Virus Classification Structure and Reproduction

In this virus classification structure and reproduction post we have briefly explained about virus classification, viral structure and reproduction.

Virus Classification Structure and Reproduction

Viruses are obligate intracellular parasites that exist in a no-man’s-land between the living and non-living worlds, exhibiting traits from both. They are now recognised to differ dramatically from bacteria, the simplest genuine organisms, in a variety of ways.


1. They cannot be observed using a light microscope

2. They have no internal cellular structure they contain either DNA or RNA, but not both.

3. They are incapable of replication unless occupying an appropriate living host cell

4. They are incapable of metabolism

5. Individuals show no increase in size.

When viruses are inside a host cell, they exhibit some of the characteristics of a living thing, such as the ability to replicate themselves, but when they are outside the cell, they are inert chemical viral structure, fueling the argument over whether they should be considered life forms.

A virus’s host range is restricted, which means it can only infect certain cell types. Nobody knows how viruses developed.


Viruses, as we’ve seen, aren’t exactly living organisms, and classifying them is a difficult task. Viruses contain species, genera, families, and orders, just like real creatures, but none of the higher classifications (class, phylum, kingdom).

Traditional biological taxonomy does not employ Latin binomials (e.g., Homo sapiens, Escherichia coli); however, a proposal for non-Latinised viral binomials has been made.

Initially, no attempt was made to infer any form of phylogenetic link between the viruses, but recent advances in viral genome sequencing have resulted in new findings in this field. Factors taken into account in the classification of viruses include: Host range, Morphology and Genome type/mode of replication.

David Baltimore presented a method in 1971 that arranged the viruses according to the mRNA synthesis processes.

As a result, there are seven major groupings. The International Commission on Taxonomy of Viruses (ICTV, founded in 1973) released a report in 2005 that recognised three orders, 73 families, 287 genera, and more than 1900 virus species. There are a slew of others that haven’t been discovered or aren’t well-characterized.

Viral Structure

Wendel Stanley’s demonstration in 1935 that a tobacco mosaic virus preparation could be crystallised demonstrated the relative chemical uniformity of viral structure, implying that they could not be conceived of in the same way as other living organisms.

Viral structure have an extremely simplistic compared to even the most primitive biological entity. An entire viral particle, or virion, consists of only two components: a nucleic acid core encircled and protected by a protein coat orcapsid, and the nucleocapsid, which is the combination of the two.

In certain types of viruses, the nucleocapsid is further surrounded by a membranous envelope, partly derived from host cell material. Most viral structure are smaller than even the smallest bacterial cells.

Viral Genome Structure

A virus’s genetic material can be RNA or DNA, and viral genome structure can be single-stranded or double-stranded in either case. Certain RNA viruses, such as the influenza virus, have an additional variation in the viral genome: instead of residing as a single molecule, it is segmented into many fragments, each of which may encode a different protein.

Because the segments of some plant viruses are present in different particles, reproduction requires a number of virions to co-infect a cell, complementing each other (multipartite genomes), The segmented form of double-stranded RNA is always present.

The viral genome structure varies widely; it can have as little as four genes or as many as 200. These genes can code for both structural and non-structural proteins, which include viral replication enzymes like RNA/DNA polymerases.

The two types of single-stranded RNA viral genome structure are (+) sense and () sense RNA. The former can function as mRNA, bind to ribosomes, and be translated into the appropriate proteins within the host cell.

As a result, it is contagious in and of itself. Minus () sense RNA, on the other hand, is only infectious in the presence of an RNA polymerase-active capsid protein.

As previously stated, this is required to convert the (-) RNA into its complementary (+) strand, which subsequently serves as a template for protein creation.

The most viral genome structure is double-stranded (dsDNA), however some smaller ones, such as parvoviruses, contain single-stranded (ssDNA).

Capsid structure

The protein coat, also known as the capsid, determines the form of a viral particle. The capsid is the outermost covering of non-enveloped viruses and is responsible for adhering the virus to a host cell’s surface.

It also protects nucleic acid against damaging environmental effects like UV light and desiccation, as well as acid and degradative enzymes found in the gastrointestinal system.

The capsid is made up of a number of components termed capsomers, each of which may include one or more protein kinds. For each viral type, the number of capsomers remains constant.

The minimal amount of protein-encoding RNA/DNA in the viral viral genome structure necessitates this repeating subunit assembly.

The capsomers have the ability to spontaneously engage with one another to form the completed capsid through a self-assembly process. If there were a huge number of different protein kinds, this would be very difficult to accomplish.

Capsomers are stacked symmetrically, resulting in the icosahedral and helical capsid geometries. Either encased or non-enveloped viruses can have both forms. Complex viruses, such as bacteriophages, possess helical and icosahedral symmetry features.

Helical capsids

When seen under an electron microscope, a variety of plant viruses, including the well-studied tobacco mosaic virus, exhibit a rod-like shape.

A helical arrangement of capsomers produces a tube or cylinder with space in the centre for the nucleic acid element, which fits into a groove on the inside.

The composition of the protein(s) that make up the capsomers determines the helix’s diameter, while the size of the nucleic acid core determines its length.

Capsids with an icosahedral shape A regular three-dimensional form with 20 triangular faces and 12 points or corners is known as an icosahedron. Overall, the capsids viral structure has a roughly spherical appearance.

The viral envelope

An enveloped Virus

Animal viruses have a considerably higher prevalence of envelopes than plant viruses. The lipid bilayer that protects an enveloped virus is derived from a prior host’s nuclear or cytoplasmic membrane.

However, proteins (typically glycoproteins) encoded by the virus’s own genome are embedded in this. These may protrude from the virion’s surface as spikes, which may aid the virus’s ability to adhere to or infiltrate its host cell.

The envelope is more vulnerable to environmental stresses than the capsid, therefore the virus requires moisture to survive. As a result, such viruses are spread through bodily fluids such as blood (e.g. hepatitis B virus) or respiratory secretions (e.g. influenza virus).

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