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| 603766968 | genome | The _________ of viruses may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, depending on the kind of virus. A virus is called a DNA virus or an RNA virus, according to the kind of nucleic acid that makes up its genome. The viral genome is usually organized as a single linear or circular molecule of nucleic acid. | |
| 603766969 | capsid, viral envelopes | The _______ is the protein shell enclosing the viral genome. They are built of a large number of protein subunits called capsomeres. The number of different kinds of proteins making up this is usually small. Some viruses have accessory structures to help them infect their hosts. A membranous envelope surrounds the capsids of flu viruses. These _________ are derived from the membrane of the host cell. They also have some host cell viral proteins and glycoproteins, as well as molecules of viral origin. | |
| 603766970 | TMV (tobacco mosaic virus) | rod-shaped with a capsid made up of one protein arranged in a helix. It and other similar viruses are referred to as *helical viruses*. RNA makes up its genome. | |
| 603766971 | adenovirus | Infects the respiratory tracts of animals, consist of 252 identical proteins arranged into a capsid with 20 triangular—an icosahedron. It and other similar viruses are referred to as *icosahedral viruses*. Glycoproteins extend from its capsid. DNA makes up its genome. | |
| 603766972 | influenza virus | Its outer viral envelope is studded with glycoprotein spikes. Its genome consists of eight different RNA molecules, each wrapped in a helical capsid | |
| 603766973 | (Bacterio)phage T4 | infect viruses. Has a complex capsid that consists of an icosahedral head in which DNA resides, a tail sheath "body" and tail fiber "legs". | |
| 603766974 | obligate intracellular | Viruses are __________________ parasites; they can only reproduce within a host cell. | |
| 603766975 | host range | Each type of virus can infect cells of only a limited variety of hosts, called the _______________ of the virus. This host specificity depends on the evolution of recognition systems by the virus. Viruses identify host cells by a "lock and key" fit between proteins on the outside of the virus and specific receptor molecules on the host's surface (which evolved for functions that benefit the host). Range is also limited to particular tissues. | |
| 603766976 | viral infection | A __________ begins when the genome of the virus enters the host cell (by endocytosis by the cell, fusing of viral to cell membrane, or injection by tail). Once inside, the viral genome commandeers its host, reprogramming the cell to copy viral nucleic acid and manufacture proteins from the viral genome. The host provides nucleotides, ribosomes, tRNAs, amino acids, ATP, and other components for making the viral components dictated by viral genes. Most DNA viruses use the *DNA polymerases* of the host cell to synthesize new genomes along the templates provided by the viral DNA. RNA viruses use special *virus-encoded polymerases* that can use RNA as a template. The nucleic acid molecules and capsomeres then self-assemble into viral particles and exit the cell. | |
| 603766977 | lytic cycle | In the lytic cycle, the phage reproductive cycle culminates in the death of the host. One of the first phage genes translated after the viral DNA enters the host cell codes for an enzyme that degrades the host cell's DNA. The phage DNA is protected because it contains a modified form of cytosine that is not recognized by the enzyme. | |
| 603766978 | virulent phage | a phage that only reproduces by the lytic cycle. The T4 phage is an example. | |
| 603766979 | receptor sites, restriction enzymes | Bacteria have not been wiped out by phages because: 1. some bacterial mutants have ________ that are no longer recognized by a particular type of phage. 2. bacteria produce restriction endonucleases, or ____________, that recognize and cut up foreign DNA, including certain phage DNA. The bacteria's DNA is methylated in a way that prevents attack by its own rest. enzymes. | |
| 603766980 | lysogenic cycle | In the ____________, the phage genome replicates without destroying the host cell. The circular viral and host DNAs are cut by viral proteins and then joined together, forming a *prophage*. One of the viral genes codes for a protein that represses most other prophage genes. As a result, the phage genome is largely silent, but every time the host divides, the prophage is passed to (the viral DNA is replicated along with the host DNA) daughter cells. A few other prophage genes may also be expressed during lysogenic cycles. Expression of these genes may alter the host's phenotype, which can have medical significance. | |
| 603766981 | temperate phages | Capable of using both lytic and lysogenic cycles; lambda is an example. When the viral genome exits the bacterial chromosome, the lytic cycle is initiated. An environment signal, such as a certain chemical or high-energy radiation, usually triggers this switchover. | |
| 603766982 | Animal | ________ viruses have both an envelope and RNA genome; in fact, nearly all animal viruses with RNA genomes have an envelope, as well as some DNA genomes. | |
| 603766983 | herpesvirus | Some viruses have envelopes that are not derived from plasma membrane. The envelope of the ____________ is derived from the nuclear envelope of the host. These double-stranded DNA viruses reproduce within the cell nucleus using viral and cellular enzymes to replicate and transcribe their DNA. In some cases, copies of the herpesvirus DNA remain behind as minichromosomes in the nuclei of certain nerve cells. There they remain for life until triggered by physical or emotional stress to leave the genome and initiate active viral production. | |
| 603766984 | Class I/II | Double-stranded and single-stranded DNA viruses, respectively. Includes Adenovrius, papovavirus, herpesvrius, poxvirus, and parvovirus. | |
| 603766985 | Class III | Double-stranded RNA; Includes Reovirus. | |
