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The replication mechanism depends on the viral genome. DNA viruses usually use host cell proteins and enzymes to make additional DNA that is used to copy the genome or be transcribed to messenger RNA (mRNA), which is then used in protein synthesis. RNA viruses, such as the influenza virus, usually use the RNA core 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] ). Of course, there are exceptions to this pattern. If a host cell does not provide the enzymes necessary for viral replication, viral genes supply the information to direct synthesis of the missing proteins. Retroviruses, such as HIV, have an RNA genome that must be reverse transcribed to make DNA, which then is inserted into the host’s DNA. To convert RNA into DNA, retroviruses contain genes that encode the virus-specific enzyme reverse transcriptase that transcribes an RNA template to DNA. The fact that HIV produces some of its own enzymes, which are not found in the host, has allowed researchers to develop drugs that inhibit these enzymes. These drugs, including the reverse transcriptase inhibitor AZT, inhibit HIV replication by reducing the activity of the enzyme without affecting the host’s metabolism.

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.

Art connection

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.

Influenza virus is packaged in a viral envelope, which fuses with the plasma membrane. This way, the virus can exit the host cell without killing it. What advantage does the virus gain by keeping the host cell alive?

Concept in action

Click through this tutorial on viruses to identify structures, modes of transmission, replication, and more.

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)

Vaccines for prevention

While we do have limited numbers of effective antiviral drugs, such as those used to treat HIV and influenza, the primary method of controlling viral disease is by vaccination, which is intended to prevent outbreaks by building immunity to a virus or virus family. A vaccine    may be prepared using weakened live viruses, killed viruses, or molecular subunits of the virus. In general, live viruses lead to better immunity, but have the possibility of causing disease at some low frequency. Killed viral vaccine and the subunit viruses are both incapable of causing disease, but in general lead to less effective or long-lasting immunity.

Questions & Answers

what is osmosis
Aon Reply
movement of water molecule from higher to lower concentration through a semipereable membrene.
Dr
what of in the case of solute
Aon
osmosis is the movement of molecules from higher concentration region to lower concentration region through semi-permeable membrane.
Broad
in case of solute means that water moves from the region with lower solutes to the region with higher solute. so it is vice versa to water.
Broad
what are the hydrophilic and hydrophobic region of the plasma membrane?
Samuel Reply
hydrophilic in other word it called water loving and hydrophobic region other word is region that does not contact with water in the plasma membrane.
Broad
recognizing living things
Emmanuel Reply
Species A has 12 pairs of chromosomes and Species B has 11 pairs of chromosomes. Explain what occurs during mitosis and during meiosis in the hybrid that allows normal development and growth from zygote to adult, but causes the adults to be sterile.
Christina Reply
what is the origin of angiosperms?
Broad
unknown group of gymnosperms of triassic period.
Aditi
ouky but what about the concept of monophyletic and polyphyletic? where angiosperms is between that two concept?
Broad
Why does water move through a membrane?
Christina Reply
How many bones are in the human skeleton
Treasure Reply
203
Oyeleke
procce of digestion of proteins a long human alimentarycanal
Carson Reply
what are the properties of lipids?
Isiah Reply
They are: Fatty acids, fats, oils, waxes, phospholipid, glycolipids, steroids and some vitamins
Rachel
explain why a fresh water fish excrete ammonia
Leonard Reply
plz answer my question
Leonard
sorry i meant it has a nucleous unlike plant cells lol
Lailah
Ammonia is the end product of protein catabolism and is stored in the body of the fish in high concentrations relative to basal excretion rates. Ammonia, if allowed to accumulate, is toxic and is converted to less toxic compounds or excreted
Rachel
What are eukaryotic cells?
Nwosueke Reply
cell with no nucleous so not a plant cell
Lailah
eukaryotic cells are membrane bound organelles that have a membrane bound nucleus
ojeen
where does the cell get energy for active transport processes?
A'Kaysion Reply
IDK maybe glucose
Lailah
what is synapsis
Adepoju Reply
ability of homology genome to pair
Broad
how many turns are required to make a molecule of sucrose in Calvin cycle
Amina Reply
why Calvin cycle occurs in stroma
Amina
why do humans enhale oxygen and exhale carbondioxide?
Maryam Reply
why do humans enhale oxygen and exhale carbondioxide? For the purpose of breaking down the food
dil
what is allele
uzoka Reply

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Source:  OpenStax, Concepts of biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11487/1.9
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