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Chapter 24 – 1 Answers VOCABULARY REVIEW 1. Virus - an infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host. 2. Capsid - the coiled or polyhedral structure, composed of proteins, that encloses the nucleic acid of a virus. 3. Retrovirus - any of a family of single-stranded RNA viruses having a helical envelope and containing an enzyme that allows for a reversal of genetic transcription, from RNA to DNA rather than the usual DNA 4. Lytic Cycle - The lytic cycle is the process in which a virus overtakes a. cell and uses the cellular machinery of its host to reproduce. 5. Lysogenic Cycle - one of the two alternative life cycles of a virus inside a host cell, whereby the virus that has infected a cell attaches itself to the host DNA and, acting like an inert segment of the DNA, replicates when the host cell divides. This method of replication is contrasted with the lytic cycle, whereby the virus that has entered a cell takes over the cell's replication mechanism, makes viral DNA and viral proteins, and then lyses (breaks open) the cell, allowing the newly produced viruses to leave the now disintegrated host cell to infect other cells. While the lysogenic cycle causes no harm to the host cell MULTIPLE CHOICE 1. 2. 3. 4. 5. D C D C C SHORT ANSWER 1. The work of Wendell Stanley suggested that viruses are made of made of DNA or RNA surrounded by a protein coat. 2. Many viruses become virulent due to opportunistic conditions. Radiation or certain chemicals can cause a prophage to become virulent. 3. Infection of a host cell requires that the cell have a surface protein that can serve as a receptor for the envelope protein of the retrovirus. The envelope protein of HIV-1 binds to CD4 molecules. 4. Flu viruses are constantly changing, and different flu viruses circulate and cause illness each season. Flu vaccines are made each year to protect against the flu viruses that have changed. In addition, a person's immune protection from vaccination declines over time. 5. This DNA then integrates into the DNA of the host cell. It then either undergoes the lytic cycle, where it makes the cell produce copies of the phage and then explode, releasing more; or it does the lysogenic cycle, where the DNA integrates, and then goes dormant, not to be activated for phage replication until the cell is stressed a certain way. This is great for genetic engineering because you can alter the phage to include what DNA you want integrated into the host cell. The phage will then perform the transformation of DNA for you. This process is called transduction. STRUCTURES AND FUNCTIONS. C. A. E. D. B. Viruses of the lytic cycle are called virulent viruses. The lytic cycle is a five-stage cycle. Attachment. The virus first attaches itself to a specific host cell. In the case of the T4 phage, a commonly studied bacteriophage that infects the bacterium Escherichia coli, this attachment is done by the tail fibers of the virus having proteins that have an affinity with the host cell wall. The virus attaches at places called receptor sites (Towle 1989). A virus also may attach by simple mechanical forces. Penetration. To infect a cell, a virus must first enter the cell through the plasma membrane and (if present) the cell wall. It then releases its genetic material (either single- or double-stranded RNA or DNA) into the cell. In the case of the T4 phage, after attachment to the host cell, the virus first releases releases an enzyme that weakens a spot in the cell wall of the host (Towle 1989). The virus then injects its genetic material much like a hypodermic needle, pressing its sheath up against the cell and injecting its DNA into the host cell through the weak spot in the cell wall. The empty capsid stays on the outside of the host cell. Other viruses enter their host cell intact, and once inside the capsid dissolves and the genetic material is released; this process is known as uncoating (Towle 1989). Once the virus has infected the cell, it also can be targeted by the immune system. Replication. The virus' nucleic acid uses the host cell’s machinery to make large amounts of viral components, both the viral genetic material (DNA or RNA) and the viral proteins that comprise the structural parts of the virus. In the case of DNA viruses, the DNA transcribes itself into messenger RNA (mRNA) molecules that are then used to direct the cell's ribosomes. One of the first polypeptides to be translated is one that destroys the hosts' DNA. In retroviruses (which inject an RNA strand), a unique enzyme called reverse transcriptase transcribes the viral RNA into DNA, which is then transcribed again into RNA. In the