Features of viruses
Viruses are infectious agents that can pass through bacterial filters, having a very simple structure. A virus is a nucleoprotein macromolecule that has the basic properties smallest of an organism
Viruses are the smallest infectious agents (20-300 nm in diameter) that can pass through bacterial filters, having a very simple structure. The virus is a nucleoprotein macromolecule that has the basic properties of an organism, but is not capable of self-reproduction, has no cellular structure, has no metabolism, and can be considered as an intermediary between viruses. living and inanimate matter.
Viruses differ from other microorganisms in the following features:
Viruses contain only one type of nucleic acid: either DNA or RNA, never both.
Viruses reproduce by replicating their only genetic material, not dividing by binary fission like bacteria.
Viruses are obligate parasites in living cells, they rely on energy sources and cellular machinery (eg ribosomes, transport RNA...) to synthesize proteins.
Viruses synthesize their components separately and then self-assemble to form new viral particles.
Viruses are not sensitive to common antibiotics.
Virus size, shape, and structure
Viruses are very small in size and can pass through bacterial filters. Therefore, the virus can only be observed through an electron microscope.
The unit for measuring the size of viruses is nanometer (nm): 1 nm = 1/1000 micrometer
Each virus has a certain size (from 20 to 300 nm) and does not change throughout the development process.
Most viruses have a certain shape, which is specific to each virus species.
Some common types of viruses:
Spherical: influenza virus, measles, polio.
Polyhedron: Adenovirus, Papovavirus.
Stick figure: tobacco mosaic virus.
Brick: smallpox virus.
Drumstick figure (pin): phage T2 of E.coli.
Viruses have a very simple structure, without a cell structure. All viral particles have two basic components: nucleic acid, which carries the viral genetic code, and the capsid, the protein sheath surrounding the nucleic acid. Nucleic acid core and capsid combine to form nucleocapsid. For some viruses, the nucleocapsid is also surrounded by a shell of lipids or lipoproteins called the envelope (envelope or peplos).
Viral nucleic acids
Each virus particle has one of two types of nucleic acids, either DNA or RNA. Nucleic acid located in the middle of the viral particle forms the core or genome of the virus.
Viral DNA molecules are mostly in the form of double-stranded DNA and a few are in the form of single-stranded DNA such as Parvoviridae. The majority of viral RNA molecules are single-stranded RNA, except for a few that are two-stranded RNA such as Reoviridae.
Nucleic acids makeup only 1-2% of the molecular weight of a virus particle, but have a particularly important function:
Nucleic acids carry the entire genetic code that is specific to each virus.
Nucleic acid determines the infectivity of the virus in the receptor cell.
Nucleic acids determine the viral replication cycle in the receptor cell
Nucleic acids are virus-specific antigens.
The capsid is the structure surrounding the nucleic acid core. The chemical nature of capsid is protein. Capsids are made up of morphological units called capsomeres, which are composed of protein molecules whose arrangement is specific to each virus. The capsomeres are arranged in a definite spatial order giving rise to the types of capsid symmetry: either torsion symmetry or bulk symmetry or complex symmetry.
The virus capsid has the following functions:
The protein coat protects the viral nucleic acid.
The capsid protein is a virus-specific antigen.
Capsid plays an important role in the attachment and cell invasion phase of the virus.
Capsid always keeps the shape and size of the virus stable.
Viruses such as Herpesviridae, Flaviviridae, Orthomyxoviridae, Paramyxoviridae,... also have an extra coat of capsid called envelope or peplos. The chemical nature of the outer shell is a complex of lipids, proteins, and carbohydrates. The outer envelope is derived from the cytoplasmic or nuclear membrane of the host cell but has been modified by the virus and has virus-specific antigenic properties. The outer shell can be destroyed by lipid-soluble solvents (such as ether, bile salts, ...)
The outer shell of the virus has the following functions:
Involved in virus attachment to receptor cells
Involved in the assembly and release of the virus from the cell after the replication cycle.
Viruses that do not have an envelope are called naked viruses.
On the envelope of some viruses, there are protruding protein spikes that may have specific functions such as agglutination of red blood cells or enzyme neuraminidase activity.
Viruses do not have a complete metabolic enzyme system like bacteria, but in the structure of some viruses, there are several proteins with enzymatic activity. The most common are polymerases such as RNA polymerase, DNA polymerase, RNA-dependent DNA polymerase (reverse transcriptase) ...
The complete viral particle capable of infecting the receptor is called a virion. Depending on the virus species, virions can have either a bare capsid or an enveloped capsid.
The viral particle that receives the host cell's genetic material during replication in place of the viral nucleic acid is called a pseudo virion. These pseudovirions, when viewed under the electron microscope, are identical to normal virions, but they are not infectious and cannot multiply. These particles can transfer cellular genes from one host cell to another.
Viroids are a small infectious agent that causes disease in plants and possibly in some slow viral infections of animals. This agent has only nucleic acids (closed-loop RNA molecules, molecular weight 70,000-120,000) without a structural protein layer.