The life cycle begins with the penetration of the virus into the host cell. Next, the virus is uncoated within the cytoplasm of the cell when the capsid is removed. Depending on the type of nucleic acid, cellular components are used to replicate the viral genome and synthesize viral proteins for assembly of new virions.
To establish a systemic infection, the virus must enter a part of the vascular system of the plant, such as the phloem.
The time required for systemic infection may vary from a few days to a few weeks depending on the virus, the plant species, and the environmental conditions. The virus life cycle is complete when it is transmitted from an infected plant to a healthy plant. Unlike the growth curve for a bacterial population, the growth curve for a virus population over its life cycle does not follow a sigmoidal curve.
During the initial stage, an inoculum of virus causes infection. In the eclipse phase , viruses bind and penetrate the cells with no virions detected in the medium. The chief difference that next appears in the viral growth curve compared to a bacterial growth curve occurs when virions are released from the lysed host cell at the same time.
Such an occurrence is called a burst , and the number of virions per bacterium released is described as the burst size. In a one-step multiplication curve for bacteriophage , the host cells lyse, releasing many viral particles to the medium, which leads to a very steep rise in viral titer the number of virions per unit volume.
If no viable host cells remain, the viral particles begin to degrade during the decline of the culture see Figure 8. Figure 8. The one-step multiplication curve for a bacteriophage population follows three steps: 1 inoculation, during which the virions attach to host cells; 2 eclipse, during which entry of the viral genome occurs; and 3 burst, when sufficient numbers of new virions are produced and emerge from the host cell.
The burst size is the maximum number of virions produced per bacterium. Ebola is incurable and deadly. The outbreak in West Africa in was unprecedented, dwarfing other human Ebola epidemics in the level of mortality. Of 24, suspected or confirmed cases reported, 10, people died. No approved treatments or vaccines for Ebola are available. While some drugs have shown potential in laboratory studies and animal models, they have not been tested in humans for safety and effectiveness.
Not only are these drugs untested or unregistered but they are also in short supply. Given the great suffering and high mortality rates, it is fair to ask whether unregistered and untested medications are better than none at all. Should such drugs be dispensed and, if so, who should receive them, in light of their extremely limited supplies?
Is it ethical to treat untested drugs on patients with Ebola? On the other hand, is it ethical to withhold potentially life-saving drugs from dying patients? Or should the drugs perhaps be reserved for health-care providers working to contain the disease? In August , two infected US aid workers and a Spanish priest were treated with ZMapp , an unregistered drug that had been tested in monkeys but not in humans.
The two American aid workers recovered, but the priest died. Later that month, the WHO released a report on the ethics of treating patients with the drug. Since Ebola is often fatal, the panel reasoned that it is ethical to give the unregistered drugs and unethical to withhold them for safety concerns. On September 24, , Thomas Eric Duncan arrived at the Texas Health Presbyterian Hospital in Dallas complaining of a fever, headache, vomiting, and diarrhea—symptoms commonly observed in patients with the cold or the flu.
After examination, an emergency department doctor diagnosed him with sinusitis, prescribed some antibiotics, and sent him home. Two days later, Duncan returned to the hospital by ambulance. His condition had deteriorated and additional blood tests confirmed that he has been infected with the Ebola virus. Further investigations revealed that Duncan had just returned from Liberia, one of the countries in the midst of a severe Ebola epidemic.
On September 15, nine days before he showed up at the hospital in Dallas, Duncan had helped transport an Ebola-stricken neighbor to a hospital in Liberia. The hospital continued to treat Duncan, but he died several days after being admitted. Figure 9. Researchers working with Ebola virus use layers of defenses against accidental infection, including protective clothing, breathing systems, and negative air-pressure cabinets for bench work.
The timeline of the Duncan case is indicative of the life cycle of the Ebola virus. The incubation time for Ebola ranges from 2 days to 21 days. This corresponds, in part, to the eclipse period in the growth of the virus population. During the eclipse phase, Duncan would have been unable to transmit the disease to others. However, once an infected individual begins exhibiting symptoms, the disease becomes very contagious. Ebola virus is transmitted through direct contact with droplets of bodily fluids such as saliva, blood, and vomit.
