Viruses are microscopic infectious agents without cellular structure. In a very basic way, viruses are made up of a protein capsule, which is called capsidenclosing one or more molecules of genetic material. There are viruses that contain RNA (ribonucleic acid), and there are viruses that contain DNA (deoxyribonucleic acid).

The main functions of the capsid are genome protection and interaction with host cells. Each virus infects a specific type of cell, and recognition is accomplished by interaction between the capsid and membrane receptors on the target cell.

A virus does not have any metabolic system of its own. It may infect a cell and hijacks its biosynthetic machinery. It is the infected cell that reads the genetic information of the virus, synthesizes new capsids and assembles them together with new copies of the viral genome.

Structure and types of capsids

The capsid is made up of numerous protein monomers, called protomerswhich join together to form the capsomeres. The capsomers are observable under the electron microscope and in them one can distinguish the three-dimensional structure that will determine the final morphology of the capsid, almost always icosahedral or helical, although some viruses have developed more complex structures, especially phages or bacteriophages, viruses that infect bacteria.

The capsid and genome together form the nucleocapsid. The nucleocapsid may be naked or surrounded by a lipid cover. The lipid cover is an element acquired from the membranes of infected cells, both from the cytoplasmic membrane and from the membrane of the organelles (Golig’s part, endoplasmic reticulum, etc). Lipid-enveloped viruses are known as “enveloped viruses.” HIV is an example of an enveloped virus.

In addition to structural proteins, other proteins with various functions can be attached to the capsid, especially adhesion proteins and cell recognition. Various proteins and enzymes can also appear inside the capsids, for example reverse transcriptase.

According to the symmetry and three-dimensional structure of the capsid, three groups can be distinguished:

  1. Icosahedral capsid
  2. helical capsid
  3. complex capsid

Icosahedral

Capsids with icosahedral structure are the most widespread among animal viruses. An icosahedron is a three-dimensional geometric figure with 12 vertices and 20 faceseach of them is an equilateral triangle with the same dimensions.

The capsomeres at the vertices are called pentones or pentamerswhile the capsomeres of the faces are called hexones or hexamers. The number and type of pentons and hexones give rise to a wide variety of icosahedral capsids.

The simplest icosahedral capsid would contain no hexones, only pentons located at vertices so close together that the penton itself forms part of the face, while other larger viruses may have several tens of hexones for each face of the icosahedron.

In general, viruses contain 12 pentons and 10(T – 1) hexones, being the number T representative of the size and complexity of the capsid. It has been found that all known viruses with T > 7 need helper proteins to assemble the capsid.

helical capsid

The helical-type capsid is common among plant viruses and bacteriophages. In this case, the capsomeres assemble and form a cylindrical structure with axial and helical symmetry.

Helical symmetry is determined by the formula P = μ * ρwhere μ is the number of structural subunits per turn, and ρ the axial increase per unit.

The best known helical virus is the tobacco mosaic virus. This virus consists of a single strand of RNA, and each capsid subunit binds internally to three nucleotides on this strand.

complex capsids

Some viruses have evolved a more complex structure than icosahedra and helices, with capsids having several structurally and functionally different parts. Complex structures are especially common among bacteriophages.

Complex capsid viruses have two, three or more parts:

  • Head: formed by a nucleocapsid of icosahedral symmetry or by a hexagonal prism and two hexagonal pyramids, one at each upper and lower end.
  • Tail: It is a protein sheath of highly variable complexity, from a simple hollow tube to a filament with a helical structure and contractile parts. Some viruses have a neck between their head and tail, and there are also some with a base plate at the end of the tail from which structures similar to spikes or spines start.
  • Paws: binding proteins to anchor to the membrane of the target cell, perforate it and inject the genetic material.

Origin and evolution

Analysis of protein and capsid structure is used, along with many other criteria, to classify viruses. For example, adenovirus, a type of virus that infects mammals, including humans, would belong to the same family as bacteriophage PRD1, which infects various gram-negative bacteria, and Chlorella Virus 1, which infects paramecium Paramecium bursaria.

It has been suggested in several studies that viral capsid proteins have evolved from highly diverse cellular proteins. Viruses would have been capturing proteins from their host cells at different stages throughout evolution, some even before cellular organisms separated into the three main domains of today: bacteria, animals, and plants, hence there are some widely distributed proteins. distributed among all viruses, even among viruses that infect extremely different and evolutionarily very distant organisms, while other proteins appear only in a small group of viruses.

Viruses are the most abundant biological entities on Earth and show great genetic diversity and replication strategies, but their origin and evolution is still the subject of intense debate. Among the possible theories of its origin:

  • Cell regression theory: the viruses would have originated from cells that parasitized other cells; These parasitic cells would have relegated more and more metabolic and reproductive tasks to their host cells until they were left with only the genetic information and a protective envelope.
  • Cellular-molecular origin theory: This theory would explain the origin of viruses as fragments of DNA or RNA from previous cells that somehow managed to replicate themselves autonomously.
  • Coevolution theory: the origin of life seems to start from organic macromolecules capable of replicating; at some point in their evolution they acquired lipid membranes and the first cells were formed. Viruses could come from macromolecules that never came to participate in the cell structure and its evolutionary line.