case of the T4 phage, the E. coli DNA is inactivated and then the DNA of the viral genome takes over, with the viral DNA making RNA from nucleotides in the host cell by using the enzymes of the host cell. The replication is often (for example, in T4) regulated in three phases of mRNA production followed by a phase of protein production (Madigan and Martinko 2006). In the early phase, the enzymes involved modify the hosts DNA replication by RNA polymerase. Among other modifications, virus T4 changes the sigma factor of the host by producing an anti-sigma factor so that the host promotors are not recognized any more but now recognize T4 middle proteins. In the middle phase, the virus nucleic acid is produced (DNA or RNA depending on virus type). In the late phase, the structural proteins are produced, including those for the head and the tail. Assembly. After many copies of viral components are made, they are assembled into complete viruses. In the case of the T4 phage, proteins coded for by the phage DNA act as enzymes for construction of the new phages (Towle 1989). The entire host metabolism is directed toward this assembly, resulting in a cell filled with new viruses. Lysis. After assembly of the new virus particles, an enzyme is produced that breaks down the bacteria cell wall from within and allows fluid to enter. The cell eventually becomes filled with viruses (typically 100-200) and liquid, and bursts, or lyses—thus giving the lytic cycle its name. The new viruses are then free to infect other cells and start the process again. 24-2 VIRAL DISEASES VOCABULARY REVIEW 1. Inactivated virus - Vaccines are made using several different processes. They may contain live viruses that have been… 2. attenuated (weakened or altered so as not to cause illness); inactivated or killed organisms or viruses; inactivated toxins (for bacterial diseases where toxins generated by the bacteria, and not the bacteria themselves, cause illness); or merely segments of the pathogen (this includes both subunit and conjugate vaccines). 3. oncogene - a gene that has the potential to cause cancer. In tumor cells, they are often mutated or expressed at high levels. MULTIPLE CHOICE 1. A 2. D 3. B 4. C 5. A SHORT ANSWER Top ten most dangerous viruses in the world 1. The most dangerous virus is the Marburg virus. It is named after a small and idyllic town on the river Lahn - but that has nothing to do with the disease itself. The Marburg virus is a hemorrhagic fever virus. As with Ebola, the Marburg virus causes convulsions and bleeding of mucous membranes, skin and organs. It has a fatality rate of 90 percent. 2. There are five strains of the Ebola virus, each named after countries and regions in Africa: Zaire, Sudan, Tai Forest, Bundibugyo and Reston. The Zaire Ebola virus is the deadliest, with a mortality rate of 90 percent. It is the strain currently spreading through Guinea, Sierra Leone and Liberia, and beyond. Scientists say flying foxes probably brought the Zaire Ebola virus into cities. 3. The Hantavirus describes several types of viruses. It is named after a river where American soldiers were first thought to have been infected with the Hantavirus, during the Korean War in 1950. Symptoms include lung disease, fever and kidney failure. 4. The various strains of bird flu regularly cause panic - which is perhaps justified because the mortality rate is 70 percent. But in fact the risk of contracting the H5N1 strain - one of the best known - is quite low. You can only be infected through direct contact with poultry. It is said this explains why most cases appear in Asia, where people often live close to chickens. 5. A nurse in Nigeria was the first person to be infected with the Lassa virus. The virus is transmitted by rodents. Cases can be endemic - which means the virus occurs in a specific region, such as in western Africa, and can reoccur there at any time. Scientists assume that 15 percent of rodents in western Africa carry the virus. 6. The Junin virus is associated with Argentine hemorrhagic fever. People infected with the virus suffer from tissue inflammation, sepsis and skin bleeding. The problem is that the symptoms can appear to be so common that the disease is rarely detected or identified in the first instance. 7. The Crimea-Congo fever virus is transmitted by ticks. It is similar to the Ebola and Marburg viruses in the way it progresses. During the first days of infection, sufferers present with pin-sized bleedings in the face, mouth and the pharynx. 