Viruses: Definition, Structure, Classification, Examples | biology dictionary (2023)

definition of virus

A virus is a string of nucleic acids (DNA or RNA) that lives in ahostCell, uses parts of the cell machinery to reproduce and releases the replicated nucleic acid strands to infect other cells. A virus is usually located in aProteinmantelÖProtein-Wrap, a protective shell that allows the virus to survive between hosts.

virus structure

A virus can take on a variety of different structures. The smallest virus measures just 17 nanometers, little more than a medium-sized protein. The largest virus is almost 1,000 times larger at 1,500 nanometers. That's too small. A human hair is about 20,000 nanometers in diameter. This means that most virus particles far exceed the capabilities of a normal light microscope. Below is ascanning electron microscope(WITHOUT) picture ofEbolaVirus.

Here you can only see the protein shell of the Ebola virus. Each virus looks like a small curved worm. However, these are not cells. Inside the protein envelope is a carefully folded RNA molecule that contains the information needed to replicate the protein envelope, the RNA molecule, and the components to take over a cell's natural processes to perform these tasks.

The exact structure of a virus depends on which species serves as the host. A virus that replicates in mammalian cells has a protein coat that allows it to attach to and infiltrate mammalian cells. The shape, structure, and function of these proteins change depending on the type of virus. A typical virus is shown below.

The virus above shows the typical structure that a virus adopts, a viral genome surrounded by a protein shield. The various proteins in the envelope allow the virus to interact with any host cell it encounters. Then part of the protein shell tears off, penetrates the cell membrane and deposits the viral genome in the cell. The protein coat can then be discarded as the viral genome now replicates within the host cell. The replicated virus molecules package themselves into their own protein coats and are released into the environment to find another host. While many virus particles have a simple shape like the one above, some are much more complicated.

The picture above shows aI do, a type of virus that specializes in bacterial cells. The protein coat of a phage is much more complex and has a multitude of specialized parts. The header contains the viral genome. The collar, sheath, baseplate, and tail fibers are all part of an intricate system for attaching and injecting the genome into a bacterial cell. Tail fibers grasp the bacterial cell and pull the ground plane towards the cell wall or membrane. The envelope and collar compress, pierce the cell and deposit the DNA in the bacterial cell.

(Video) Virus structure and classification | Cells | MCAT | Khan Academy

Some virus molecules lack any protein coat or have never been identified as such. In some types of plant viruses, the virus is transmitted from one cell to another within the plant. When the seeds are created in the plant, the virus spreads to the seeds. This way, the virus can live inside cells throughout its existence and will never need a protein coat to protect it from the environment. Other virus molecules have even larger and more complex protein envelopes and are specialized for multiple hosts.

Is a virus alive?

This is a tricky question. A cell is considered alive because it contains all the necessary components to replicate its DNA, grow, and divide into new cells. This is the process that all life goes through, be it a unicellular organism or a multicellular organism. Some people don't consider a virus alive because a virus doesn't contain all the necessary mechanisms to replicate itself. You would say that without a host cell, a virus cannot replicate itself and therefore is not alive.

However, based on the definition of life above, it appears that once a virus is inside a host cell, it has all the mechanisms it needs to survive. The protein coat that exists on the outside of a cell is the equivalent of aBacterial spores, a small bacterial capsule forms around it to survive in harsh conditions. Scientists who argue that a virus is a living organism note the similarity between a virus in a protein shell and a bacterial spore. None of the organisms are active inside its protective shell, they only become active when they reach favorable conditions.

In fact, a virus only affects us because it is activated in our cells. Also, a virus tends to evolve with its host. Most dangerous viruses have recently jumped to a new species. The biochemistry they have evolved to live in other species is incompatible with the new species, and cellular damage and death ensue. This causes a range of reactions depending on the cells infected. The HI virus, for example, only attacks immune cells. This leads to a complete loss of immune function in patients. With the common cold virus, the virus attacks and damages the cells in the airways while doing its job.

