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.

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.

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.

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.

Test

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

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

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

references

• 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.