Viruses can rightly be called the true owners of our planet. Wherever scientists carry out research, they stumble upon already known viruses or discover new ones that can survive at extremely low and high temperatures, in acidic or alkaline environments. Viruses have a unique ability that is not characteristic of any other living organisms - to store genetic information on RNA. What are viruses made of? How do they invade a living cell? How do they replicate their genetic material?
Virus Overview
A virus outside a living cell does not show any signs of life. Such a particle is called a virion. Virions are simple and complex. Simple consists of the outer shell (capsid) and the genetic material of the virus (DNA or RNA). Complex viruses have one more envelope, supercapsid, to protect the genome from adverse factors. The capsid consists mainly of proteins and a small amount of lipids - cholesterol and phospholipids. The supercapsid also contains glycoproteins, which are directly involved in the penetration of the virus into the cell. Viruses multiply by expressing their genome in an infected cell.
We study the features of the structure of viruses in more detail.
Capsid structure
The virion particle is protected from external influences by a dense shell - a capsid. This is the structure by which the virus genome is protected from many damage before it enters the cell. In addition to proteins and lipids, enzymes are present in the shell. Capsid consists of capsomeres - protein molecules that can be distinguished through a microscope.
The genome and the protein coat protecting it are collectively called the nucleocapsid.
Supercapsid
Simple viruses are protected from external factors only by a capsid, while complex viruses have an additional lipoprotein membrane - supercapsid. Like a capsid, it consists mainly of proteins and lipids.
On the surface of supercapsids of many viruses there are glycoprotein growths (peplomeres). Glycoproteins are involved in the identification of cells and the binding of the virus to the target cell. Then the viral envelope merges with the host membrane, while the capsid and the viral genome penetrate into the cell and integrate into it. Supercapsid is formed at the stage of assembly of viral particles and the exit of the virus from the cell.
Capsid function
The main function of supercapsid and capsid is to protect the genetic material of the virus. As well as the delivery of the virus genome to the cell and interaction with the host’s immune system.
The outer shell of the virus is designed to protect genetic material from lethal chemical and physical factors. These include radiation, sudden changes in pH or temperature, the action of proteolytic and nucleolytic enzymes.
Upon delivery of the virus genome into the cell, the capsid (or supercapsid) binds to the external receptors of the host cell. On the surface of viruses having a supercapsid, peplomers are responsible for the adsorption of the virus on the surface of the host cell. On the surface of the peplomeres of some viruses, for example, myxoviruses or influenza virus, there is a hemagglutinin protein that causes red blood cell agglutination. On the surface of supercapsid processes, neuraminidase, which destroys the cell membrane, is also present.
Viruses without a supercapsid membrane typically penetrate the host cell membrane completely. The remaining viruses enter the cell after the fusion of the outer membrane with the plasma membrane. In this case, only the viral genome enters the cytoplasm of the cell.
Capsid Symmetry Types
Since the capsid is a complex structure consisting of small subunits, it is obvious that these structural components can be organized in different ways. That is why there are various types of capsid symmetry.
One of the most common capsid types in nature is icosahedral. It is characteristic of adenoviruses and many bacteriophages. Capsomers are arranged so that they form a non-convex polyhedron with 12 vertices.
Another type of capsid is spiral. With this type of symmetry, protein subunits are arranged as if in a spiral around the axis of symmetry. Spiral capsid is characteristic of jaundice and tobacco mosaic viruses. Such an organization makes the virus rod-shaped.
A mixed type of symmetry, in which part of the capsid has an icosahedral shape, and part spiral, occurs in bacteriophages.
Virus genome
Unlike most cells that contain both DNA and RNA, virions contain only one nucleic acid, which forms their genome. Most human viruses are RNA-containing. The ability to store genetic material on RNA is a unique feature of viruses.
The virus may contain single-stranded or double-stranded nucleic acid. Double-stranded DNA in the genome are herpes viruses and adenoviruses, single-stranded parvoviruses, double-stranded RNA rotaviruses, single-stranded RNA astroviruses.
Some viruses, such as HIV, contain an enzyme that allows you to build DNA on an RNA matrix — reverse transcriptase.
Forms of viruses
Viruses are very diverse, both in their chemical composition and in form. Most viruses (herpes, smallpox) have a spherical shape. The virus capsid in this case has icosahedral symmetry. Rod-shaped viruses (jaundice, tobacco mosaic) are also common . Bullet-like viruses (rabies virus) are found. Unusual are star-shaped astroviruses.