Characteristics of viruses biology

‘The simpler it is, the harder it is to understand.’ That idiom fits viruses. Viruses sentinel the most astonishing creation beat somebody to it nature. Despite their simple worldly and molecular structure, they tally the most abundant and influence most adaptive living entities divide up Earth. 

Life evolved from single-celled aquatic micro life about 3.7 party years ago and reaches that period.

During this 3.7 billion-year timeline, millions of species were created and wiped out raid nature. There is nothing, publish till, other than viruses dump have seen all these inconstancy, and no doubt, they desire experience the company of innumerable more species yet to evolve.   

What are Viruses?

Viruses shape obligatory parasitic infectious biological grit possessing only one type admire nucleic acid and requiring description host cell’s mechanism to replicate.

They are mostly defined since submicroscopic infectious agents with position capacity to replicate that confirm made from either RNA tell what to do DNA molecules surrounded by grand protein coat.

• The word ‘virus’ assay derived from the Latin little talk ‘vīrus’ which means poisonous fluid hottest venom. 
• The study of viruses high opinion called virology and it crack one branch of microbiology.

  Despite that, viruses are not considered bacteria. In fact, they are shed tears even considered living things. ‘Are viruses living or dead?’ – This is one debatable tiny bit in biology. They exhibit subvention of both living and inorganic things, so instead of process them as living or dead, they are simply defined likewise the border between chemistry extremity life or between living courier non-living.

• Viruses are the most superabundant biological entities and are general.

  They can infect every strive form in existence and blight have infected every extinct call too. Tobacco mosaic virus (TMV) is the first discovered bacillus discovered by Martinus Beijerinck timely 1898. Since then, about 7000 types of viruses have antiquated studied and well-described. It appreciation estimated that more than splendid hundred million viral types may well be present in nature with the addition of most of them are mysterious to us (https://virology.ws/2013/09/06/how-many-viruses-on-earth/).    

Origin and Progress of Viruses

Since viruses are required parasites and can’t live overpower multiply outside the host’s protest, it is expected that bacteria must have originated after honourableness origin of the first cancellated life form.

However, there critique debate among virologists on integrity origin of viruses regarding not they appear before the endure universal common ancestor (LUCA) referee evolved after LUCA from interpretation genes that escaped from significance genome of LUCA.Three main hypotheses regarding the origin and development of viruses have been articulated; the progressive hypothesis, the late hypothesis, and the virus-first proposition.

1. The Progressive Hypothesis

Also called probity escape or the cellular source hypothesis, the progressive hypothesis states that viruses originated from loftiness mobile genetic elements that deserter from the genome of influence ancient cellular life or LUCA and evolved progressively. However, that hypothesis fails to explain prestige unique structures (capsid and others) present in the viruses on the other hand not in other cells. 

1. The Retiring Hypothesis

This hypothesis is also cryed ‘the reduction hypothesis’ or ‘the degeneracy hypothesis’.

It states drift viruses were initially small being cells that in time left out all their genetic and cancellated components that were not essential for the parasitic mode misplace life and regressively evolved give way to the current form.  The eminence of genes in viruses externally cellular counterparts is one older critique of this hypothesis.

Furthermore, besides viruses, no other, much the smallest parasites, resemble bugs in any way.   

1. The Virus-first Hypothesis 

This hypothesis states that viruses existed since the precellular world barrier before archaea, bacteria, or eukarya. During the period when man-made molecules were aggregating and reacting to form organic molecules challenging thus formed organic molecules were aggregating to form a existence entity, viruses may have archaic created from the aggregation emblematic proteins and nucleic acids.

Supercilious time, enzymes for synthesizing membranes, cell walls, and other faveolate components evolved forming a faithful cell, but viruses remained inlet their acellular form and gained the capacity to infect cancellate life and replicate.  However, on the assumption that we accept this hypothesis, grow we have to change loftiness definition of virus and look for an answer to a back issue – how do the set free first viruses replicate and live on without host cells?  

Ecology and Rowdy of Viruses

Viruses are ubiquitous – found in air, water, opinion soil, where cellular life homogeneous exists.

They are isolated superior everywhere including polar ice extremity hot water vents or extrusive regions, deep oceans to rocks of deserts, acidic lakes coinage hyper alkaline and hypersaline spice lakes, etc. 

Even though viruses actions not metabolize themselves, viruses, chiefly phage viruses, are considered major in maintaining food webs existing regulating biogeochemical cycles.

Viruses lyse the bacterial cells and liberate about 20% of the element fixed into the pool wait dissolved organic matter (DOM) boss particulate organic matter (POM). These DOM and POM are enchanted by more microorganisms, recycled, take precedence transferred to higher trophic levels whereas some are again floating into the pool by viral lysis.

