Biological Sciences
Lytic Cycle
The lytic cycle is a process in which a virus infects a host cell, takes over the cell's machinery to replicate its own genetic material, assembles new virus particles, and then causes the host cell to burst, releasing the newly formed viruses. This cycle results in the destruction of the host cell and the spread of the virus to other cells.
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6 Key excerpts on "Lytic Cycle"
eBook - ePub- Britannica Educational Publishing, Kara Rogers(Authors)
- 2010(Publication Date)
- Britannica Educational Publishing(Publisher)
CHAPTER 5Cycles and Patterns of Viral InfectionT he cycles and patterns of viral infection vary depending on the type of virus involved. In order to replicate, however, all viruses must first find a suitable host cell. Whereas some viruses then lie dormant for an indefinite period of time, others set to work appropriating the cells’ genetic replication machinery. The latter is accomplished primarily through the ability of a virus to integrate into host cell DNA. The subsequent manipulation of the host’s genetic machinery results in the generation of numerous viral progeny. When the host cell dies, the progeny are freed to infect new host cells, repeating the infection and replication cycle.The mechanisms of viral infection sometimes involve phenomena such as lysogeny, in which a bacteriophage lies dormant within a host bacterial cell and eventually awakens, replicates, and causes the cell to break open (lyse), and malignant transformation, in which animal cells infected with certain viruses are transformed into cancer cells. In addition to cancer, viral infections in humans also underlie various other diseases, ranging from gastrointestinal illness to influenza to AIDS. Thus, the development of vaccines to prevent infection and of antiviral drugs to treat infection form important areas of research at the crossroads of virology and medicine.THE CYCLE OF INFECTION
Viruses can reproduce only within a host cell. The parental virus (virion) gives rise to numerous progeny, usually genetically and structurally identical to the parent virus. The actions of the virus depend both on its destructive tendencies toward a specific host cell and on environmental conditions. In the vegetative cycle of viral infection, multiplication of progeny viruses can be rapid. This cycle of infection often results in the death of the cell and the release of many virus progeny. Certain viruses, particularly bacteriophages, are called temperate (or latent) because the infection does not immediately result in cell death. The viral genetic material remains dormant or is actually integrated into the genome of the host cell. Cells infected with temperate viruses are called lysogenic because the cells tend to be broken down when they encounter some chemical or physical factor, such as ultraviolet light. In addition, many animal and plant viruses, the genetic information of which is not integrated into the host DNA, may lie dormant in tissues for long periods of time without causing much, if any, tissue damage. Viral infection does not always result in cell death or tissue injury; in fact, most viruses lie dormant in tissue without ever causing pathological effects, or they do so only under other, often environmental, provocations.
eBook - PDF- Nigel J. Dimmock, Andrew J. Easton, Keith N. Leppard(Authors)
- 2015(Publication Date)
- Wiley-Blackwell(Publisher)
The process of translation uses ribosomes provided by the host cell and it is this requirement for the translation machinery, as well as the need for molecules for biosynthesis, that makes viruses obligate intracellular parasites. The newly-synthesized Chapter 1 Towards a definition of a virus 7 Fig. 1.2 A diagrammatic representation of the six phases common to all virus multiplication cycles. See text for details. genomes may then be used as templates for further rounds of replication and as templates for transcription of more virus mRNA in an amplification process which increases the yield of virus from the infected cells. When the new genomes are produced, they come together with the newly-synthesized virus proteins to form progeny virus particles in a process called assembly. Finally, the particles must leave the cell in a release phase after which they seek out new potential host cells to begin the process again. The particles produced within the cell may require further processing to become infectious and this maturation phase may occur before or after release. Combining the consideration of the steps which make up a virus multiplication cycle with the information in the graph of the results of a single step growth curve, it can be seen that during the eclipse phase the virus is undergoing the processes of attachment, entry, uncoating and biosynthesis. At this time, the cells contain all of the elements necessary to produce viruses but the original infecting virus has been dismantled and no new infectious particles have yet been produced. It is only after the assembly step that we see virus particles inside the cell before they are released and appear in the medium. 1.4 Viruses can be defined in chemical terms The first virus was purified in 1933 by Schlessinger using differential centrifugation. Chemical analysis of the purified bacteriophage showed that it consisted of approximately equal proportions of protein and deoxyribonucleic acid (DNA).
