Some enveloped viruses enter the cell when the viral envelope fuses directly with the cell membrane. Once inside the cell, the viral capsid is degraded and the viral nucleic acid is released, which then becomes available for replication and transcription.
The replication mechanism depends on the viral genome. The viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins, and to assemble new virions.
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. Reverse transcription never occurs in uninfected host cells; the needed enzyme, reverse transcriptase, is only derived from the expression of viral genes within the infected host cells.
The fact that HIV produces some of its own enzymes not found in the host has allowed researchers to develop drugs that inhibit these enzymes. This approach has led to the development of a variety of drugs used to treat HIV and has been effective at reducing the number of infectious virions copies of viral RNA in the blood to non-detectable levels in many HIV-infected individuals.
Ebola is also a leading cause of death in gorillas. Transmitted by bats and great apes, this virus can cause death in 70—90 percent of the infected within two weeks. Using newly developed vaccines that boost the immune response, there is hope that immune systems of affected individuals will be better able to control the virus, potentially reducing mortality rates. Another way of treating viral infections is the use of antiviral drugs.
These drugs often have limited ability to cure viral disease but have been used to control and reduce symptoms for a wide variety of viral diseases. For most viruses, these drugs inhibit the virus by blocking the actions of one or more of its proteins.
It is important that the targeted proteins be encoded for by viral genes and that these molecules are not present in a healthy host cell. In this way, viral growth is inhibited without damaging the host. There are large numbers of antiviral drugs available to treat infections, some specific for a particular virus and others that can affect multiple viruses.
Antivirals have been developed to treat genital herpes herpes simplex II and influenza. For genital herpes, drugs such as acyclovir can reduce the number and duration of the episodes of active viral disease during which patients develop viral lesions in their skins cells.
As the virus remains latent in nervous tissue of the body for life, this drug is not a cure but can make the symptoms of the disease more manageable. Other antiviral drugs, such as Ribavirin, have been used to treat a variety of viral infections.
By far the most successful use of antivirals has been in the treatment of the retrovirus HIV, which causes a disease that, if untreated, is usually fatal within 10—12 years after being infected. Anti-HIV drugs have been able to control viral replication to the point that individuals receiving these drugs survive for a significantly longer time than the untreated. Drugs have been developed that inhibit the fusion of the HIV viral envelope with the plasma membrane of the host cell fusion inhibitors , the conversion of its RNA genome to double-stranded DNA reverse transcriptase inhibitors , the integration of the viral DNA into the host genome integrase inhibitors , and the processing of viral proteins protease inhibitors.
Still, even with the use of combination HAART therapy, there is concern that, over time, the virus will evolve resistance to this therapy. Thus, new anti-HIV drugs are constantly being developed with the hope of continuing the battle against this highly fatal virus. Viruses are acellular entities that can usually only be seen with an electron microscope.
Viruses are diverse, infecting archaea, bacteria, fungi, plants, and animals. Viruses consist of a nucleic-acid core surrounded by a protein capsid with or without an outer lipid envelope. Viral replication within a living cell always produces changes in the cell, sometimes resulting in cell death and sometimes slowly killing the infected cells.
There are six basic stages in the virus replication cycle: attachment, penetration, uncoating, replication, assembly, and release. A viral infection may be productive, resulting in new virions, or nonproductive, meaning the virus remains inside the cell without producing new virions. Viruses cause a variety of diseases in humans. Many of these diseases can be prevented by the use of viral vaccines, which stimulate protective immunity against the virus without causing major disease.
Viral vaccines may also be used in active viral infections, boosting the ability of the immune system to control or destroy the virus. Antiviral drugs that target enzymes and other protein products of viral genes have been developed and used with mixed success. Combinations of anti-HIV drugs have been used to effectively control the virus, extending the lifespan of infected individuals. Learning Objectives By the end of this section, you will be able to: Describe how viruses were first discovered and how they are detected Explain the detailed steps of viral replication Describe how vaccines are used in prevention and treatment of viral diseases.
Figure This figure shows three relatively complex virions: the bacteriophage T4, with its DNA-containing head group and tail fibers that attach to host cells; adenovirus, which uses spikes from its capsid to bind to the host cells; and HIV, which uses glycoproteins embedded in its envelope to do so.
