Once replication has been completed and the host cell is exhausted of all resources in making viral progeny, the viruses may begin to leave the cell by several methods. Different types of virus have varying sites of synthesis and replication. Virus assembly depends on the site of synthesis and such sites are the nucleus, endoplasmic reticulum, and the Golgi apparatus aka Golgi body. Aside from this, assembly also occurs in the viroplasm which is an inclusion body in a cell.
When the virus has replicated and multiplied, they would want to leave the infected cell and infect other cells. However, they require an envelope to enclose the DNA as well as to bind with the other healthy cells so that they can infect. Budding is a method which viruses use to exit the cell. Prior to budding, the virus may put its own receptor onto the surface of the cell in preparation for the virus to bud through, forming an envelope with the viral receptors already on it.
This process will slowly use up the cell membrane and eventually lead to the demise of the cell. This is also how antiviral responses are able to detect virus infected cells. Other methods for exit would be cell lysis. This method releases the virus from the infected cell by bursting its membrane and this kills the cell as well.
Microviridae and Leviviridae inhibit cell wall biosynthesis and induce cytolysis. Occasional lysis : Many viruses can induce cell lysis under special circumstance. Occlusion body: In the late phase of host infection some viruses induce the formation of a crystalline protein matrix that ends up with cell lysis.
The viruses trapped in the occlusion body are often involved in a host-to-host infection. This strategy is used by Baculoviridae , insects infecting Iridoviridae , Cypovirus and insects infecting Poxviridae. Immune response: In vertebrate hosts, infected cell lysis can be induced by natural killer cells or cytotoxic T cells responding to the infection. Contact Us Home.
Cell lysis Cell lysis is a common outcome of viral infection. The adenovirus death protein E J Virol. Functional and structural characterization of 2B viroporin membranolytic domains.
The SV40 late protein VP4 is a viroporin that forms pores to disrupt membranes for viral release. PLoS Pathog. 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. Other viruses cause long-term chronic infections, such as the virus causing hepatitis C, whereas others, like herpes simplex virus, cause only intermittent symptoms. Still other viruses, such as human herpes viruses 6 and 7, which in some cases can cause the minor childhood disease roseola, often successfully cause productive infections without causing any symptoms at all in the host; these patients have an asymptomatic infection.
In hepatitis C infections, the virus grows and reproduces in liver cells, causing low levels of liver damage. The damage is so low that infected individuals are often unaware that they are infected, with many infections only detected by routine blood work on patients with risk factors such as intravenous drug use. Since many of the symptoms of viral diseases are caused by immune responses, a lack of symptoms is an indication of a weak immune response to the virus. This allows the virus to escape elimination by the immune system and persist in individuals for years, while continuing to produce low levels of progeny virions in what is known as a chronic viral disease.
Chronic infection of the liver by this virus leads to a much greater chance of developing liver cancer, sometimes as much as 30 years after the initial infection. As mentioned, herpes simplex virus can remain in a state of latency in nervous tissue for months, even years. Under certain conditions, including various types of physical and psychological stress, the latent herpes simplex virus may be reactivated and undergo a lytic replication cycle in the skin, causing the lesions associated with the disease.
Once virions are produced in the skin and viral proteins are synthesized, the immune response is again stimulated and resolves the skin lesions in a few days by destroying viruses in the skin. As a result of this type of replicative cycle, appearances of cold sores and genital herpes outbreaks only occur intermittently, even though the viruses remain in the nervous tissue for life. Latent infections are common with other herpes viruses as well, including the varicella-zoster virus that causes chickenpox.
Chicken pox virus : a Varicella-zoster, the virus that causes chickenpox, has an enveloped icosahedral capsid visible in this transmission electron micrograph. Its double-stranded DNA genome incorporates into the host DNA and reactivates after latency in the form of b shingles, often exhibiting a rash.
Plant viruses can cause damage to stems, leaves, and fruits and can have a major impact on the economy because of food supply disruptions. As plant viruses have a cell wall to protect their cells, their viruses do not use receptor-mediated endocytosis to enter host cells as is seen with animal viruses.
This damage is often caused by weather, insects, animals, fire, or human activities such as farming or landscaping. Additionally, plant offspring may inherit viral diseases from parent plants. When plant viruses are transferred between different plants, this is known as horizontal transmission; when they are inherited from a parent, this is called vertical transmission.
Symptoms of viral diseases vary according to the virus and its host. One common symptom is hyperplasia: the abnormal proliferation of cells that causes the appearance of plant tumors known as galls. Other viruses induce hypoplasia, or decreased cell growth, in the leaves of plants, causing thin, yellow areas to appear. Still other viruses affect the plant by directly killing plant cells; a process known as cell necrosis. Other symptoms of plant viruses include malformed leaves, black streaks on the stems of the plants, altered growth of stems, leaves, or fruits, and ring spots, which are circular or linear areas of discoloration found in a leaf.
Oak tree galls : Galls are abnormal plant growth or swellings comprised of plant tissue. Galls are usually found on foliage or twigs.
These unusual deformities are caused by plant growth-regulating chemicals or stimuli produced by an insect or other arthropod pest species. The chemicals produced by these causal organisms interfere with normal plant cell growth. Plant viruses can seriously disrupt crop growth and development, significantly affecting our food supply.
They are responsible for poor crop quality and quantity globally, and can bring about huge economic losses annually. Other viruses may damage plants used in landscaping. Some viruses that infect agricultural food plants include the name of the plant they infect, such as tomato spotted wilt virus, bean common mosaic virus, and cucumber mosaic virus. In plants used for landscaping, two of the most common viruses are peony ring spot and rose mosaic virus.
There are far too many plant viruses to discuss each in detail, but symptoms of bean common mosaic virus result in lowered bean production and stunted, unproductive plants.
In the ornamental rose, the rose mosaic disease causes wavy yellow lines and colored splotches on the leaves of the plant. Privacy Policy.
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