Viruses have always been the focal point of concern for man because of their great infectivity, great numbers in which they exist, ultramicroscopic size, ever-changing genetic nature due to mutations etc and their status /existence (living or nonliving).
In this blog, we will try to shed some light on various aspects of virus viz. structure, infectivity, virulency, genetic changes or mutations, and nature of Coronavirus.
Structure of virus
All viruses contain two components –
1) A nucleic acid genome – DNA or RNA and not both
2) A protein coat or capsid – that covers the genome
Together they are called Nucleocapsid.
3) In addition, many animal viruses contain a lipid envelope
The entire intact infectious virus particle is called VIRION. So, a Virion consists of an RNA or DNA core, with a protein coat, sometimes with external envelopes.
The core confers infectivity and the capsid provides specificity to the virus.
The Protein capsid is 2 types –
1) ICOSAHEDRAL– (a) Identical protein subunits form an icosahedron (made up of equilateral triangles fused together in a spherical shape) coat or capsid.
(b) Such viruses are released into the environment when the host cell dies, breaks down or lyses, thus releasing the virion.
(c) Ex- polio rhinovirus
2) HELICAL– (a) The proteins are arranged in a circular fashion, creating a disc like shape. The disc shapes are attached helically creating a tube, with room for nucleic acid in the middle.
(b) Ex- tobacco mosaic virus
Two types of virus based on structure
1) ENVELOPED Virus- (a) in these viruses, the nucleocapsid is surrounded by a lipid bilayer called envelope, which is derived from the host cell membrane during replication after infection.
(b) The envelope is formed when the virus is exiting the host cell via budding. (Non-enveloped viruses kill the host cell in order to escape).
(c) Ex- herpes simplex, chickenpox, influenza virus, hepatitis C virus
(d) Some viruses have projections from the envelope known as ‘SPIKES’ which help in attachment of virus to the host cell. Ex- HIV, coronavirus
2) COMPLEX Virus – (a) These virus structures have a combination of icosahedral and helical shape and may have head- tail morphology.
(b) This head- tail morphology is unique to viruses that only infect bacteria and are known as bacteriophages.
(c) Here, the head is icosahedral with the helical tail. The tail is used to attach to the bacterium.
(d) Ex – poxvirus
Classification of Viruses
The virus may contain RNA ( so called RNA viruses)or DNA (so called DNA viruses) in their genome. RNA viruses comprise about 70% of all viruses on earth.
The RNA single strand may be –
(1) a sense strand ( plus strand) which can function as a mRNA or
(2) an anti-sense strand (minus strand) which cannot function as mRNA for protein translation.
So, RNA & DNA viruses can be classified as –
1)Plus strand RNA viruses
2) Minus strand RNA viruses
3) Double- stranded RNA viruses
4) Single-stranded DNA viruses
5) double-stranded DNA viruses
Genetic changes in viruses (Mutations)
Viruses are continuously changing as a result of genetic selection. They undergo subtle genetic changes through Mutation and major genetic changes through Recombination.
Mutation occurs when an ERROR is incorporated in the viral genome. Recombination occurs when two related viruses exchange genetic material during co-infection of a host cell creating a NOVEL virus.
Alterations in the genetic material of a virus may lead to change in the function of viral proteins. Such changes may result in the creation of new viral serotypes or viruses of altered virulence.
Mutations arise when the virus replicates inside host cells and mistakes are made in copying its genetic code.
Mutations in virus happen by chance all the time. Most have little effect, and some hamper the virus, but over time, single or multiple mutations can potentially buildup, that make the virus more successful by enabling it to spread more easily.
Mutations can also make the virus more dangerous, for example, by making it more efficient at infecting cells.
Mutations can make a virus either more or less virulent.
A virus that is less virulent could infect far more hosts because the hosts are well enough to come in contact with many other potential hosts.
Seasonal influenza virus mutates so rapidly that we need a different vaccine each year.
The faster a virus mutates, the quicker it changes its behaviour.
Mutations may cause the virus’s outer surface to appear different to a host previously infected with the ancestor viral strain. So, antibodies produced by previous infection cannot effectively fight the mutated virus and disease results.
Significance Of Proteins –
Proteins are large molecules made up by 20 small molecules called amino acids. All living organisms have the same 20 amino acids ( as described in the previous blog), but they are arranged in different ways and this determines the different function for each protein.
7 types of proteins are:-
• contractile proteins
• hormonal proteins
• structural proteins
• storage proteins
• transport proteins
Characteristics of Coronavirus
• family of Plus strand RNA viruses
• encode a complex RNA- dependent RNA polymerase
• coronavirus is a non-reverse transcribing RNA virus
• coronavirus is an ENVELOPED Virus
• coronavirus has large genomes
• coronavirus is a plus strand single-stranded RNA virus with helical nucleocapsid
• coronavirus is a recombinant virus
• it causes diseases in mammals and birds
RNA virus versus DNA virus –
DNA viruses are able to attack both human cellular and metabolic processes simultaneously.
RNA viruses interact with human PROTEINS functioning in specific cellular processes and intra cellular transport.
How do viruses multiply?
Viruses multiply in the host cell, where they find the machinery, the PROTEINS with which they can copy their genetic material before infecting other cells.
Which Viruses show higher mutation rate?
• RNA viruses mutate faster than the DNA viruses as RNA nucleotides are more unstable than DNA nucleotides.
• Single-stranded viruses mutate faster than double- stranded viruses.
• The lesser the genome size, the higher is the mutation rate.