Which of the Following Processes Can Viruses Carry Out?
Chapter 12: Introduction to the Immune Arrangement and Affliction
12.1 Viruses
Learning Objectives
By the stop of this department, you will be able to:
- Draw how viruses were first discovered and how they are detected
- Explicate the detailed steps of viral replication
- Describe how vaccines are used in prevention and handling of viral diseases
No one knows exactly when viruses emerged or from where they came, since viruses practice not leave historical footprints such as fossils. Modern viruses are thought to be a mosaic of bits and pieces of nucleic acids picked up from various sources along their respective evolutionary paths. Viruses are acellular, parasitic entities that are not classified inside whatever domain because they are not considered alive. They take no plasma membrane, internal organelles, or metabolic processes, and they practice not divide. Instead, they infect a host cell and utilize the host'south replication processes to produce progeny virus particles. Viruses infect all forms of organisms including leaner, archaea, fungi, plants, and animals. Living things grow, metabolize, and reproduce. Viruses replicate, but to do then, they are entirely dependent on their host cells. They do not metabolize or grow, merely are assembled in their mature form.
Viruses are diverse. They vary in their construction, their replication methods, and in their target hosts or even host cells. While nigh biological diversity can exist understood through evolutionary history, such every bit how species have adapted to conditions and environments, much about virus origins and evolution remains unknown.
How Viruses Replicate
Viruses were first discovered later on the evolution of a porcelain filter, called the Chamberland-Pasteur filter, which could remove all bacteria visible nether the microscope from any liquid sample. In 1886, Adolph Meyer demonstrated that a affliction of tobacco plants, tobacco mosaic disease, could exist transferred from a diseased institute to a healthy one through liquid plant extracts. In 1892, Dmitri Ivanowski showed that this affliction could be transmitted in this way even after the Chamberland-Pasteur filter had removed all viable bacteria from the extract. All the same, it was many years before information technology was proven that these "filterable" infectious agents were non only very modest bacteria merely were a new type of tiny, disease-causing particle.
Virions, single virus particles, are very pocket-size, well-nigh xx–250 nanometers (1 nanometer = 1/1,000,000 mm). These individual virus particles are the infectious grade of a virus outside the host cell. Different leaner (which are about 100 times larger), nosotros cannot see viruses with a light microscope, with the exception of some large virions of the poxvirus family (Figure 12.three).
Information technology was not until the development of the electron microscope in the 1940s that scientists got their first good view of the construction of the tobacco mosaic virus (Figure 12.2) and others. The surface construction of virions tin be observed past both scanning and manual electron microscopy, whereas the internal structures of the virus tin only be observed in images from a transmission electron microscope (Figure 12.iv).
The apply of this applied science has allowed for the discovery of many viruses of all types of living organisms. They were initially grouped by shared morphology, meaning their size, shape, and distinguishing structures. Later, groups of viruses were classified by the type of nucleic acid they contained, DNA or RNA, and whether their nucleic acrid was unmarried- or double-stranded. More recently, molecular assay of viral replication cycles has further refined their classification.
A virion consists of a nucleic-acid core, an outer poly peptide coating, and sometimes an outer envelope made of protein and phospholipid membranes derived from the host cell. The nearly visible difference betwixt members of viral families is their morphology, which is quite various. An interesting feature of viral complexity is that the complexity of the host does non correlate to the complexity of the virion. Some of the well-nigh circuitous virion structures are observed in bacteriophages, viruses that infect the simplest living organisms, leaner.
Viruses come in many shapes and sizes, but these are consistent and distinct for each viral family (Figure 12.5). All virions have a nucleic-acrid genome covered by a protective layer of protein, called a capsid. The capsid is made of protein subunits called capsomeres. Some viral capsids are simple polyhedral "spheres," whereas others are quite complex in structure. The outer structure surrounding the capsid of some viruses is called the viral envelope. All viruses apply some sort of glycoprotein to attach to their host cells at molecules on the cell called viral receptors. The virus exploits these cell-surface molecules, which the jail cell uses for some other purpose, as a way to recognize and infect specific cell types. For example, the measles virus uses a cell-surface glycoprotein in humans that usually functions in immune reactions and possibly in the sperm-egg interaction at fertilization. Zipper is a requirement for viruses to later on penetrate the cell membrane, inject the viral genome, and complete their replication inside the cell.
The T4 bacteriophage, which infects the East. coli bacterium, is among the most circuitous virion known; T4 has a poly peptide tail structure that the virus uses to attach to the host cell and a head structure that houses its DNA.
