How does a virus infect another cell

After entering the body in the case of coronavirus, this is thought to occur through the nose, mouth, or eyes , a virus attaches itself to a host cell and inserts its genetic instructions. The virus can then hijack the host cell's functions to produce the components needed for it to create copies of itself.

Those components self-assemble into new viruses, which eventually burst from the host cell and go on to infect other cells, either in the original host or in a new host. SARS-CoV-2 is an enveloped virus, meaning that it has a lipid membrane an outer covering made of a fat-like substance.

As you will learn in the next module, animal viruses are associated with a variety of human diseases. Some of them follow the classic pattern of acute disease , where symptoms get increasingly worse for a short period followed by the elimination of the virus from the body by the immune system and 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, only cause intermittent symptoms. Still other viruses, such as human herpesviruses 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, and thus we say 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, and many infections are detected only by routine blood work on patients with risk factors such as intravenous drug use. On the other hand, 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, all the while producing 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 already discussed, 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 or weeks 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 herpesviruses as well, including the varicella-zoster virus that causes chickenpox. Some animal-infecting viruses, including the hepatitis C virus discussed above, are known as oncogenic viruses : They have the ability to cause cancer.

These viruses interfere with the normal regulation of the host cell cycle either by introducing genes that stimulate unregulated cell growth oncogenes or by interfering with the expression of genes that inhibit cell growth. Cancers known to be associated with viral infections include cervical cancer, caused by human papillomavirus HPV Figure 5 , liver cancer caused by hepatitis B virus, T-cell leukemia, and several types of lymphoma. View this video to learn the various stages of the replicative cycles of animal viruses and click on the flash animation links.

All Rights Reserved. Skip to content By the end of this section, you will be able to do the following: List the steps of replication and explain what occurs at each step Describe the lytic and lysogenic cycles of virus replication Explain the transmission of plant and animal viruses Discuss some of the diseases caused by plant and animal viruses Discuss the economic impact of plant and animal viruses. Steps of Virus Infections A virus must use its host-cell processes to replicate.

Attachment A virus attaches to a specific receptor site on the host cell membrane through attachment proteins in the capsid or via glycoproteins embedded in the viral envelope. Entry Viruses may enter a host cell either with or without the viral capsid.

Replication and Assembly The replication mechanism depends on the viral genome. Egress The last stage of viral replication is the release of the new virions produced in the host organism, where they are able to infect adjacent cells and repeat the replication cycle. Figure 1: The influenza reproductive cycle.

In influenza virus infection, glycoproteins on the capsid attach to a host epithelial cell. Following this, the virus is engulfed. RNA and proteins are then made and assembled into new virions. Bacteriophages Figure 2: Bacteriophages attached to a host cell transmission electron micrograph.

In bacteriophage with tails, like the one shown here, the tails serve as a passageway for transmission of the phage genome. Graham Beards; scale-bar data from Matt Russell. Figure 3: 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. Table 1: Some Common Symptoms of Plant Viral Diseases Symptom Appears as Hyperplasia Galls tumors Hypoplasia Thinned, yellow splotches on leaves Cell necrosis Dead, blackened stems, leaves, or fruit Abnormal growth patterns Malformed stems, leaves, or fruit Discoloration Yellow, red, or black lines, or rings in stems, leaves, or fruit Plant viruses can seriously disrupt crop growth and development, significantly affecting our food supply.

Animal Viruses Animal viruses, unlike the viruses of plants and bacteria, do not have to penetrate a cell wall to gain access to the host cell. Figure 4: A latent virus infection. Its double-stranded DNA genome becomes incorporated in the host DNA and can reactivate after latency in the form of b shingles, often exhibiting a rash. Erskine Palmer, B. Figure 5: HPV, or human papillomavirus, has a naked icosahedral capsid visible in this transmission electron micrograph and a double-stranded DNA genome that is incorporated into the host DNA.

The virus, which is sexually transmitted, is oncogenic and can lead to cervical cancer. For example, cold and flu viruses will attack cells that line the respiratory or digestive tracts. All viruses have some type of protein on the outside coat or envelope that "feels" or "recognizes" the proper host cell s.

This protein attaches the virus to the membrane of the host cell. Some enveloped viruses can dissolve right through the cell membrane of the host because both the virus envelope and the cell membrane are made of lipids. Those viruses that do not enter the cell must inject their contents genetic instructions, enzymes into the host cell.

Those viruses that dissolve into a cell simply release their contents once inside the host. In either case, the results are the same. A study published in by Massachusetts Institute of Technology researchers found that improving the rates of hand-washing by travelers passing through airports could significantly reduce the spread of viruses and other infectious diseases [source: MIT ].

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