However, not all viral infections are harmful to the host. A virus that kills the host will be less successful over time than a virus that does no harm to the host. A healthy host increases the number of viral molecules released into the environment, which is the ultimate goal of the virus. In fact, some virus particles can actuallyto benefitthe host. A good example is a form of the herpes virus found in mice. When this virus infects a mouse, it provides the mouse with a good defense against bacteria that transmit plague. Although the mechanism is unclear, the virus somehow prevents the bacteria from taking hold in the mouse's system.

In this way, it is easy to see that a virus is very similar to a bacterium. The bacteria create and maintain the tools needed to reproduce the DNA where the virus steals it. This is the only real difference between a virus and a bacterium. Because of this, many scientists consider a virus to be a living organism. scientists studying viruses,virologists, remember that virus particles (living or not) have probably evolved alive since the first cells appeared. Because of this, there is a specialized virus in almost every species on the planet.

virus classification

Scientists classify viruses based on how well they replicate their genome. Some viral genomes are made up of RNA, others of DNA. Some viruses use a single strand, others double strand. The complexity involved in replication and packaging of these different molecules divides viruses into seven different categories.

(Video) Virus structure and Classification : DNA and RNA virus

Class I virus genomes are made up of double-stranded DNA, just like the human genome. This makes it easier for these virus molecules to use the cell's natural machinery to make proteins from the virus's DNA. However with itDNA Polymerase(the molecule that copies DNA) in order to be active, the cell must divide. Some class I virus molecules contain stretches of DNA that cause the cell to begin actively dividing. These virus molecules can cause cancer.humanes PapillomavirusIt is a class I sexually transmitted virus and can cause cervical cancer.

A class II virus contains only a single strand of DNA. Before it can be read by host DNA polymerase enzymes, it must be converted into double-stranded DNA. It does this by hijacking the host cell.Histone(DNA proteins) and DNA polymerase. Instead of waiting or forcing the cell to divide, class II viral DNA encodes a protein calledRepresentative. This replication enzyme replicates the genome of the original single-stranded virus. Other proteins are made from DNA and are used to create protein coats using cellular machinery. The single-stranded DNA is then packaged in these protein shells and new virus packages are formed.

Class III virus genomes are constructed from double-stranded RNA. Although uncommon, these viral packages contain their own protein,RNA-Polymerase. This protein can createBoten-RNA(mRNA) of double-stranded viral RNA. Therefore, the virus RNA remains in the virus capsule and only the mRNA enters the host cytoplasm. Here the mRNA is converted into proteins, some of which contain more RNA polymerase. This RNA polymerase creates new double-stranded RNA, which is encapsulated by proteins and released from the cell.

Class IV viruses are single-stranded RNAs that are nearly identical to the mRNA produced by the host cell. In these viruses, the entire protein envelope is engulfed by an uninfected host cell. The small RNA genome escapes the protein coat and enters the cytoplasm. This mRNA-like strand encodes a largePolyprotein, which is created by the hostsRibosomes. Polyprotein naturally breaks down into different parts. Some generate protein coats while others read and replicate the original viral RNA strand. The virus continues to replicate and create new fully packaged virus particles. When the cell is completely filled, it ruptures and releases the virus particles into the blood or the environment. Up to 10,000 virus particles can be released from a single cell.

Class V viral genomes are also single-stranded RNA. However, they run in the opposite direction of normal mRNA. Therefore, the cell's machinery cannot read them directly. These virus molecules contain an RNA polymerase molecule that can be read upside down. These virus molecules have large capsules surrounded by cell membrane and proteins. When the virus approaches a cell, its membrane proteins attach to the cell and are drawn into the cytoplasm. Here it breaks, releasing the retrograde viral RNA and associated proteins. These small complexes produce regular mRNA that creates new viral complexes. These unfinished complexes migrate to the cell surface, where they coat the cell membrane with the proteins they make. When finished, they wrap themselves around this membrane and separate from the cell.