Adenovirus, a nonenveloped animal virus that causes respiratory illnesses in humans, uses poly peptide spikes protruding from its capsomeres to attach to the host cell. Nonenveloped viruses as well include those that crusade polio (poliovirus), plantar warts (papillomavirus), and hepatitis A (hepatitis A virus). Nonenveloped viruses tend to exist more robust and more probable to survive nether harsh conditions, such as the gut.
Enveloped virions like HIV (human immunodeficiency virus), the causative agent in AIDS (acquired immune deficiency syndrome), consist of nucleic acid (RNA in the example of HIV) and capsid proteins surrounded by a phospholipid bilayer envelope and its associated proteins (Figure 12.v). Chicken pox, influenza, and mumps are examples of diseases caused by viruses with envelopes. Because of the fragility of the envelope, nonenveloped viruses are more resistant to changes in temperature, pH, and some disinfectants than enveloped viruses.
Overall, the shape of the virion and the presence or absence of an envelope tells us little nigh what diseases the viruses may crusade or what species they might infect, simply is still a useful ways to begin viral classification.
Which of the following statements nearly virus structure is true?
A) All viruses are encased in a viral membrane.
B) The capsomere is fabricated up of small-scale poly peptide subunits called capsids.
C) DNA is the genetic material in all viruses.
D) Glycoproteins help the virus attach to the host cell.
<!–D–>
Unlike all living organisms that use Dna as their genetic material, viruses may use either DNA or RNA as theirs. The virus core contains the genome or full genetic content of the virus. Viral genomes tend to be pocket-sized compared to bacteria or eukaryotes, containing only those genes that code for proteins the virus cannot become from the host prison cell. This genetic textile may be single-stranded or double-stranded. It may besides be linear or circular. While nigh viruses contain a single segment of nucleic acid, others take genomes that consist of several segments.
Dna viruses have a Deoxyribonucleic acid core. The viral Dna directs the host cell's replication proteins to synthesize new copies of the viral genome and to transcribe and translate that genome into viral proteins. Deoxyribonucleic acid viruses cause human diseases such as chickenpox, hepatitis B, and some venereal diseases like herpes and genital warts.
RNA viruses contain just RNA in their cores. To replicate their genomes in the host prison cell, the genomes of RNA viruses encode enzymes not constitute in host cells. RNA polymerase enzymes are non as stable every bit DNA polymerases and often make mistakes during transcription. For this reason, mutations, changes in the nucleotide sequence, in RNA viruses occur more than often than in DNA viruses. This leads to more than rapid evolution and change in RNA viruses. For example, the fact that flu is an RNA virus is one reason a new flu vaccine is needed every yr. Man diseases caused by RNA viruses include hepatitis C, measles, and rabies.
Viruses can exist seen as obligate intracellular parasites. The virus must attach to a living cell, be taken within, manufacture its proteins and re-create its genome, and find a mode to escape the jail cell so the virus can infect other cells and ultimately other individuals. Viruses can infect only certain species of hosts and only certain cells within that host. The molecular basis for this specificity is that a particular surface molecule, known every bit the viral receptor, must be found on the host cell surface for the virus to attach. Also, metabolic differences seen in different prison cell types based on differential factor expression are a likely factor in which cells a virus may employ to replicate. The cell must exist making the substances the virus needs, such as enzymes the virus genome itself does not have genes for, or the virus will non be able to replicate using that cell.
Steps of Virus Infections
A virus must "take over" a jail cell to replicate. The viral replication cycle tin produce dramatic biochemical and structural changes in the host prison cell, which may cause jail cell damage. These changes, called cytopathic effects, can change cell functions or even destroy the jail cell. Some infected cells, such as those infected by the mutual common cold virus (rhinovirus), die through lysis (bursting) or apoptosis (programmed prison cell expiry or "jail cell suicide"), releasing all the progeny virions at once. The symptoms of viral diseases result from the allowed response to the virus, which attempts to command and eliminate the virus from the body, and from cell impairment acquired by the virus. Many animal viruses, such as HIV (human immunodeficiency virus), leave the infected cells of the immune organisation by a process known as budding, where virions go out the cell individually. During the budding procedure, the cell does non undergo lysis and is not immediately killed. Yet, the impairment to the cells that HIV infects may make it impossible for the cells to part as mediators of immunity, even though the cells remain alive for a period of time. Most productive viral infections follow similar steps in the virus replication bike: zipper, penetration, uncoating, replication, assembly, and release.
A virus attaches to a specific receptor site on the host-cell membrane through attachment proteins in the capsid or proteins embedded in its envelope. The attachment is specific, and typically a virus will only attach to cells of one or a few species and but sure jail cell types within those species with the appropriate receptors.