| 603766986 | Class IV | Single-stranded RNA. Serves as mRNA. Includes picornavirus (common cold), cornoavirus, flavivirus (yellow fever, west nile virus), togavirus (rubella virus) | |
| 603766987 | Class V | *Single-stranded RNA*. Serves as a template for *mRNA sythesis*. Has its own viral enzymes that carry out the viral RNA -> more RNA process. Includes filovirus (ebola virus), orthomyxovirus (influenza virus), paramyxovirus (measles, mumps), rhabdovirus (rabies) | |
| 603766988 | Class VI | *Single-stranded RNA*. Serves as a template for DNA synthesis. Example: retrovirus (HIV, AIDS, leukemia) | |
| 603766989 | Retrovirus | Make up class VI. Have the most complicated reproductive cycles. Have enzymes called *reverse transcriptase* which transcribes its RNA template into DNA, providing an RNA -> DNA template flow, which is usually the opposite. | |
| 604785284 | HIV | A retrovirus that causes AIDs, double-stranded viral DNA is synthesized from the viral RNA genome, and then integrated into the host cell's DNA, forming a *provirus*, where it is a permanent resident of the cell. mRNA is synthesized from the provirus, which may be used to put in new viruses or for synthesizing viral proteins. | |
| 604785285 | after | Because viruses depend on cells for their own propagation, it is reasonable to assume that they evolved _______ the first cells appeared. Most molecular biologists favor the hypothesis that viruses originated from fragments of cellular nucleic acids that could move from one cell to another. | |
| 604785286 | plasmids, transposons | Candidates for the original sources of viral genomes include: 1. __________: small, circular DNA molecules that are separate from chromosomes; found in bacteria and in eukaryote yeast, they can replicate independently of the rest of the cell and are occasionally transferred between cells.and transposable elements. 2. __________: DNA segments that can move from one location to another within a cell's genome. Both are mobile genetic elements, like viruses. | |
| 604785287 | nucleic acids | Perhaps the earliest viruses were naked bits of _____________ that passed between cells via injured cell surfaces. The evolution of capsid genes may have facilitated the infection of undamaged cells. | |
| 604785288 | host | A viral genome usually has more in common with the genome of its _______ than with those of viruses infecting other hosts. However, some viruses have genetic sequences that are quite similar to seemingly distantly related viruses (plant virus vs. animal virus). This genetic similarity may reflect the persistence of groups of viral genes that were *evolutionarily successful* during the early evolution of viruses and their eukaryotic host cells. | |
| 604785289 | mimivirus | Largest virus yet discovered. Has double-stranded DNA in an icosahedral capsid about the size of a small bacteria. What is significant about this virus is that some of its genes code for proteins involved in translation, DNA repair, protein folding, and polysaccharide synthesis. | |
| 604785290 | Antibiotics, nucleosides | ___________, which can kill bacteria by inhibiting enzymes or processes specific to bacteria, are powerless against viruses, which have few or no enzymes of their own. Most antiviral drugs resemble ___________ and interfere with viral nucleic acid synthesis. An example is acyclovir, which impedes herpesvirus reproduction by inhibiting the viral polymerase that synthesizes viral DNA. Azidothymidine (AZT) curbs HIV reproduction by interfering with DNA synthesis by reverse transcriptase. | |
| 604785291 | mutation, species | The emergence of these new viral diseases is due to three processes: 1. _________: RNA viruses tend to have high mutation rates because replication of their nucleic acid lacks proofreading. Some mutations create new viral strains with sufficient genetic differences from earlier strains. 2. spread of existing viruses from one _________ to another 3. dissemination of a viral disease from a small, isolated population: Technological and social factors, including affordable international travel, blood transfusion technology, sexual promiscuity, and the abuse of intravenous drugs allowed a previously rare disease to become a global scourge. | |
| 604785292 | influenza | There are three types of ________ virus: types B and C, which infect only humans and have never caused an epidemic, and type A, which infects a range of animals. | |
| 604785293 | horizontal transmission | In _____________, a plant is infected with the virus by an external source. Plants are more susceptible if their protective epidermis is damaged, perhaps by wind, chilling, injury, or insects. Insects are often carriers of viruses, transmitting disease from plant to plant. | |
| 604785294 | vertical transmission | In______________, a plant inherits a viral infection from a parent. This may occur by asexual propagation or in sexual reproduction via infected seeds. | |
| 604785295 | Viroids | __________ , smaller and simpler than even viruses, consist of tiny molecules of naked circular RNA that infect plants. Their several hundred nucleotides do not encode for proteins but can be replicated by the host's cellular enzymes. These small RNA molecules can disrupt plant metabolism and stunt plant growth, perhaps by causing errors in the regulatory systems that control plant growth. They show that *molecules* can act as infectious agents to spread disease. | |
| 604785296 | Prions | _________ are infectious *proteins* that spread disease, and are likely transmitted in food. Their slow incubation period of at least ten years prevents sources of infection from being identified until long after the first cases appear. Prions are virtually indestructible. They are not destroyed or deactivated by heating to normal cooking temperatures--there is no known cure for them. | |
| 604785297 | prion | According to the leading hypothesis, a _____ is a misfolded form of a normal brain protein. When it gets into a cell with the normal form of the protein, it can convert the normal protein into its misfolded version, creating a chain reaction that increases their numbers. |
