Viruses: structure and function

Viruses: Structure, Function, and Uses A virus is a small parasite that cannot reproduce by itself. Once it infects a susceptible cell, however,  a virus can  direct  the  cell  machinery  to  produce  more  viruses. Most  viruses have either RNA or DNA as   their   genetic   material.   The nucleic   acid may   be   single­   or   double­ stranded. The entire infectious virus particle, called a virion, consists of the nucleic acid and an outer shell of protein. The simplest viruses contain only enough RNA or DNA to encode four proteins. The most complex can encode 100 – 200 proteins.     The study of plant viruses inspired some of the first experiments in molecular biology. In 1935, Wendell Stanley purified and partly crystallized tobacco mosaic virus (TMV); other plant viruses were crystallized soon thereafter. Pure proteins had been crystallized only a short time before Stanley’s work, and it was considered very surprising at the time that a replicating organism could be crystallized. A wealth of subsequent research with bacterial viruses and animal viruses has provided detailed understanding of viral structure, and virus­infected cells have proved extremely useful as model systems  for   the   study  of   basic   aspects   of  cell   biology.   In   many   cases, DNA viruses  utilize cellular enzymes for synthesis of their DNA genomes and mRNAs; all viruses utilize normal cellular ribosomes, tRNAs, and translation factors for synthesis of their proteins. Most viruses comman­deer the cellular  machinery for macromolecular  synthesis during the  late  phase of infection,  directing  it  to  synthesize  large  amounts of  a  small  number  of  viral  mRNAs  and proteins instead of the thousands of normal cellular macromolecules. For instance, animal cells infected   by   influenza   or   vesicular   stomatitis   virus   synthesize   only   one   or   two   types   of glycoproteins, which are encoded by viral genes, whereas uninfected cells produce hundreds of glycoproteins. Such virus­infected cells have been used extensively in studies on synthesis of cell­surface   glycoproteins.   Similarly,   much   information   about   the   mechanism   of   DNA replication has come from studies with bacterial cells and animal cells infected with simple DNA viruses, since these viruses depend almost entirely on cellular proteins to replicate their DNA. Viruses also often express proteins that modify host­cell processes so as to maximize viral replication. For example, the roles of certain cellular factors in initiation of protein synthesis were revealed because viral proteins interrupt their action. Finally, when certain genes carried by cancer­causing viruses integrate into chromosomes of a normal animal cell, the normal cell can be converted to a cancer cell. Since many viruses can infect a large number of different cell types, genetically modified viruses often are used to carry foreign DNA into a cell. This approach provides the basis for a growing list of experimental gene therapy treatments. Because of the extensive use of viruses in cell biology research and their potential as therapeutic agents, we describe the basic aspects of viral structure and function in this section. Questions: What is virus? How many proteins сan they  encode? What do viruses often do to maximize viral replication? Who is Wendell Stanley? Dictionary: Reproduce / воспроизводить, повторять, репродуцировать    [  ri pr dju s ] ˌ ː əˈ ː Machinery / механизм, машинное оборудование, аппарат     [ m i n ri ]   əˈʃ ː ə Infectious / инфекционный    [  n fek s ]   ɪ ˈ ʃə Crystallized / кристаллизованный, закристаллизовавшийся, выкристаллизованный   [  kr s.t ˈ ɪ ə ɪ l.a zd ]   Purified / очищенный          [  pj ˈ ʊə ɪ .r .fa  ]   ɪ Subsequent / последующий, более поздний       [  s b.s .kw nt ]   ˈ ʌ ɪ ə ə Influenza / грипп          [  n.flu en.z  ] ˌɪ ˈ Tasks 1. Which of the following statements about viruses is FALSE? They are obligate intracellular parasites. They consist of nucleic acids surrounded by a protein coat. They reproduce and thus are considered alive. 2. Viruses are large organisms and must often be killed with baseball bats. True                                                         False 3. Which of these diseases is NOT caused by a virus? AIDS                                                                  Measles Flesh                                                                  Eating Bacteria Rabies 4. Viruses can have either DNA or RNA for their nucleic acids. True                                                         False 1. A virus consists of: RNA or DNA and a cell membrane RNA or DNA and a protein coat RNA and DNA and a protein coat proteins, cell membrane and RNA 2. How do viruses reproduce? they divide by mitosis sexually, by external fertilization replication outside the host inserting DNA into the host cell 3. Which of the following is smallest? eukaryote cell bacteria bacteriophage cell membrane protein 4. Retroviruses are different from viruses in that: they have RNA instead of DNA they can become dormant they are symmetrical all of these 5. Vaccines can be used to prevent viral infection by: creating an immune response in the host destroying any viruses that enter the host creating a blocking protein on the cell preventing replication of the virus 1. RNA or DNA and a protein coat 2. inserting DNA into the host cell 3. bacteriophage 4. they have RNA instead of DNA 5. creating an immune response in the host