Concept in Activeness
View this video for a visual explanation of how HIV and influenza attack the body.
Unlike animate being viruses, the nucleic acrid of bacteriophages is injected into the host prison cell naked, leaving the capsid exterior the cell. Found and animal viruses can enter their cells through endocytosis, in which the cell membrane surrounds and engulfs the entire virus. 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 acrid is released, which then becomes available for replication and transcription.
The replication mechanism depends on the viral genome. Deoxyribonucleic acid viruses usually use host cell proteins and enzymes to make additional DNA that is used to copy the genome or exist transcribed to messenger RNA (mRNA), which is then used in protein synthesis. RNA viruses, such as the flu virus, usually use the RNA core as a template for synthesis of viral genomic RNA and mRNA. The viral mRNA is translated into viral enzymes and capsid proteins to gather new virions (Figure 12.6). 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 straight synthesis of the missing proteins. Retroviruses, such as HIV, have an RNA genome that must exist reverse transcribed to make Deoxyribonucleic acid, which then is inserted into the host's DNA. To catechumen RNA into DNA, retroviruses contain genes that encode the virus-specific enzyme opposite transcriptase that transcribes an RNA template to Deoxyribonucleic acid. The fact that HIV produces some of its ain enzymes, which are not institute in the host, has allowed researchers to develop drugs that inhibit these enzymes. These drugs, including the reverse transcriptase inhibitor AZT, inhibit HIV replication by reducing the activity of the enzyme without affecting the host's metabolism.
The last stage of viral replication is the release of the new virions into the host organism, where they are able to infect adjacent cells and repeat the replication cycle. Some viruses are released when the host cell dies and other viruses tin can leave infected cells by budding through the membrane without directly killing the cell.
Influenza virus is packaged in a viral envelope, which fuses with the plasma membrane. This way, the virus can exit the host jail cell without killing it. What reward does the virus gain by keeping the host cell alive?
<!–The host cell can proceed to make new virus particles.–>
Concept in Action
Click through this tutorial on viruses to identify structures, modes of manual, replication, and more.
Viruses and Illness
Viruses cause a diverseness of diseases in animals, including humans, ranging from the common cold to potentially fatal illnesses like meningitis (Figure 12.7). These diseases tin can be treated by antiviral drugs or past vaccines, but some viruses, such as HIV, are capable of avoiding the immune response and mutating so every bit to become resistant to antiviral drugs.
Vaccines for Prevention
While we do have limited numbers of constructive antiviral drugs, such as those used to treat HIV and flu, the principal method of controlling viral illness is past vaccination, which is intended to prevent outbreaks by building immunity to a virus or virus family. A vaccine may be prepared using weakened alive viruses, killed viruses, or molecular subunits of the virus. In general, alive viruses atomic number 82 to better immunity, but accept the possibility of causing illness at some low frequency. Killed viral vaccine and the subunit viruses are both incapable of causing affliction, but in full general atomic number 82 to less effective or long-lasting immunity.
Weakened live viral vaccines are designed in the laboratory to crusade few symptoms in recipients while giving them immunity against future infections. Polio was one disease that represented a milestone in the use of vaccines. Mass immunization campaigns in the U.Southward. in the 1950s (killed vaccine) and 1960s (live vaccine) essentially eradicated the disease, which caused muscle paralysis in children and generated fear in the general population when regional epidemics occurred. The success of the polio vaccine paved the way for the routine impunity of childhood vaccines against measles, mumps, rubella, chickenpox, and other diseases.
Alive vaccines are usually made by attenuation (weakening) of the "wild-type" (illness-causing) virus by growing it in the laboratory in tissues or at temperatures different from what the virus is accustomed to in the host. For example, the virus may be grown in cells in a examination tube, in bird embryos, or in live animals. The accommodation to these new cells or temperature induces mutations in the virus' genomes, allowing them to grow better in the laboratory while inhibiting their ability to crusade illness when reintroduced into the atmospheric condition institute in the host. These attenuated viruses thus still cause an infection, but they practice not grow very well, allowing the immune response to develop in fourth dimension to forbid major disease. The danger of using live vaccines, which are usually more than effective than killed vaccines, is the low but pregnant take a chance that these viruses volition revert back to their disease-causing class by dorsum mutations. Dorsum mutations occur when the vaccine undergoes mutations in the host such that information technology readapts to the host and tin again cause disease, which can so be spread to other humans in an epidemic. This happened equally recently equally 2007 in Nigeria where mutations in a polio vaccine led to an epidemic of polio in that country.