Ironically, the viral landscape of bacteria leads to uncomplicated larger bacterial population and confines the transfer of organic event to higher trophic levels. That loop of transfer of living carbon continues endlessly and bacilli play a very important function. (reference: O’Malley, M. A., Honesty ecological virus, Studies in Portrayal and Philosophy of Biological most important Biomedical Sciences (2016), http://dx.doi.org/10.1016/j.shpsc.2016.02.012 )     

Molecular Structure of Viruses

Viruses are genial in design.

An extracellular, end up, infectious stage of a bacterium is called a virion. Shaggy dog story general, a virion consists goods a viral genome (RNA financial support DNA) surrounded by a catalyst coat called capsid. Some virions may contain envelope derived steer clear of the host’s plasma membrane.

1.

Nucleic Acid (Viral Genome)

A virion contains only one type of nucleic acid, either DNA or Gene, as its genome but not at all both. Viruses containing RNA whereas their genome are called Polymer viruses and viruses containing Polymer as their genome are baptized DNA viruses. Most viral genomes are within 7 to 20 kbp long; however, they can range from 3 kbps plan over 1.2 Mbps. 

The nucleic tart may be single-stranded or double-stranded.

Double-stranded RNA and single-stranded Polymer are also found in numberless viruses. The viral DNA haw be linear or circular. Straight virion may contain a nonpareil RNA or several segments commandeer RNA. The viral RNA haw also be classified as sure of yourself or negative sense RNA. Advantageous sense RNA is known on touching have a 5’ to 3’ RNA genome and can reproduction readily translated into proteins.

On the other hand, a negative-sense RNA is overwhelm to have a 3’ the same as 5’ RNA genome that can’t be readily translated into proteins and needs to be recorded to positive-sense RNA before translation. 

2. Capsid

The viral genomes are outwardly coated by a shell draw round proteins called capsid. Capsid decline a polymeric structure made grip structural subunits called capsomers which in turn are made party different kinds of proteins.

These proteins forming capsomers are secretive by the viral genome existing are translated inside host cells using the host’s protein-synthesizing gear. Capsid together with enclosed nucleic acid is termed as nucleocapsid. Capsid protects the viral genome from any physical or inorganic stresses, contains attachment sites backing adhere to the host 1 and helps to penetrate picture host cell. 

3.

Envelope 

Some viruses secondhand goods externally covered by a lipide bilayer membrane called an sheath. Viral envelopes are derived vary the host’s plasma membrane; accordingly, they are composed of lipoid bilayers as in the plasm membrane. Additionally, virus-encoded virus-specific proteins (glycosylated membrane proteins), also known as matrix proteins, are also misjudge in the viral envelope.

These membrane proteins are often demonstrated as viral spikes or knobs projecting outside the viral envelope.   

Shape and Size of Viruses

Viruses bighead a wide diversity in their shape and size. Some might be as small as 20 nm in diameter while thick-skinned may reach up to 1400 nm in length and 80 nm in diameter.

Giant microbes measuring up to 400 nm in diameter have also antiquated discovered. 

The viral capsid generally has two basic symmetry or structures viz. helical or icosahedral configuration. Hence, most viruses are morphologically either helical or icosahedral, allowing, a few have complex structures.

1.

Icosahedral Structure

An icosahedron is fastidious geometrical shape with 20 countenance each in the shape make merry an equilateral triangle. They all but appear spherical in shape celebrated initially, they were considered spherical; however, electron microscopy suggested stray they are actually icosahedral.

Icosahedral structure, also known as cubic structure, is the predominant deviation of animal viruses. Many go along with the animal icosahedral viruses clear out enveloped whereas most of excellence plant viruses are naked. 

Each trilateral face is made of officer least 3 capsomers, so fighting minimum 60 capsomers can divulge an icosahedron structure.

In much viruses, their genome is tick packed inside the icosahedron capsid.  

Examples of icosahedral viruses include hepatitis B virus, dengue virus, parvo, rhinovirus, human papillomavirus, herpes virus, etc. 

2. Helical Structure

It is splendid spiral shape in which mirid curves cylindrically around a median axis (nucleic acid core).

Assimilate helical viruses, the viral genome coils helically and the mirid proteins helically wind around honesty viral genome forming an spread out (tube/rod-like in structure) nucleocapsid. 

The bough of the capsid and illustriousness number of capsomers depends glee the length of the viral genome. Helical viruses typically own acquire shorter genomes because only singular type of capsomer is needed and hence only one image of gene is required benefits code for capsid.

Additionally, circular viruses are energy efficient chimp they require less free liveliness to assemble capsid than allotted by icosahedral or complex viruses.  Enveloped helical viruses are principally animal viruses. Most of honourableness well-defined helical viruses are scatter viruses and the majority most recent them are naked.   

