Structure of Viruses: Overview, Types, Size, Structure

Structure of Viruses: A virus is referred to as an agent, that is simple but highly infectious and is only capable of multiplying itself in the living cells of bacteria, plants, or animals. The viruses are much small than bacteria and contain single- double-stranded RNA or RNA. The structure of the virus changes its form/structure based on the organism it communicates with. Continue reading to find out more about the types, sizes, and structure of viruses.

Since the virus comprises a nucleic acid, a protective protein shell, a.k.a. capsid, does the protection for it. Even the structure of capsid differs based on the type and structure of the virus. A viral capsid is formed of proteins that are present in viral genes inside the genome, a haploid set of chromosomes. Some viruses are made up of proteins and phospholipids on their envelopes, generated from a piece of the host’s cell membrane.

Structure of Viruses: Define Viruses

Viruses can be defined as ultramicroscopic, obligate intracellular parasites that can stay alive only in host cells. Viruses are called particles because they are non-cellular organisms, they depend on host cell components to carry various functions. Viruses cannot reproduce outside the host body nor can they produce energy. They remain as a non-living things outside the host cell. They use the DNA of host cells to replicate.

Discovery of Viruses

The term virus was given by the Dutch microbiologist, Martinus Willem Beijerinck in the year \(1897\) which is a Latin word meaning poison. Dmitri Iwanowski in \(1892\) while studying tobacco mosaic disease discovered an agent smaller than bacteria. When the infected tobacco plants were passed through filters the filtrate could induce mosaic disease in healthy plants. Tobacco mosaic virus was the first to be found. Later with further growth in technologies aided a lot to study viruses. The study of viruses is called virology.

Structure of Virus Diagram

Fig: Structure of Virus

What is the Size and Structure of a Virus?

Viruses are observed in a wide range of shapes and sizes, but these shapes are constant in a particular viral family. The basic structure of all viruses includes nucleic acid, either DNA (Deoxyribonucleic Acid) or RNA (Ribonucleic Acid) called a genome which is enveloped by a protein coat called a capsid. In some cases, the capsid is surrounded by an envelope which is usually a bilayer of glycoprotein and lipoproteins. Some species of the virus also contain enzymes. The classification of viruses is done based on several factors and the most important factor is the genome of a virus.
Let us know these components of the Virus and their functions in detail:

1. Genome

a. The central part of the virus that contains nucleic acid is called Genome. Viruses may have DNA or RNA as their genetic material, but never both.

b. Viruses that contain DNA are called DNA viruses and the ones that carry RNA are called RNA viruses.

c. The genetic material of viruses can be double-stranded DNA (like adenovirus structure), single-stranded DNA, double-stranded RNA, or single-stranded RNA.

i. ssDNA is the genetic material for \({\rm{\varphi X174}}\) Phage, etc.

ii. dsDNA is the genetic material in Poxvirus, Cauliflower Mosaic Virus, Herpes Virus, etc.

iii. dsRNA is the genetic material for Wound Tumour Virus, Rice Dwarf Virus, etc.

iv. ssRNA is the genetic material for Mumps Virus, Measles Virus, HIV, Influenza Virus, Rabies Virus, Rhinovirus, TMV, etc.

Functions of Viral Genome:

a. Viral genome has codes with genetic information for the synthesis of various proteins.

b. The main motive of the virus is to replicate its genetic material to produce more of its kind.

2. Capsid

a. The capsid, also known as the shell or coat of viruses, is the outer protein coat of the virus.

b. This protein coat is made of up small subunits called capsomeres which in return are made up of protomere.

c. The number of capsomeres differs in different species of viruses.

d. The genome and capsid of a virus are collectively called virus nucleo-capsid.

Functions of Capsid:

The capsid has three important functions:

a. It protects the viral genome from getting digested by enzymes.

b. It bears special sites on its exterior that enable the virion to connect to a host cell.

c. It also produces proteins that facilitate the virion to insert the host cell membrane and, in some cases, to inject the infectious viral genome into the cytoplasm of the cell.

3. Envelope

a. Envelope is the structure that surrounds the Nucleocapsid of a virus. Viruses without an envelope are called naked viruses.

b. It is basically a hybrid structure made up of lipids derived from host cells and proteins derived from viruses.

c. The glycoprotein of the envelope projects out as spikes in some viral structures. These spikes are called peplomers and help in binding the virus to the host cell.

Functions of Envelope:

a. Like capsid, the envelope also helps in viral genome protection, attachment and penetration to the host cell.

b. Glycoproteins found on the envelope surface help to recognise and bind to receptor sites on the host cell membrane.

c. In some cases, they possess spikes made of glycoprotein that help them to attach to specific cell surfaces.

d. Viruses with envelopes are adapted for changes and may cause constant infections.

4. Enzyme

Some species of viruses contain various enzymes that help them to catalyze various processes. Reverse transcriptase, lysozyme, viral protease, and integrase are some examples of enzymes found in certain species of viruses.

Functions of Enzymes:

a. Some species of virus contain a special enzyme that helps them to infect the host. For example, a bacteriophage has a lysozyme enzyme that helps to make a hole in a bacterial cell for entry of the viral genome.

b. A few viruses contain their own enzyme called nucleic acid polymerase that transcribes the viral genetic material into mRNA during the replication process. For example, Retrovirus possesses an RNA dependent DNA polymerase called Reverse transcriptase that replicates inside the host cell as a DNA intermediate.

