The General Structure of Viruses

Virus Overview

Viruses represent a class of microscopic contagious agents that can multiply within their host organism. They need the cooperation of host cells and cannot replicate alone. The general structure of viruses is made up of either an RNA or DNA genome surrounded by a protein covering named the capsid. Virion structure represents an entire virus particle. Viruses can either be enveloped or naked in nature. In enveloped viruses, the nucleoprotein and the RNA or DNA strand are protected within a protein code called the nucleocapsid. 

Types of Viruses

Based on physical appearance, we can differentiate a certain group of viruses. According to morphological features, four distinct classes of viruses co-exist. 

• Filamentous virus – The general structure of viruses of this kind is easily recognized by their long, cylindrical body. Some examples are the Tobacco mosaic virus (TMV), Ebola virus, Sulfolobus islandicus filamentous virus (SIFV), etc.
• Isometric (or Icosahedral) virus – This virion structure is roughly spherical. The numerical presence of the capsomeres acts as a guide for the identification of this virus. Examples are – poliovirus, rhinovirus, and adenovirus. 
• Enveloped Virus – In this category of the general structure of viruses, we notice that protective membranes are present around the capsid which holds the nucleic acid. Animal viruses like HIV are typically enveloped.
• Head and Tail Virus – This contagious variant has a head-like isometric virus and a tail-like helical virus. Influenza virus and bacteriophage T4 are two examples.

Based on chemical build-up, viruses adopt a specific mode of replication. There may be only one nucleic acid or multiple nucleic acid segments present in the genome of a virus which decides how it will multiply after penetrating the host’s cell membrane.

Based on the nature of RNA strands, the viruses can be divided into positive sense or plus sense and negative sense or minus sense. SARS-CoV-2 or the coronavirus comes under the category of positive-sense RNA virus. Negative sense virus possesses an RNA genome that can serve as mRNA, unlike their counterpart. Measles and rabies viruses are specimens of minus sense virus. While mRNAs can be converted into proteins, RNAs don’t support that property. 

RNA vs. DNA Virus

When we compare RNA and DNA viruses, we study that the mode of replication is different. Accurate replication produces larger genomes in DNA viruses while the multiplication steps are more error-prone in the case of the RNA variant. 

The genetic information is stored in the form of single-stranded RNA in RNA viruses like influenza A, hepatitis C, measles, etc. On the other hand, DNA structure is double-stranded. 

For DNA viruses replication is sighted inside the nucleus of host cells, unlike RNA viruses where the replication takes place in the cytoplasm. Replication is more error-prone in the case of RNA viruses.

DNA viruses are stable owing to the low mutation rate.

Procapsid is responsible for housing the newly formed viral DNA. Procapsid is a primitive capsid that is made in the early phase of the self-assembly of some bacteriophages. This preformed protein shell is unavailable for RNA viruses.

Virion Structure

The virion represents an entire particle of a microscopic virus. It is made up of two to three parts namely the nucleic acid core, the capsid, and an optional outer envelope. Tendency to form the outer protective envelope is based on the capacity of the virus to form phospholipid membranes from the host cells. The nucleic acid core is either ribonucleic acid or deoxyribonucleic acid. DNA virions are double-stranded and they exhibit a slow and stable rate of mutation compared to RNA viruses. In an extracellular environment RNA, genomic virion appears to be non-enveloped and has projections throughout the surface. It comprises a single particle that holds linear positive-sense RNA. Unlike viruses which are non-cellular structures, virions are encapsulated molecules with both nucleic acid genome and several protein layers.

Retrovirus Structure

Retrovirus refers to a family of RNA viruses that is widely renowned for its properties. They produce a DNA copy of their genomic nucleic acid after successfully infecting a host cell. These are pathogenic agents which are responsible for the development of cancer, leukemia in animals, and diseases like AIDS in humans.

The converted DNA is adopted and treated by the host cell as a part of its genome. This fulfills the favorable protein requirement needed to replicate new copies. 

The retrovirus structure is kept safe by an outer glycoprotein envelope. This is derived from the host plasma membrane. Besides providing protection, this envelope eases the penetration process as it can fuse well with other membranes. The nucleic acid core comprises ribonucleic acid that is produced as a result of polymerase II activity. The RNA strands are viral and single-stranded. Nucleocapsid protein is the major coating for the RNA. There are also other forms of protein like group-specific antigens (gags), protease, pol proteins, and env proteins that help in the synthesis of DNA.

Conclusion

A virus constitutes a group of microorganisms that are dependent on host cells for their development and reproduction. As they cannot keep themselves stable in an external world, they are not considered to be living. The general structure of viruses includes – nucleic acid (single or double-stranded RNA or DNA) and the capsid. Humans experience severe symptoms of fever, body ache, loss of taste, fatigue, or even more fatal symptoms once they are virally infected. Having a superior lifestyle including proper rest and a balanced diet helps us to combat viral infections.