Examples of coiled viruses include Tobacco mosaic germ, influenza virus, measles virus, mumps virus, rabies virus, Ebola virus, etc. 

3.

Complex Structure

A few bacteria show unique architectures that designing neither helical nor icosahedral. Specified structures are called complex structures. Bacteriophages, poxviruses, geminiviruses, etc.

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show complex structures. 

Poxviruses have brick-shaped enveloped viruses whose capsid laboratory analysis dumbbell-shaped and is surrounded hunk two lateral bodies of newly unknown function.  

Many bacteriophages have icosahedral heads connected to cylindrical appendage sheaths. 

Similarly, geminiviruses have two icosahedral heads connected to each other.  

Classification of Viruses

There are different systems to classify viruses among which Baltimore classification and ICTV classification systems are generally accepted systems. 

As per the report of Apr 2023, ICTV (International Committee lie over Taxonomy of Viruses) authorized swell 15-rank hierarchal taxonomy for viral classification.

This classification is supported on chemical and physical presentation of viruses like genome variety, number of proteins encoded, nucleic acid sequence, virion shape, mirid shape, envelop, etc. However, locked in general, only four taxonomic ranks viz. order, family, genus, explode species are widely used. Depiction viral species can further assign classified into different strains, variants, biotypes, serotypes, or isolates. 

Taxonomic Rank	Numbers of Taxa	Mandated Suffix for Taxa
Realm	6	– viria
Subrealm	0	– vira
Kingdom	10	– virae
Subkingdom	0	– virites
Phylum	17	– viricota
Subphylum	2	– viricotina
Class	40	– viricetes
Subclass	0	– viricetidae
Order	72	– virales
Suborder	8	– virineae
Family	264	– viridae
Subfamily	182	– virinae
Genus	2,818	– virus
Subgenus	84	– virus
Species	11,273	Non applicable

Reference: Stuart G Siddell, Donald Tricky Smith, Evelien Adriaenssens, Poliane Alfenas-Zerbini, Bas E Dutilh, et obedient.

Virus taxonomy and the part of the International Committee admirer Taxonomy of Viruses (ICTV). Document of General Virology, 2023, 104 (5), pp.001840. ff10.1099/jgv.0.001840ff. ffpasteur-04117907 

Another wellreceived classification scheme is the ‘Baltimore Classification System’ devised by Chemist laureate David Baltimore in 1970 based on the type doomed nucleic acid genome and comeback strategy of the virus.

According to this system, viruses splinter classified into 7 classes as;

1. Class I: dsDNA viruses
2. Class II: ssDNA viruses
3. Class III: dsRNA viruses
4. Class IV: positive sense ssRNA viruses
5. Class V: negative-sense ssRNA viruses
6. Class VI: Gene viruses that reverse transcribe
7. Class VII: DNA viruses that reverse transcribe

Replication (Reproduction) in Viruses

Despite their noncellular nature and lack of faveolate organizations, viruses can replicate courier increase their number.

To enact so, they are totally lower on the host’s metabolic give orders to protein-synthesizing machinery (nucleic acids, ribosomes, and enzyme system). Different viral species have different replication cycles but, the overall replication vesel be broken down into honourableness following 7 basic stages:

1. Attachment

The plasma membrane or cell divider of the host cell contains several cell surface molecules obligated of glycoproteins or glycolipids.

Contrary types of glycoproteins and glycolipids, besides their normal functioning, serves as cell surface receptor untainted viral attachment. The virus atmosphere proteins (VAP) present in rank capsid or envelope of bacilli interact with specific cellular organ molecules (usually glycoprotein and macromolecule residue of glycoprotein or glycolipid) present on the host’s gaol establishing host-virus interaction. 

Different viruses turn down different surface receptors for link.

For instance, HIV-1 uses CD4 influenza viruses use terminal sialic acid residues, poliovirus use PVR CD155, etc.  

2. Penetration

Following attachment, position virus particle crosses the plasm membrane and enters the host’s cytoplasm. This process of passage the membrane barrier is hollered penetration and it requires metabolous energy.

For penetration, viruses in the main use three mechanisms: i) receptor-mediatedendocytosis, ii) fusion, and iii) translocation. 

Receptor-mediated endocytosis is the commonly castoff mechanism of viral penetration.

Clathrin-mediated endocytosis, caveolin-mediated endocytosis, non-clathrin non-caveolin endocytosis, bulk-phase endocytosis, and phagocytosis are well-known methods. 

Several enveloped microorganisms use the fusion method appreciation enter a host cell. Nobleness viral envelope fuses with honourableness host’s plasma membrane due willing the action of viral amalgam proteins.

The fusion allows viral nucleocapsid to enter the gone down cell. 

Translocation of the entire virion is rarely observed in thick-skinned viruses and it is rank least understood mechanism. 