Notice that HIV has proteins called matrix proteins, internal to the envelope, which help stabilize virion shape. A All viruses are encased in a viral membrane. B The capsomere is made up of small protein subunits called capsids. C DNA is the genetic material in all viruses. D Glycoproteins help the virus attach to the host cell. Hiccups during the copying process of viruses cause recombination to take place: the exchange of segments of viral RNA.
As well as providing insight into how viruses direct the host cell to create new virus Print Email Share. Boy or Girl? Can't Find Your Keys? Living Well. View all the latest top news in the environmental sciences, or browse the topics below:. Keyword: Search. When infection of a cell by a bacteriophage results in the production of new virions, the infection is said to be productive. Lytic versus lysogenic cycle : A temperate bacteriophage has both lytic and lysogenic cycles.
In the lytic cycle, the phage replicates and lyses the host cell. In the lysogenic cycle, phage DNA is incorporated into the host genome, where it is passed on to subsequent generations. Environmental stressors such as starvation or exposure to toxic chemicals may cause the prophage to excise and enter the lytic cycle.
With lytic phages, bacterial cells are broken open lysed and destroyed after immediate replication of the virion. As soon as the cell is destroyed, the phage progeny can find new hosts to infect. An example of a lytic bacteriophage is T4, which infects E. Lytic phages are more suitable for phage therapy. Some lytic phages undergo a phenomenon known as lysis inhibition, where completed phage progeny will not immediately lyse out of the cell if extracellular phage concentrations are high.
In contrast, the lysogenic cycle does not result in immediate lysing of the host cell. Those phages able to undergo lysogeny are known as temperate phages. Their viral genome will integrate with host DNA and replicate along with it fairly harmlessly, or may even become established as a plasmid.
The virus remains dormant until host conditions deteriorate, perhaps due to depletion of nutrients; then, the endogenous phages known as prophages become active. At this point they initiate the reproductive cycle, resulting in lysis of the host cell. An example of a bacteriophage known to follow the lysogenic cycle and the lytic cycle is the phage lambda of E. Viruses that infect plant or animal cells may also undergo infections where they are not producing virions for long periods.
An example is the animal herpes viruses, including herpes simplex viruses, which cause oral and genital herpes in humans. In a process called latency, these viruses can exist in nervous tissue for long periods of time without producing new virions, only to leave latency periodically and cause lesions in the skin where the virus replicates.
Even though there are similarities between lysogeny and latency, the term lysogenic cycle is usually reserved to describe bacteriophages.
Animal viruses have their genetic material copied by a host cell after which they are released into the environment to cause disease. Animal viruses, unlike the viruses of plants and bacteria, do not have to penetrate a cell wall to gain access to the host cell.
When a protein in the viral capsid binds to its receptor on the host cell, the virus may be taken inside the cell via a vesicle during the normal cell process of receptor-mediated endocytosis.
An alternative method of cell penetration used by non-enveloped viruses is for capsid proteins to undergo shape changes after binding to the receptor, creating channels in the host cell membrane. Enveloped viruses also have two ways of entering cells after binding to their receptors: receptor-mediated endocytosis and fusion. Many enveloped viruses enter the cell by receptor-mediated endocytosis in a fashion similar to some non-enveloped viruses.
On the other hand, fusion only occurs with enveloped virions. These viruses, which include HIV among others, use special fusion proteins in their envelopes to cause the envelope to fuse with the plasma membrane of the cell, thus releasing the genome and capsid of the virus into the cell cytoplasm. After making their proteins and copying their genomes, animal viruses complete the assembly of new virions and exit the cell.
On the other hand, non-enveloped viral progeny, such as rhinoviruses, accumulate in infected cells until there is a signal for lysis or apoptosis, and all virions are released together.
Animal viruses are associated with a variety of human diseases. Some of them follow the classic pattern of acute disease, where symptoms worsen for a short period followed by the elimination of the virus from the body by the immune system with eventual recovery from the infection. Examples of acute viral diseases are the common cold and influenza.
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