Reproduction: DNA Animal Viruses


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Concept in Action

These receptors are like keys allowing the viral cell to fuse with a cell or attach itself to a cell. The way that these proteins are acquired is through similar process to that of an infection cycle. Therefore, HIV can enter T cells and macrophages. 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. 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.


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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.


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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 are viruses that affect plants. Like all other viruses, plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host. Plant viruses are pathogenic to higher plants.

There are many types of plant virus, but often they only cause a loss of yield, and it is not economically viable to try to control them.

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How do animal DNA viruses get to the nucleus?

Plant viruses are often spread from plant to plant by organisms vectors. These are normally insects, but some fungi, nematode worms and single-celled organisms have been shown to be vectors. When control of plant virus infections is considered economical, for perennial fruits for example , efforts are concentrated on killing the vectors and removing alternate hosts such as weeds. Plant viruses are harmless to humans and other animals because they can only reproduce in living plant cells. To enter the cells, proteins on the surface of the virus interact with proteins of the cell.

Attachment, or adsorption, occurs between the viral particle and the host cell membrane. A hole forms in the cell membrane, then the virus particle or its genetic contents are released into the host cell, where viral reproduction may commence. At this stage, a distinction between susceptibility and permissibility of a host cell is made.

Permissibility determines the outcome of the infection. After control is established and the environment is set for the virus to begin making copies of itself, replication occurs quickly by the millions. After a virus has made many copies of itself, it usually has exhausted the cell of its resources.

The Viral Life Cycle - Biology LibreTexts

The host cell is now no longer useful to the virus, therefore the cell often dies and the newly produced viruses must find a new host. The process by which virus progeny are released to find new hosts, is called shedding. This is the final stage in the viral life cycle.

This hiding is deemed latency. During this time, the virus does not produce any progeny, it remains inactive until external stimuli—such as light or stress—prompts it to activate. Viruses can be spread by direct transfer of sap, and by contact of a wounded plant with a healthy one. Such contact may occur during agricultural practices, when damage is caused by tools or hands, or naturally, when an animal feeds on the plant.

Generally Tobacco mosaic virus TMV , potato viruses, and cucumber mosaic viruses are transmitted via sap. Tobacco mosaic virus and Cauliflower mosaic virus CaMV are frequently used in plant molecular biology. Of special interest is the CaMV 35S promoter, which is a very strong promoter most frequently used in plant transformations. The infection causes characteristic patterns mottling and discoloration on the leaves hence the name.

TMV was the first virus to be discovered. Although it was known from the late 19th century that an infectious disease was damaging tobacco crops, it was not until that the infectious agent was determined to be a virus. Skip to main content. Search for:. Viral Replication General Features of Virus Replication Virologists describe the formation of viruses during the infection process in target host cells as viral replication.

Learning Objectives Outline the features of viral replication. Key Takeaways Key Points Viral populations do not grow through cell division, because they are acellular. Instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves, and they assemble in the cell.

Steps of Virus Infections

The life cycle of viruses differs greatly between species but there are six basic stages in the life cycle of viruses: attachment, penetration viral entry , uncoating, replication, and lysis. Key Terms lysis : The disintegration or destruction of cells leukocyte : A white blood cell. Steps of Virus Infections Viral infection involves the incorporation of viral DNA into a host cell, replication of that material, and the release of the new viruses. Learning Objectives List the steps of viral replication and explain what occurs at each step.

Key Takeaways Key Points Viral replication involves six steps: attachment, penetration, uncoating, replication, assembly, and release. During attachment and penetration, the virus attaches itself to a host cell and injects its genetic material into it. During release, the newly-created viruses are released from the host cell, either by causing the cell to break apart, waiting for the cell to die, or by budding off through the cell membrane.

Key Terms virion : a single individual particle of a virus the viral equivalent of a cell glycoprotein : a protein with covalently-bonded carbohydrates retrovirus : a virus that has a genome consisting of RNA. Learning Objectives Explain viral tropism. Key Takeaways Key Points Viruses must bind to specific cell surface receptors in order to enter a cell. In virology, Tissue tropism is the cells and tissues of a host which support growth of a particular virus or bacteria. Key Terms dendritic cell : Any cell, having branching processes, that forms part of the mammalian immune system. Part of the innate immune system.

Animal Viruses Animal viruses have their genetic material copied by a host cell after which they are released into the environment to cause disease. Learning Objectives Describe various animal viruses and the diseases they cause. Key Takeaways Key Points Animal viruses may enter a host cell by either receptor -mediated endocytosis or by changing shape and entering the cell through the cell membrane.

Viruses cause diseases in humans and other animals; they often have to run their course before symptoms disappear. Examples of viral animal diseases include hepatitis C, chicken pox, and shingles. Key Terms receptor-mediated endocytosis : a process by which cells internalize molecules endocytosis by the inward budding of plasma membrane vesicles containing proteins with receptor sites specific to the molecules being internalized. An essential text for all virology laboratories. This picornavirus is the etiological agent of an acute systemic vesicular disease that affects cattle worldwide.

Here we have addressed several aspects dealing with the molecular biology of this highly variable and transmissible virus that are relevant to understand the viral infectious cycle. Particular emphasis has been given to the peculiarities of its genome organization, as well as its control of gene expression. Soon after infection, the single stranded positive RNA that constitutes the viral genome is efficiently translated using a cap-independent mechanism driven by the internal ribosome entry site element IRES. This process occurs concomitantly with the inhibition of cellular protein synthesis, caused by the expression of viral proteases.

Processing of the viral polyprotein is achieved cotranslationally by viral encoded proteases, giving rise to the different mature viral proteins. Viral RNA as well as viral proteins interact with different components of the host cell, acting as key determinants of viral pathogenesis. In depth knowledge of the molecular basis of the viral cycle is needed to control viral pathogenesis and disease spreading.

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Molecular Biology of Pestiviruses.

Reproduction: DNA Animal Viruses Reproduction: DNA Animal Viruses
Reproduction: DNA Animal Viruses Reproduction: DNA Animal Viruses
Reproduction: DNA Animal Viruses Reproduction: DNA Animal Viruses
Reproduction: DNA Animal Viruses Reproduction: DNA Animal Viruses
Reproduction: DNA Animal Viruses Reproduction: DNA Animal Viruses
Reproduction: DNA Animal Viruses Reproduction: DNA Animal Viruses

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