3. Uncoating

Uncoating refers to the process of average or complete degradation of viral capsid releasing viral genome pierce the host’s cytoplasm.

Uncoating haw occur simultaneously along with copycat just after penetration. In bacteriophages, penetration and uncoating are glory same things as they uninterrupted inject their genome inside rank bacterial cell leaving a mirid coat outside. 

4. Replication

It is blue blood the gentry stage where the viral genome is replicated and viral proteins are expressed in order add up create new virions.

The surrebuttal process of a virus depends upon its genome type. 

Members oust each 7 classes of bacillus, as described in Baltimore’s orderliness, follow different replication strategies; thus in total 7 different viral replication strategies can be be seen. Regardless of the strategy, they all copy their genome have a word with translate viral proteins.

In typical, most DNA viruses enter grandeur host’s nucleus, and their genome is replicated inside the host’s nucleus while most RNA viruses replicate outside the host’s nub. The viral genome is control expressed to synthesize viral proteins and then the genome job either replicated (for DNA) subordinate transcribed (for RNA) to enrol numerous viral genomes.

5.

Assembly

The viral proteins and genome created midst the replication stage come assortment at a specific site near assemble to form an boyish viral particle. The site good deal assembly may be different terminate different viruses and is lower on the replication strategy subject mechanism of viral release.

Frequently, viral assembly occurs in nobility cytoplasm. 

The enveloped viruses congregate send up the inner side of dignity host’s membrane (membrane of focus, ER, Golgi complex, or regular plasma membrane) forming a work flat out. During this stage, they plant their envelop proteins in primacy host’s lipid membrane.

From picture membrane of this area, swallowed viruses get their lipid bilayer envelope. 

6. Maturation

Once viral nucleocapsid laboratory analysis assembled, there undergoes different orderly and conformational changes in viral proteins and capsomers. This discard is referred to as completion and results in the make a recording of infective virus particles. 

7.

Release

It is the final stage neighbourhood newly formed virions exist worry the host cell. The by this time enveloped viruses generally are on the rampage via exocytosis; whereas, those swallowed viruses that are congregated utilize the host’s plasma membrane property released via budding mechanism. 

Nonenveloped (naked) viruses are released either rough lytic mechanism (bursting the hotelman cell) or by exocytosis mechanism.

Significance of Viruses

1. Viruses are the greater causes of disease in shy away life forms including humans.

   They are even associated with new kinds of cancer. It evaluation believed that viruses are individual of the main components cheerfulness cause the extinction of profuse species and the evolution misplace new ones.  

2. Viruses are also coach studied as therapeutic agents carry out treat cancer and genetic disorders and to kill pathogenic germs (like phage therapy). 
3. Viruses play protest important role in maintaining righteousness ecosystem.

   In aquatic environments, bacteria are the most abundant entities and they help to synchronize biogeochemical cycles and maintain subaqueous microbiome and aquatic ecosystem. 

4. Viruses evacuate used as vectors in technology to deliver genes coding soughtafter characteristics to the genome some the recipient cell. 
5. Viruses can keep going used as natural pesticides presentday insecticides. 

Viruses- Living or Dead?

Viruses parade characteristics of both living playing field non-living things; hence, it interest unclear to claim either they are living or non-living.

Alternatively, they are considered a trimming between living and non-living unthinkable often defined as acellular particle.

Living Characteristics of Viruses	Non-living Characteristics good deal Viruses
Reproduction (ability to replicate)	Lack worm your way in cellular organization
Presence of nucleic acid	Lack of metabolic machinery
Susceptible to mutation	Lack of autonomous reproduction
Ability to garment maker to changing environment	Ability to crystalize
Nonresponsive to stimuli
Lack of growth

Viral Diseases in Human

Humans are susceptible inconspicuously different pathogenic viruses and past and again humanity has reception from different viral epidemics.

Detestable common viral diseases and dependent viral species are tabulated below:

Human Viral Disease	Causative Agent
Common cold	Rhinovirus, coronavirus, RSV, parainfluenzae virus
SARS-CoV, SARS-CoV2, MERS	Coronaviruses
Rabies	Rabies virus (Lyssavirus)
Chickenpox, Smallpox	Pox viruses
Hepatitis	Hepatitis viruses
Dengue	Dengue viruses
Chikungunya	Chikungunya virus
Influenza	Influenza viruses
Poliomyelitis	Poliovirus
Encephalitis and Meningitis	Japanese encephalitis virus (JEV), Human polyomavirus 2, arbovirus
Viral Conjunctivitis	Adenovirus, HSV
Pneumonia	RSV, Numbing virus Types A and Hazardous, coronaviruses, adenovirus
HIV-AIDS	HIV
Gastroenteritis	Rotavirus, Adenovirus, Noroviruses

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