Duncan could conceivably have transmitted the disease to others at any time after he began having symptoms, presumably some time before his arrival at the hospital in Dallas. Once a hospital realizes a patient like Duncan is infected with Ebola virus, the patient is immediately quarantined, and public health officials initiate a back trace to identify everyone with whom a patient like Duncan might have interacted during the period in which he was showing symptoms.
Public health officials were able to track down 10 high-risk individuals family members of Duncan and 50 low-risk individuals to monitor them for signs of infection. None contracted the disease. What is the name for the transfer of genetic information from one bacterium to another bacterium by a phage?
Skip to main content. Acellular Pathogens. Search for:. The Viral Life Cycle Learning Objectives Describe the lytic and lysogenic life cycles Describe the replication process of animal viruses Describe unique characteristics of retroviruses and latent viruses Discuss human viruses and their virus-host cell interactions Explain the process of transduction Describe the replication process of plant viruses.
Think about It Is a latent phage undetectable in a bacterium? Think about It Which phage life cycle is associated with which forms of transduction? Think about It In what two ways can a virus manage to maintain a persistent infection?
Think about It What is the structure and genome of a typical plant virus? Think about It What aspect of the life cycle of a virus leads to the sudden increase in the growth curve? Unregistered Treatments Ebola is incurable and deadly.
Ebola in the US On September 24, , Thomas Eric Duncan arrived at the Texas Health Presbyterian Hospital in Dallas complaining of a fever, headache, vomiting, and diarrhea—symptoms commonly observed in patients with the cold or the flu.
For additional information about Ebola, please visit the CDC website. Key Concepts and Summary Many viruses target specific hosts or tissues. Some may have more than one host. Many viruses follow several stages to infect host cells. These stages include attachment, penetration, uncoating, biosynthesis, maturation, and release. Bacteriophages have a lytic or lysogenic cycle.
The lytic cycle leads to the death of the host, whereas the lysogenic cycle leads to integration of phage into the host genome. Bacteriophages inject DNA into the host cell, whereas animal viruses enter by endocytosis or membrane fusion.
Animal viruses can undergo latency , similar to lysogeny for a bacteriophage. The majority of plant viruses are positive-strand ssRNA and can undergo latency, chronic, or lytic infection, as observed for animal viruses. The growth curve of bacteriophage populations is a one-step multiplication curve and not a sigmoidal curve, as compared to the bacterial growth curve.
Bacteriophages transfer genetic information between hosts using either generalized or specialized transduction. Multiple Choice Which of the following leads to the destruction of the host cells?
Lysogenic Cycle: Lysogenic cycle has a prophage stage. Lysogenic Cycle: The lysogenic virus is non virulent. Lytic Cycle: Host cell is lysed during the release of the viral particles in the lytic cycle. Lysogenic Cycle: Host cell is not lysed by the lysogenic cycle. Lytic Cycle: Viral particles are liberated in the lytic cycle. Therefore, lytic cycle produces a progeny of virus. Lysogenic Cycle: Typically, viral particles are not liberated in the lysogenic cycle. Therefore, the lysogenic cycle does not produce a viral progeny.
Lytic Cycle: Lytic cycle occurs within a short period of time. Lysogenic Cycle: Lysogenic cycle takes time. Lytic Cycle: Lytic cycle cannot follow the lysogenic cycle. Lysogenic Cycle: Lysogenic cycle can follow the lytic cycle. Lytic Cycle: Lytic cycle shows the symptoms of viral replication. Lysogenic Cycle: Lysogenic cycle does not show symptoms of viral replication. Lytic Cycle: Lytic cycle does not allow genetic recombination in the host bacterium.
Lysogenic Cycle: Lysogenic cycle allows genetic recombination of the host bacterium. Lytic cycle and lysogenic cycle are two mechanisms of the viral reproduction. In the lytic cycle, the host cell is lysed by the release of the new progeny of the virus. So many different types of viruses exist that nearly every living organism has its own set of viruses that try to infect its cells. Even the smallest and simplest of cells, prokaryotic bacteria, may be attacked by specific types of viruses.
Bacteriophages are viruses that infect bacteria. Bacteriophages may have a lytic cycle or a lysogenic cycle, and a few viruses are capable of carrying out both. When infection of a cell by a bacteriophage results in the production of new virions, the infection is said to be productive.
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.
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