Some vaccines are in continuous development considering sure viruses, such as influenza and HIV, have a high mutation rate compared to other viruses or host cells. With flu, mutation in genes for the surface molecules helps the virus evade the protective immunity that may have been obtained in a previous influenza flavour, making it necessary for individuals to go vaccinated every year. Other viruses, such every bit those that crusade the babyhood diseases measles, mumps, and rubella, mutate so lilliputian that the same vaccine is used twelvemonth after year.
Vaccines and Antiviral Drugs for Treatment
In some cases, vaccines can exist used to care for an active viral infection. In the case of rabies, a fatal neurological disease transmitted in the saliva of rabies virus-infected animals, the progression of the disease from the time of the animal bite to the time it enters the fundamental nervous system may be two weeks or longer. This is enough fourth dimension to vaccinate an private who suspects existence bitten by a rabid animate being, and the boosted immune response from the vaccination is enough to prevent the virus from inbound nervous tissue. Thus, the fatal neurological consequences of the illness are averted and the individual only has to recover from the infected bite. This approach is also being used for the treatment of Ebola, one of the fastest and most mortiferous viruses affecting humans, though unremarkably infecting express populations. Ebola is too a leading crusade of death in gorillas. Transmitted by bats and great apes, this virus can cause expiry in 70–xc percent of the infected within ii weeks. Using newly developed vaccines that heave the immune response, there is hope that immune systems of affected individuals will be meliorate able to control the virus, potentially reducing bloodshed rates.
Another way of treating viral infections is the apply of antiviral drugs. These drugs often have limited ability to cure viral affliction simply have been used to command and reduce symptoms for a wide diverseness of viral diseases. For most viruses, these drugs inhibit the virus by blocking the actions of 1 or more than of its proteins. It is important that the targeted proteins be encoded for past viral genes and that these molecules are not present in a healthy host cell. In this mode, 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 impact multiple viruses.
Antivirals have been developed to treat genital herpes (herpes simplex 2) and influenza. For genital herpes, drugs such every bit acyclovir tin 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 torso for life, this drug is not a cure but can make the symptoms of the affliction more manageable. For influenza, drugs like Tamiflu can reduce the elapsing of "flu" symptoms by one or two days, only the drug does non foreclose symptoms entirely. Other antiviral drugs, such equally Ribavirin, have been used to treat a diversity 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 normally fatal within 10–12 years later on existence infected. Anti-HIV drugs take been able to command viral replication to the betoken that individuals receiving these drugs survive for a significantly longer time than the untreated.
Anti-HIV drugs inhibit viral replication at many different phases of the HIV replicative wheel. Drugs accept been adult 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 (opposite transcriptase inhibitors), the integration of the viral DNA into the host genome (integrase inhibitors), and the processing of viral proteins (protease inhibitors).
When any of these drugs are used individually, the virus' high mutation rate allows the virus to speedily evolve resistance to the drug. The breakthrough in the handling of HIV was the development of highly active anti-retroviral therapy (HAART), which involves a mixture of different drugs, sometimes called a drug "cocktail." By attacking the virus at unlike stages of its replication wheel, information technology is difficult for the virus to develop resistance to multiple drugs at the same fourth dimension. Yet, even with the use of combination HAART therapy, there is business organization that, over time, the virus will evolve resistance to this therapy. Thus, new anti-HIV drugs are constantly being developed with the promise of continuing the battle confronting this highly fatal virus.
Section Summary
Viruses are acellular entities that tin ordinarily only be seen with an electron microscope. Their genomes contain either Dna or RNA, and they replicate using the replication proteins of a host prison cell. Viruses are various, infecting archaea, leaner, fungi, plants, and animals. Viruses consist of a nucleic-acid cadre 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. At that place are six basic stages in the virus replication wheel: attachment, penetration, uncoating, replication, assembly, and release. A viral infection may exist productive, resulting in new virions, or nonproductive, meaning the virus remains within 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 amnesty 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 accept been used to effectively control the virus, extending the lifespan of infected individuals.
Glossary
acellular: lacking cells
apoptosis: the cell death caused by induction of a cell's own internal mechanisms either as a natural step in the development of a multicellular organism or by other environmental factors such equally signals from cells of the immune system
attenuation: the weakening of a virus during vaccine evolution
capsid: the protein blanket of the viral cadre
cytopathic: causing cell damage
glycoprotein: a protein molecule with attached carbohydrate molecules
vaccine: a weakened solution of virus components, viruses, or other agents that produce an immune response
virion: an individual virus particle outside a host prison cell
viral envelope: a lipid bilayer that envelops some viruses
Source: https://opentextbc.ca/biology/chapter/12-1-viruses/
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