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The replication mechanism depends on the viral genome (DNA or RNA). DNA viruses usually use host cell proteins and enzymes to make additional DNA that is then used to copy the genome or be transcribed to messenger RNA (mRNA). The mRNA is then used in protein synthesis. RNA viruses, such as the influenza virus, usually use the RNA as a template for synthesis of viral genomic RNA and mRNA. The viral mRNA is translated into viral enzymes and capsid proteins to assemble new virions ( [link] ).

The last stage of viral replication is the release of the new virions into the host organism, where they are able to infect adjacent cells and repeat the replication cycle. Some viruses are released when the host cell dies and other viruses can leave infected cells by budding through the membrane without directly killing the cell.

The illustration shows the steps of an influenza virus infection. In step 1, influenza virus becomes attached to a receptor on a target epithelial cell. In step 2, the cell engulfs the virus by endocytosis, and the virus becomes encased in the cell’s plasma membrane. In step 3, the membrane dissolves, and the viral contents are released into the cytoplasm. Viral mRNA enters the nucleus, where it is replicated by viral RNA polymerase. In step 4, viral mRNA exits to the cytoplasm, where it is used to make viral proteins. In step 5, new viral particles are released into the extracellular fluid. The cell, which is not killed in the process, continues to make new virus.
In influenza virus infection, glycoproteins attach to a host epithelial cell. As a result, the virus is engulfed. RNA and proteins are made and assembled into new virions.

Lytic and lysogenic pathways

Cell death may be immediate or delayed after attachment and penetration by the virus. For example, bacteriophages, viruses that infect bacteria, may or may not kill their host immediately. There are two viral replication strategies; when the virus kills the host cell it is called the lytic cycle , and when the virus does not kill the host but replicates when the host replicates it is called the lysogenic cycle ( [link] ).

viral replication strategies
The two viral reproductive strategies, the lytic cycle and the lysogenic cycle

Lytic cycle

The lytic cycle causes death of the host cell and the term refers to the last stage of the infection when the cell lyses (breaks open) and releases new virions that were produced within the cell. These new virions can infect healthy cells and the cycle is repeated ( [link] ).

So why haven't all the bacteria in the world been destroy by bacteriophages? The answer is natural selection of defense mechanisms by bacteria. Mutations of bacterial surface proteins that are not recognized by a particular phage allow the bacteria to survive by preventing attachment. Without going into detail, bacteria have internal defenses that allow them to cut up viral DNA before it can infect the cell. Then one might ask, why hasn't all the bacteriophages in the world gone extinct by not being able to reproduce. Once again, the answer is natural selection. Viruses mutate to bypass the defense mechanisms of the bacteria. This illustrates that the parasite-host relationship is in a constant evolutionary duel.Similar co-evolutionary strategies characterize the interactions of viruses and animals, or viruses and plants.

Lysogenic cycle

There is another reason why bacteria are not extinct because of bacteriophages. Many bacteriophages do not kill their host but coexist within their host, and when this occurs it is called the lysogenic cycle. After penetration, the viral DNA or RNA can either be incorporated into the host DNA, or the viral genome can be a self-replicating entity. Once this occurs, the viral genome is replicated along with the host cell's DNA, but the virus does not destroy the cell as it does in the lytic cycle ( [link] ). However, at some point the viral genes are turned on and can trigger the virus to enter the lytic cycle and kill the host cell ( [link] ). Cell starvation or cell damage (e.g. from radiation) may trigger a lysogenic infection to turn into a lytic infection thereby killing the host cell. The next generation of viruses, depending on the host cell condition, can use either of the viral replication strategies, lysogenic or lytic, on the next host.

Viruses and disease

Viruses cause a variety of diseases in animals, including humans, ranging from the common cold to potentially fatal illnesses like meningitis ( [link] ). These diseases can be treated by antiviral drugs or by vaccines, but some viruses, such as HIV, are capable of avoiding the immune response and mutating so as to become resistant to antiviral drugs.

The illustration shows an overview of human viral diseases. Viruses that cause encephalitis or meningitis, or inflammation of the brain and surrounding tissues, include measles, arbovirus, rabies, JC virus, and LCM virus. The common cold is caused by rhinovirus, parainfluenza virus, and respiratory syncytial virus. Eye infections are caused by herpesvirus, adenovirus, and cytomegalovirus. Pharyngitis, or inflammation of the pharynx, is caused by adenovirus, Epstein-Barr virus, and cytomegalovirus. Parotitis, or inflammation of the parotid glands, is caused by mumps virus. Gingivostomatitis, or inflammation of the oral mucosa, is caused by herpes simplex type I virus. Pneumonia is caused by influenza virus types A and B, parainfluenza virus, respiratory syncytial virus, adenovirus, and SARS coronavirus. Cardiovascular problems are caused by coxsackie B virus. Hepatitis is caused by hepatitis virus types A, B, C, D, and E. Myelitis is caused by poliovirus and HLTV-1. Skin infections are caused by varicella-zoster virus, human herpesvirus 6, smallpox, molluscum contagiosum, human papillomavirus, parvovirus B19, rubella, measles, and coxsackie A virus. Gastroenteritis, or digestive disease, is caused by adenovirus, rotavirus, norovirus, astrovirus, and coronavirus. Sexually transmitted diseases are caused by herpes simplex type 2, human papillomavirus, and HIV. Pancreatitis B is caused by coxsackie B virus.
Viruses are the cause of dozens of ailments in humans, ranging from mild illnesses to serious diseases. (credit: modification of work by Mikael Häggström)

Questions & Answers

how can chip be made from sand
Eke Reply
is this allso about nanoscale material
are nano particles real
Missy Reply
Hello, if I study Physics teacher in bachelor, can I study Nanotechnology in master?
Lale Reply
no can't
where is the latest information on a no technology how can I find it
where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
has a lot of application modern world
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Principles of biology. OpenStax CNX. Aug 09, 2016 Download for free at http://legacy.cnx.org/content/col11569/1.25
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