the same time the 3-dimensional morphological subunits that can be observed, which may or may not correspond to individual proteins, are called capsomere

Capsomere

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For the leaf bug, see Miridae
Miridae

The large and diverse insect family Miridae contains the plant bugs, leaf bugs, and grass bugs, and may also be known as capsid bugs....
.

A capsid is the protein shell of a virus. It consists of several oligomer

Oligomer

Protein

Protomer

Capsomere

Capsomeres make up the capsid, an outer covering of protein that does not protect the genetic material of a virus.Capsomere: protein subunits of the capsid. They are self-assembled to form the capsid....
s. The capsid encloses the genetic material of the virus.

Capsids are broadly classified according to their structure. The majority of viruses have capsids with either helical or icosahedral structure. Some viruses, such as bacteriophages, have developed more complicated structures. The icosahedral shape, which has 20 equilateral triangular faces, approximates a sphere

Sphere

A sphere is a symmetrical geometrical object. In non-mathematical usage, the term is used to refer either to a round ball or to its two-dimensional surface....
, while the helical shape is cylindrical. The capsid faces may consist of one or more proteins. For example, the foot-and-mouth disease

Foot-and-mouth disease

Foot-and-mouth disease or hoof-and-mouth disease is a infectious disease and sometimes fatal virus disease of cloven-hoofed animals, including domestic animals such as cattle, Domestic water buffalo, Domestic sheep, goats and pigs, as well as antelope, bison and other wild Bovidaes, and deer....
virus capsid has faces consisting of three proteins named VP1-3.

Some viruses are enveloped, meaning that the capsid is coated with a lipid membrane known as the viral envelope
Viral envelope

Many viruses have viral envelopes covering their protein capsids. The envelopes are typically derived from portions of the host cell membranes , but include some viral glycoproteins....
. The envelope is acquired by the capsid from an intracellular membrane in the virus' host; some examples would include the inner nuclear membrane, the golgi

Golgi

Golgi may refer to the physician Camillo Golgi or one of the structures named after him:*Golgi apparatus in some cells*Golgi tendon organ...
membrane, or the cell's outer membrane

Cell membrane

The cell membrane is the interface between the cellular machinery inside the cell and the fluid outside.It is a semipermeable lipid bilayer found in all cell ....
.

Once the virus has infected the cell, it will start replicating itself, using the mechanisms of the infected host cell. During this process, new capsid subunits are synthesized according to the genetic material of the virus, using the protein biosynthesis

Protein biosynthesis

Protein synthesis is the process in which cell build proteins. The term is sometimes used to refer only to protein translation but more often it refers to a multi-step process, beginning with amino acid synthesis and transcription which are then used for translation ....
mechanism of the cell. During the assembly process, a portal subunit is assembled at one vertex of the capsid. Through this portal, viral DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetics instructions used in the development and functioning of all known living organisms and some viruses....
or RNA

RNA

Ribonucleic acid is a type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nucleobase, a ribose sugar, and a phosphate....
is transported into the capsid. The structure and assembly of the herpes virus capsid portal protein has been imaged via cryo-electron microscopy

Cryo-electron microscopy

Electron cryomicroscopy is a form of electron microscopy where the sample is studied at cryogenic temperatures . CryoEM is developing popularity in structural biology....
.

It is this capsid or protein shell which makes protective vaccines a possibility. Structural analyses of major capsid protein (MCP) architectures have been used to categorise viruses into families. For example, the bacteriophage PRD1, Paramecium bursaria Chlorella algal virus, and mammalian adenovirus have been placed in the same family.