Morphology in Structure of Viruses

Fig: Structure of Viruses

The arrangement of capsomeres in the capsid of a virus is called the symmetry of the virus. There are mainly three types of symmetry observed in viruses that resemble two primary shapes that are rod-shaped and spherical shaped. The third one is the complex symmetry seen in virus-like phages. Let us learn these structures with an example.

1. Rod Shaped
a. Helical Symmetry
2. Spherical Shaped
a. Cubicle or Icosahedral Symmetry
i. Pentagonal
ii. Hexagonal
3. Complex Symmetry

1. Rod Shaped Capsid

a. In this type of symmetry, the capsomere in the capsid and viral genome are woven together to form a helical or spiral tube-like structure.

b. Most of the helical viruses are enveloped and all are RNA viruses.

c. An example of a virus with such symmetry includes the tobacco mosaic virus (TMV).

Structure of Tobacco Mosaic Virus (TMV)

1. TMV is a single-stranded RNA virus that infects a wide range of plants, especially tobacco. It was the first virus to be discovered.

2. It has a rod-like appearance with a capsid made from coat proteins and one molecule of genomic single-strand RNA that contains \(6400\) bases long.

3. The coat protein self-assembles into the rod-like helical structure around the RNA and looks like a hairpin loop structure.

2. Spherical Shaped Capsid

a. Spherical shape virus has icosahedral or cubical symmetry. It is a polygon with \(12\) vertices (corner), \(20\) facets (sides) and \(30\) edges.

b. Each of these \(20\) facets have an equilateral triangle.

c. Icosahedral capsid may have pentagonal or hexagonal capsomeres at the vertices.

d. Examples include Coronavirus, Papovavirus, Herpes virus, etc.

Coronavirus

Fig: Structure of Coronavirus

1. It is a spherical shape virus that causes respiratory tract infections in humans which can be mild to lethal.

2. It is a single-stranded RNA virus and has a string of nucleoproteins that helps in the replication of the viral genome.

3. It has spikes on the surface of the enveloped virus and gives a crown-like appearance under a microscope. These spikes help viruses to attach to the host cell.

Complex Symmetry

a. Some viruses are composed of several separate capsomeres with separate shapes and symmetry forming complex symmetry.

b. They neither have icosahedral nor helical symmetry due to the complexity of their capsid structure.

c. Examples include Poxvirus, Bacteriophage, etc.

Bacteriophage

Bacteriophage is known to be the most complicated virus in terms of structure. It possesses an icosahedral head and helical tail. Such a structure is called binal symmetry.

1. Bacteriophage is the virus that infects bacteria, these are of different types.

2. Some of these contain DNA and some have RNA as genetic material.

3. There are three different structural forms of these viruses, that are an icosahedral (\(20\)-sided) head with a tail, an icosahedral head without a tail and a filamentous form.

Structure of Viruses Summary

Viruses are indeed the most fascinating organisms among all other groups due to their characteristic of being alive and functional only in the body of other organisms. Therefore, they have not been included in Whittaker’s five-kingdom classification. Viruses are found in different shapes and sizes. In this article, we will learn what viruses are and about the typical structural components of a virus and its types according to morphology in structures.

Viruses are intracellular, obligate parasites that are on the borderline between living and non-living. They contain only nucleic acid and proteins enclosed in the capsid. Viruses attach themselves to the host cell, cause incisions and insert their genome into the host body. Viruses use host cell proteins and enzymes to replicate their own genome. These genomes are packed with protein capsids and ready to spread infections to other organisms.

FAQs on Structure of Viruses

Here are a few questions regarding the structure of viruses:

Q.1. What is the shape and structure of a virus?
Ans: The shape of the virus varies due to the structure of its capsid. The capsid can be filamentous or rod-shaped, isometric or icosahedral, spherical or complex with an icosahedral head and helical tail.

Q.2. What is the chemical structure of viruses?
Ans: The centre portion of the virus consists of a single molecule of nucleic acids either DNA or RNA that is surrounded by capsids made of protein units called capsomers. Some viruses also contain certain enzymes of their own.

Q.3. What are the three vital structures of viruses?
Ans: There are mainly three types of symmetry observed in viruses that resemble two primary shapes that are rod-shaped and spherical shaped. The third one is the complex symmetry seen in virus-like phages.

Q.4. What is meant by DNA-virus or RNA-virus?
Ans: Unlike cellular life forms, viruses can only have DNA or RNA (and never both). If the genome of the virus is made up of DNA, it is called DNA-virus, like the Herpes virus. Conversely, if the genome is made up of RNA, it is called RNA-virus, like HIV.

Q.5. Why are viruses considered to be on the borderline between living and nonliving?
Ans: Unlike any cellular organisms, viruses do not have any cellular nature and can be crystallised, which is a nonliving character. On the other hand, they can multiply using host machinery inside any host cell, which is a living character. That is why viruses are considered to be on the borderline between living and nonliving.

Q.6. What structure is unique to a virus?
Ans: Bacteriophage is known to be the most complicated or unique virus in terms of structure. It possesses an icosahedral head and helical tail. Such a structure is called binal symmetry.

We hope this detailed article on the Structure of Viruses helps you in your preparation. If you have any questions regarding the structure of these viruses, do let us know in the comments section below and we will get back to you at the earliest.

For more interesting articles on poliovirus structure, rotavirus structure, retrovirus structure, etc., keep an eye on Embibe.