Class VI viruses have the same genomes as class V viruses, but they use a different method of replication. Class VI virus particles are known asRetrovirus. Instead of creating mRNA from viral RNA, these viral molecules work with a different protein. known asreverse transcriptasethis enzyme is able to generate DNA from the RNA of the virus. The viral RNA is converted into double-stranded DNA. This DNA then produces a new virus. DNA can be incorporated into and become part of the host's DNAendogenized. This means that the DNA stays in the cell for life. If the cell is in agermline, like a sperm or egg cell, the virus becomes a permanent part of the host's genome. It is estimated that 5 to 8% of the human genome remains retroviral DNA.

The final class, Class VII, includes thePararetroviruses. Similar to class VI, these viral genomes use reverse transcriptase. However, these viral genomes are packaged as DNA, not RNA. These viruses insert themselves directly into the host's genome, which begins to convert the viral DNA into RNA. Most of this RNA will be mRNA, which is used to make a polyprotein. Part of the polyprotein is reverse transcriptase. This reverse transcriptase works on pieces of RNA known asimprint genome. It reads these RNA molecules and makes the original viral DNA. This is then packaged in viral protein envelopes. Class VII viruses are common in plants and can travel between cells using the virusesPlasmodesmos, or they may be carried by herbivorous insects that feed on plants. Aphids transmit many plant diseases because their proboscis pierces plant cell walls and drinks the cytoplasm.

Examples of Viruses

Polio Virus

The poliovirus that crippled President Franklin Roosevelt is a class III virus. This double-stranded RNA virus encodes 12 proteins. Like other class III virus genomes, it reproduces by releasing mRNA strands into the host cell cytosol that encode new virus molecules. Interestingly, the polio virus wasn't deadly until people started treating their water. Before chlorinated water, polio survived in most water sources. Therefore, most babies were exposed to polio at an early age.

(Video) Viral Structure and Functions

Infants usually have no symptoms of polio and the immune system responds to the virus. However, after chlorinated water was introduced, most children did not suffer from polio. However, the disease has not been eradicated. Many people were exposed to polio outbreaks in adulthood, which are still ongoing. These people suffered greatly from the disease because the immune system did not react quickly enough. Like FDR, they were usually permanently compromised by the virus' effects on bone health. Fortunately, the polio vaccine, one of the first ever developed, is easily made by using heat to kill the live polio virus. The dead protein shells allow the body to develop immunity to the virus without infecting the cells.

rabies virus

The rabies virus is a class V virus with a spherical protein envelope. This virus consists of single-stranded linear RNA. The genome of the rabies virus encodes five proteins of 12,000 nucleotides. Interestingly, the symptoms of rabies in many animals include increased aggression. This trait, caused by where the virus attacks and the damage it inflicts, causes animals to bite other animals more often than normal. Collected rabies virus particles accumulate in the saliva. So if an infected animal bites another, the virus will be transmitted to the new animal.

The rabies virus is almost always fatal in humans if not treated promptly. Almost 15 million post-exposure rabies vaccines are administered annually. The vaccine essentially loads the body with dead virus, allowing for a strong immune response against the virus. This can stop the virus before it takes root on the system. When this happens, there is little chance of recovery. Dogs are commonly vaccinated prior to exposure, providing their owners with general protection in the event they are bitten by an animal infected with the virus.


1. Which of the following classes of viral genomes can be directly reproduced by cellular machines?
A.Class I
B.Class III
C.Class VI

Answer to question #1

Athat's right. Class I viral genomes are composed of DNA and are double-stranded. This means that the viral genome is ready to be copied into mRNA without the intermediate steps found in other virus classes.

2. Human rhinovirus A causes the common cold. The rhinovirus genome is a single-stranded RNA, similar to the mRNAs produced by the host cell. What class do rhinoviruses belong to?
A.Class VII
B.Class II
C.Class IV

(Video) Viruses (Updated)

Answer to question #2

Cthat's right. Class IV includes all mRNA-like viral genomes. These viruses can be directly translated into proteins by the host's ribosomes, bypassing the steps taken by other viruses.

