Viruses, like bacteria, can be detected at every point on our planet. They are found in hot springs, at the bottom of the oceans and even in Arctic ice. The chemical composition of viruses has long surprised scientists, since it differs significantly from the composition of all previously known organisms: only viruses can store genetic information on the RNA matrix, transcribe DNA on the RNA matrix, and integrate their genome into the DNA of a living cell under the influence of integrase enzymes. In this article, we will analyze in detail the structure, genome, and chemical composition of viruses.
Virus Overview
Being outside a living cell, viruses do not show any signs characteristic of living organisms. Inactive viruses in this state are called virions. In the virion, there are no cellular organelles characteristic of cells of other living organisms - the plasma membrane, mitochondria, ribosomes, nucleolus, nucleus, and others. The virion includes a shell of protein subunits - a capsid, an extra shell, which not all viruses have - a supercapsid and a genome.
The genetic material of the virus is realized only when it enters a living cell. Depending on the type of viral genome, nucleic acids replicate either in the host cell DNA or in the mitochondria in the cytoplasm.
The structure of a simple virion
Simple virion consists of genetic material and the outer shell - the capsid. Capsid consists of protein subunits called capsomers. The way Caspomers are organized determines the spatial structure of the virus. The chemical composition of a capsid is represented by one or more types of proteins. The capsid form can be icosahedral (typical for adenoviruses), spiral (tobacco mosaic virus), or complex (found in proxiviruses and rhabdoviruses). Capsid can consist of either one or several types of proteins. Capsid subunits largely determine the morphology and chemical composition of viruses.
Capsid protects the genetic material of the virus from mechanical damage, the effects of temperature changes, pH, exposure to radiation and chemicals. The capsid together with the genome of the virus is called the nucleocapsid.
Complex virion structure
A complexly organized virion has an additional structure - the supercapsid membrane, which is located above the capsid.
The structure and chemical composition of viruses containing the supercapsid membrane is significantly different from the composition of simple viruses. The supercapsid membrane is formed from the cell membrane of the host cell and consists of 95% lipids and proteins. The supercapsid contains a small amount of glycoproteins, complex proteins in which the protein part is bound to the carbohydrate by covalent bonds.
Supercapsid, like capsid, has a protective function. The glycoproteins in the supercapsid are used to identify and bind to specific receptors on the surface of the host cell.
Viral proteins
Bacterial proteins can be capsid, supercapsid, or genomic. Capsid and supercapsid proteins have protective functions. Genomic proteins are covalently linked to the genome and form ribo- or deoxyribonucleoproteins with viral RNA or DNA molecules. These proteins are involved in the compaction of nucleic acids, as well as in repair, transcription and translation.
The chemical composition of viruses is complex. Especially diverse in their structure and composition are viral enzymes. Depending on the function performed, they are divided into two large classes:
- enzymes necessary for replication of the viral genome;
- enzymes that facilitate the penetration of viral nucleic acid into the cell and provide the subsequent exit of virions from the cell.
The first class of enzymes includes RNA and DNA-dependent RNA polymerase, DNA polymerase , reverse transcriptase, integrase, DNA beta-glycosyl transferase and many others.
The second class includes neuraminidase, which is part of glycoproteins, hemagglutinin esterase, endolysin and some others.
Viral lipids
Lipids are one of the main components of the chemical composition of viruses and are found in large quantities in the supercapsid membrane. A supercapsid is formed from the plasma membrane of the host cell; therefore, the composition of the lipid composition determines the chemical composition of this membrane. Viral lipids are mainly represented by phospholipids (50-60%) and cholesterol (20-30%), since it is precisely these lipids that are present in the largest quantities in the plasmalemma. Phosphoinositol may be present in trace amounts.
Lipids are an essential component of the composition of the supercapsid membrane. They contribute to the formation of the surface charge of the cell due to the charged groups in the phospholipids, and also give the supercapsid the flexibility necessary to withstand external mechanical damage. Lipids also serve as a good additional insulator for the genetic material of viruses in the event of sharp changes in temperature or acidity of the medium, and ensure the maintenance of a constant chemical composition of the cell. Due to the thick layer of lipids and proteins, supercapsid-coated viruses are more resistant to detergents than simple virions.
Carbohydrates in the virus
Carbohydrates in the virus are usually associated with lipids or capsid proteins (they are called glycolipids or glycoproteins, respectively). Glycoproteins form spiky outgrowths on the surface of the cell, which have the properties of hemagglutinins (cause agglutination of red blood cells) or destroy neuraminic acid, which is part of the cell walls, using neuraminidase.
Genetic material
The genetic material of viruses can be represented by either single or double stranded DNA, or single or double stranded RNA. More than any living organisms, RNA is not the main carrier of genetic information. DNA viruses replicate in the cell nucleus, since cell DNA polymerase is required for this process. RNA viruses replicate in the cytoplasm, on the ribosomes of the host cell.
There are viruses that can convert an RNA molecule into a DNA molecule using reverse transcriptase. The most famous representative of this class of viruses is the human immunodeficiency virus. The viral DNA molecule synthesized on the RNA matrix under the action of the integrase enzyme is built into the chromosome of the host cell and transcribed along with normal DNA regions.
Bacterial viruses: bacteriophages
Bacteriophages are special viruses, as they only infect bacterial cells. The structure and chemical composition of viruses and bacteriophages are very similar. However, the second has an additional process of fibrillar proteins. The genetic material of bacteriophages can be represented by both DNA and RNA.
The penetration of a bacteriophage into a bacterial cell leads to its lysis. Thus, bacteriophages regulate the size of the bacterial population. In addition, these viruses provide the genetic diversity of bacteria. Thanks to bacteriophages, the transduction process is carried out: fragments of a bacterial chromosome or plasmid are packed into the head of the bacteriophage, come out of its original bacterial cell and fed into another bacterial cell, where it is replicated. So in a bacterial cell gets a new genetic material for it.