8. The Machupo virus is associated with Bolivian hemorrhagic fever, also known as black typhus. The infection causes high fever, accompanied by heavy bleedings. It progresses similar to the Junin virus. The virus can be transmitted from human to human, and rodents often the carry it. 9. Scientists discovered the Kyasanur Forest Virus (KFD) virus in woodlands on the southwestern coast of India in 1955. It is transmitted by ticks, but scientists say it is difficult to determine any carriers. It is assumed that rats, birds and boars could be hosts. People infected with the virus suffer from high fever, strong headaches and muscle pain which can cause bleedings. 10. Dengue fever is a constant threat. If you're planning a holiday in the tropics, get informed about dengue. Transmitted by mosquitoes, dengue affects between 50 and 100 million people a year in popular holiday destinations such as Thailand and India. But it's more of a problem for the 2 billion people who live in areas that are threatened by dengue fever. 2. Chickenpox and shingles are two diseases that are both caused by the varicella-zoster virus. People who have been sick with chickenpox are also at risk for shingles. Initial exposure to the varicella-zoster virus usually occurs during childhood and causes chickenpox. The chickenpox symptoms resolve, but the virus remains resident and can reactivate later in life, causing shingles. Vaccinations are available for both chickenpox and shingles. 3. VIRAL CONTROL a. Immunoprophylaxis against viral illnesses includes the use of vaccines or antibody-containing preparations to provide immune protection against a specific disease. b. Active Prophylaxis (Vaccines) - Active immunization involves administering a virus preparation that stimulates the body's immune system to produce its own specific immunity. Viral vaccines now available for use include the following types: (1) attenuated live viruses; (2) killed viruses; (3) recombinant produced antigens. A vaccinee is a person who has been vaccinated. c. Immune Response to Vaccines: Vaccination evokes an antibody response and stimulates T lymphocytes. Vaccine effectiveness is assessed in terms of percentage of recipients protected and the duration and degree of protection. Most effective viral vaccines protect more than 90 percent of recipients and produce fairly durable immunity. d. Passive Prophylaxis - Passive immunity is conferred by administering antibodies formed in another host. Human immunoglobulins remain a mainstay of passive prophylaxis (and occasionally therapy) for viral illnesses; they are usually used to protect individuals who have been exposed to a disease and cannot be protected by vaccination. e. Sanitation and Vector Control - Many viral diseases are controlled by reducing exposure to the virus by (1) eliminating nonhuman reservoirs, (2) eliminating the vector, and (3) improving sanitation. f. Antiviral Chemotherapy - There are three types of antiviral agents: (1) virucidal agents, which directly inactivate viruses, (2) antiviral agents, which inhibit viral replication, and (3) immunomodulators, which boost the host immune response. g. Interferons Virus-infected cells and cells induced with other agents, e.g., doublestranded polynucleotides, can secrete proteins called interferons, which protect normal cells from viral infection. Therapeutic administration of interferon alpha has proven effective for several human viral illnesses. h. Cytokines - Cytokines are molecules produced by cells which modify the biological responses of the same or other cells. 4. Cancers occur when the genetic material within cells, the cells' DNA, develops mutations that cause the cell to divide uncontrollably. These mutations sometimes arise when the DNA is damaged. However, viruses can have effects similar to these mutations when they insert themselves into the DNA, and the end result is the same – uncontrolled cell growth. Two types of human papillomaviruses (HPV) are thought to cause cervical cancer this way. 5. If individuals from a community venture into a previously undeveloped forest, they may encounter an animal that harbors a previously unknown virus. If these individuals become infected, they can carry this infection to their community 6. Viruses (retroviruses) use have RNA that is reverse transcribed into DNA and ligated into the host genome. Now if you develop a drug that attacks the transcriptional machinery of the cell, you will most likely kill the host also because you are targeting the enzyme RNA polymerase. As a result, you will be killing off both the virus and host cell in the process. Which you do not want. You want to kill the viruse only. You can target the reverse transcriptase enzyme, which will kill the virus, but not the host. STRUCTURES AND FUNCTIONS A. PROTEIN SPIKES B. RNA C. REVERSE TRANSCRIPTASE ENZYME D. ENVELOPE E. CAPSID