3. Your friend claims viruses are the same as allergies because both make your nose runny. Which of the following statements will convince your friend otherwise?
A.Only viruses trigger an immune response
B.A virus doesn't just cause a reaction, it replicates itself in your cells.
C.Why argue? your friend is right

Answer to question #3

Bthat's right. Both substances trigger an immune response. The immune system is responsible for recognizing itself to others. The difference is that allergens, like pollen and dust, don't replicate themselves inside the cells once they've taken over.


  • Nelson, D. L. e Cox, M.M. (2008).principles of biochemistry.Nova York: WH Freeman and Company.
  • Rossinck, MJ (2016).Virus.Princeton: Princeton University Press.
  • Widmaier, E.P., Raff, H. and Strang, K.T. (2008).Vanders Human Physiology: The Mechanisms of Body Function(11ª edition). Boston: McGraw-Hill Higher Education.


What is the definition structure and classification of virus? ›

Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat. Viruses may be viewed as mobile genetic elements, most probably of cellular origin and characterized by a long co-evolution of virus and host.

What are some examples of the different classifications of viruses? ›

The classification based on different shapes and symmetry of viruses are as follows:
  • Complex virus. E.g Poxvirus.
  • Radial symmetry virus. E.g.Bacteriophage.
  • Cubical or icosahedral symmetry shaped virus. E.g. Reovirus, Picornavirus.
  • Rod or Spiral shaped or helical symmetry virus. E.g. Paramyxovirus, orthomyxovirus.

What is the structure of virus answer? ›

Viral Structure. In the simpler viruses the virion consists of a single molecule of nucleic acid surrounded by a protein coat, the capsid; the capsid and its enclosed nucleic acid together constitute the nucleocapsid.

What is the first dictionary definition of virus? ›

: any of a large group of submicroscopic, infectious agents that are usually regarded as nonliving, extremely complex molecules or sometimes as very simple microorganisms, that typically contain a protein coat surrounding an RNA or DNA core of genetic material but no semipermeable membrane, that are capable of growth ...

What are the main structures of viruses? ›

All viruses contain nucleic acid, either DNA or RNA (but not both), and a protein coat, which encases the nucleic acid. Some viruses are also enclosed by an envelope of fat and protein molecules. In its infective form, outside the cell, a virus particle is called a virion.

What is virus structure and function? ›

Viruses are a unique type of pathogen that lack cytoplasmic membrane, cytosol, or functional organelles and use the metabolic machinery of host cells to produce more viral molecules. They can exist extracellularly as a virion or intracellularly as nucleic acids that induce the host to synthesize viral components.

What are the five classification of viruses? ›

Key Concepts. The first broad division used in virus classification separates viruses into vertebrate viruses, invertebrate viruses, plant viruses, bacterial viruses, and algae, fungi, yeast, and protozoan viruses.

What are 10 examples of viruses? ›

Examples of Computer Virus
  • Morris Worm.
  • Nimda.
  • SQL Slammer.
  • Stuxnet.
  • CryptoLocker.
  • Conficker.
  • Tinba.

What is a virus short answer? ›

A virus is an infectious microbe consisting of a segment of nucleic acid (either DNA or RNA) surrounded by a protein coat. A virus cannot replicate alone; instead, it must infect cells and use components of the host cell to make copies of itself.

What is the structure of viruses quizlet? ›

The structure of a virus particle is that it consists of two basic parts-a core of either DNA or RNA (never both) and a protein covering called a capsid.

What are 5 characteristics of viruses? ›

Characteristics of a Virus:
  • They reproduce at a spectacular rate, but only in live host cells.
  • They can be transformed.
  • They are acellular, i.e., they have no cytoplasm or cellular organelles.
  • They do not conduct any metabolism on their own and must replicate using the metabolic machinery of the host cell.

What is the definition and examples of virus? ›

virus, infectious agent of small size and simple composition that can multiply only in living cells of animals, plants, or bacteria. The name is from a Latin word meaning “slimy liquid” or “poison.”

What is viruses description and examples? ›

Viruses are biological entities that can only thrive and multiply in a host, which is a living organism such as a human, an animal, or a plant. Some viruses cause disease. For example, severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2, causes the disease COVID-19.

What is the meaning and examples of virus? ›

A submicroscopic infectious agent that is unable to grow or reproduce outside a host cell. It is non-cellular but consisting of a core of DNA or RNA surrounded by a protein coat. A virus requires a living cell for it to grow and reproduce similar to an obligate intracellular parasite.

What 4 types of viral structures are there? ›

Viruses are classified into four groups based on shape: filamentous, isometric (or icosahedral), enveloped, and head and tail. Many viruses attach to their host cells to facilitate penetration of the cell membrane, allowing their replication inside the cell.

What are the 7 characteristics of viruses? ›

  • Non living structures.
  • Non-cellular.
  • Contain a protein coat called the capsid.
  • Have a nucleic acid core containing DNA or RNA (one or the other - not both)
  • Capable of reproducing only when inside a HOST cell.

What is a Class 3 virus? ›

Class III: Double stranded RNA (dsRNA) viruses

Double stranded RNA viruses replicate in the core capsid in the host cell cytoplasm and do depend as heavily on host polymerases as DNA viruses.

What is the size and structure of a virus? ›

Viruses are usually much smaller than bacteria with the vast majority being submicroscopic, generally ranging in size from 5 to 300 nanometers (nm). Helical viruses consist of nucleic acid surrounded by a hollow protein cylinder or capsid and possessing a helical structure.

How do viruses reproduce? ›

Viruses cannot replicate on their own, but rather depend on their host cell's protein synthesis pathways to reproduce. This typically occurs by the virus inserting its genetic material in host cells, co-opting the proteins to create viral replicates, until the cell bursts from the high volume of new viral particles.

Why is virus structure important? ›

First, one of the most important roles of virus structure is a protective role. The capsid structure protects the viral genome from physicochemical damage, such as nucleases, and radiation (eg, ultraviolet). Second, the role of the virus structure is to recognize the cellular receptor for the entry.

What is a virus made out of? ›

Viruses are bundles of nucleic acid—DNA or RNA—that are enclosed by a protein shell known as a capsid. By some measures the most abundant life form on earth,1 viruses lurk everywhere; experts estimate that they are 10 times more numerous than bacteria.

What is the virus life cycle? ›

The virus life cycle could be divided into six steps: attachment, penetration, uncoating, gene expression and replication, assembly, and release.

What are the 4 most common viruses? ›

Common examples of contagious viral diseases include the flu, the common cold, HIV, and herpes.

How many types of viruses explain it? ›

The main types of computer virus are as follows: Boot Sector Virus. Direct Action Virus. Multipartite Virus.

What are the 4 parts of a virus and define each quizlet? ›

What are the basic components of a virus? Protein shell (capsid), contains the virus genome (either RNA or DNA). Capsomeres, individual subunits of the capsid. Nucleocapsid, complete complex of nucleic acid plus capsid protein.

What are 3 examples of viruses? ›

Examples of viral infections
  • COVID-19.
  • Influenza (the flu)
  • HIV, which can lead to AIDS.
  • Meningitis (there is also bacterial meningitis)
  • Pneumonia (there is also bacterial pneumonia)
  • Human papillomavirus (HPV)
  • Herpes.
  • Rotavirus.

What are 5 common diseases caused by viruses? ›

Viruses are responsible for causing many diseases, including:
  • AIDS.
  • Common cold.
  • Ebola.
  • Genital herpes.
  • Influenza.
  • Measles.
  • Chickenpox and shingles.
  • Coronavirus disease 2019 (COVID-19)

What is a virus definition for kids? ›

A virus is a very tiny type of germ. Viruses (say: VY-ruh-sez) can get inside the body and make a person sick. Viruses cause colds, chickenpox, measles, the flu, and many other illnesses. Antibiotics only work on bacteria, not viruses.

What is the size and structure of a virus quizlet? ›

Normally 20 to 1000 nm in length. complete, fully developed, infectious viral particle composed of nucleic acid and surrounded by a protein coat outside of the host cell, and is a vehicle of transmission from one host cell to another. Capsid is polyhedral and sheath is helical.

What is a virus quizlet? ›

What is a virus? An extremely tiny infectious agent that is only able to live inside a cell, and a tiny bundle of genetic material - either DNA or RNA - carried in a shell called a viral coat or capsid which is made up of protein.

Which structures are found in all viruses quizlet? ›

Which of the following are found in all viruses? Capsid, nucleic acid core, proteins such as enzymes.

What are the main properties of viruses? ›

Viruses have several common characteristics: they are small, have DNA or RNA genomes, and are obligate intracellular parasites. The virus capsid functions to protect the nucleic acid from the environment, and some viruses surround their capsid with a membrane envelope.

What is true of viruses? ›

Viruses are smaller than the bacteria. Viruses consists of a genetic material (DNA or RNA) as well as a protein coat. Viruses require the host cell to reproduce. They are inactive outside the host cell.

What is an example of virus in biology? ›

The rabies virus is a Class V virus, with a bullet-shaped protein coat. This virus is made of linear, single-stranded RNA. The rabies virus genome codes for five proteins, from 12,000 nucleotides. Interestingly, the symptoms of rabies in many animals include increased aggression.

What is the full meaning of its virus? ›

The full meaning of the virus is Vital Information Resources Under Siege.

What is the classification of virus and bacteria? ›

Bacteria and viruses are microscopic microbes. Bacteria are prokaryotes. They are living cells which can be either beneficial or harmful to other organisms. But, viruses considered to be particles that are somewhere between living and non-living cells.

What are the classification of viruses of medical importance? ›

There are two medically important groups: (1) the oncovirus group, which contains the sarcoma and leukemia viruses (e.g., human T-cell leukemia virus [HTLV]) and (2) the lentivirus (“slow virus”) group, which includes human immunodeficiency virus (HIV) and certain animal pathogens (e.g., visna virus).

What are the criteria for classification of viruses? ›

The order of presentation of virus families and genera follows four criteria: (1) the nature of the viral nucleic acid; (2) the strandedness of the nucleic acid; (3) the use of a reverse transcription process (DNA or RNA); and (4) the positive or negative sense of gene coding on the encapsidated genome.

What are characteristics of a virus? ›

  • Non living structures.
  • Non-cellular.
  • Contain a protein coat called the capsid.
  • Have a nucleic acid core containing DNA or RNA (one or the other - not both)
  • Capable of reproducing only when inside a HOST cell.

What are the 7 classification of viruses? ›

The seven classes of viruses in the Baltimore Classification System are as follows: Class I: Double-stranded DNA (dsDNA) viruses. Class II: Single-stranded DNA (ssDNA) viruses. Class III: Double-stranded RNA (dsRNA) viruses.

What are the 7 classes of viruses? ›

  • Group I: double-stranded DNA viruses.
  • Group II: single-stranded DNA viruses.
  • Group III: double-stranded RNA viruses.
  • Group IV: positive sense single-stranded RNA viruses.
  • Group V: negative sense single-stranded RNA viruses.
  • Group VI: single-stranded RNA viruses with a DNA intermediate in their life cycle.

What is the structure of bacteria? ›

In bacteria, the cell wall forms a rigid structure of uniform thickness around the cell and is responsible for the characteristic shape of the cell (rod, coccus, or spiral). Inside the cell wall (or rigid peptidoglycan layer) is the plasma (cytoplasmic) membrane; this is usually closely apposed to the wall layer.

What is virus type? ›

The main types of computer virus are as follows: Boot Sector Virus. Direct Action Virus. Multipartite Virus. Polymorphic Virus.

What are the two categories of viruses? ›

Baltimore Classification

Group I viruses contain double-stranded DNA (dsDNA) as their genome. Their mRNA is produced by transcription in much the same way as with cellular DNA. Group II viruses have single-stranded DNA (ssDNA) as their genome.

How many classifications of diseases are there? ›

There are two related classifications of diseases with similar titles, and a third classification on functioning and disability. The International Classification of Diseases (ICD) is the classification used to code and classify mortality data from death certificates.


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