eBook - PDF- Julianne Zedalis, John Eggebrecht(Authors)
- 2018(Publication Date)
- Openstax(Publisher)
The viral replication cycle does not affect the structure of the host cell. b. The viral replication cycle cannot affect genetic material of the host cell. c. The viral replication cycle has seven basic steps. d. The viral replication cycle can change cell functions or even destroy the host cell. 11. What happens in the replication cycle? a. During the budding process, virions leave the host cell individually b. During the budding process, the host cell bursts. c. During the budding process, the virus connects with a permissive host cell. d. During the budding process, the host cell dies immediately. 12. In the _____ cycle, the virus replicates and bursts out of the host cell. a. lytic b. lysogenic c. cytopathic d. latency 13. How is the Lytic Cycle different from the lysogenic cycle? a. The phage infects a cell in the Lytic Cycle. b. The Lytic Cycle contains the formation of a prophage. c. In the Lytic Cycle, new phages are produced; immediately in the lysogenic cycle phage DNA is merged into the host genome. d. The phages move on to infect other cells in the lysogenic phase. 14. Which of the following statements is false? 878 Chapter 21 | Viruses This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 a. Enveloped viruses and naked viruses both may enter cells using the fusion method. b. Many enveloped viruses enter the cell by receptor-mediated endocytosis. c. Naked viruses enter the cell by receptor- mediated endocytosis. d. Undergoing shape changes and creating channels in the host cell membrane is an alternative method of cell penetration used by naked viruses. 15. An apple tree has yellow splotches on the leaves. This is a symptom of _____. a. cell necrosis b. discoloration c. hyperplasia d. hypoplasia 16. What happens during the release step in the viral replication cycle? a. During the release step, genetic information is transferred through the lytic and lysogenic cycles.
eBook - PDF- Frank J. Fenner, B. R. McAuslan, C. A. Mims(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
I INTRODUCTION In Chapter 3 we described the C H A PT E R 5 chemistry and structure of vir-I ions. But the term virus en-■ · ■· · compasses much more than a T h e M u l t i p l i c a t i o n Of rather dry description of virus D N A VirUS6S particles. For virions are only I the inert forms assumed by I viruses to travel from one host cell to the next; they undergo no metabolism and they do not reproduce. All dynamic events associated with viruses occur within host cells. This chapter and the next are concerned with these events most of which are facets of the process of viral multiplication. Because DNA and RNA viruses differ in important details of their mechanisms of multiplication, particularly with regard to transcription and replication of their genomes, we shall consider these two groups separately. It will be useful first to give a brief outline of the multiplica-tion cycle as it applies to all viruses. THE MULTIPLICATION CYCLE The study of the multiplication of animal viruses has been greatly influenced by earlier work with bacterial viruses, the concept of a single growth cycle being developed from studies of cytocidal infections with virulent bacterio-phages. Ellis and Delbriick (1939) developed the one-step growth experiment by manipulating the conditions in a population of infected cells so that the be-havior of the population as a whole could be used to test hypotheses about what was happening in any individual cell at a given time after infection. In essence, the one-step growth experiment, as it is used in animal virology, consists of the simultaneous infection of a population of cells with at least one infectious virus particle per cell, usually 10-100 to ensure that every cell is infected and that there is a high degree of synchrony of subsequent events in the infected cell population.
eBook - ePub- David P. Clark(Author)
- 2009(Publication Date)
- Academic Cell(Publisher)
intracellular parasites ; that is to say that they must actually enter the cells of the host organism to replicate. Note that not all intracellular parasites are viruses. Certain disease-causing bacteria and protozoans may enter the cells of higher organisms and live inside them as parasites. However, these parasites are nonetheless living cells themselves and contain their own ribosomes to make their own proteins. This chapter does not attempt to cover the realm of virology systematically. Rather, examples are given to illustrate novel aspects of molecular biology found among the viruses.Life Cycle of a Virus
A virus alternates between two forms, an inert virus particle, the virion, which survives outside the host cell, and an active intracellular stage. The life cycle of a typical virus goes through the following stages (Fig. 17.03 ):a. Attachment of virion to the correct host cell b. Entry of the virus genome c. Replication of the virus genome d. Manufacture of the virus proteins e. Assembly of new virus particles (virions) f. Release of new virions from the host cellFigure 17.03 Virus Life CycleThe life cycle of a virus starts when the viral DNA or RNA enters the host cell. Once inside, the virus uses the host cell to manufacture more copies of the virus genome and to make the protein coats for assembly of virus particles. Once multiple copies of the virus have been assembled, the host cell is burst open to allow the viral progeny to escape and find new host cells to infect.Virus genes subvert the host cell into manufacturing more virus particles.Attachment of a virus requires a protein on the virus particle to recognize a molecule on the surface of the target cell. Sometimes this receptor is another protein; sometimes it is a carbohydrate. Often it is a glycoprotein, that is a protein with carbohydrate groups attached. On some virus particles, the recognition proteins form spikes or prongs sticking out from the surface. Most bacterial and plant viruses abandon their protein coat when they infect a new host cell. Only the genetic material (DNA or RNA) enters the cell. Animal viruses vary in regard to when exactly they disassemble their protein coat.
eBook - PDF- A. O. Betts, C. J. York, A. O. Betts, C. J. York(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
4. The Multiplication of Viruses 129 6. RELEASE OF BAGTERIOPHAGE New bacteriophage progeny are liberated by the lysis of the infected cells. Normally cells are lysed from within by lysozyme-like molecules that are synthesized late in the replicative cycle and incorporated into virus particles. 7. SUMMARY OF BAGTERIOPHAGE REPLICATION Bacteriophages adsorb to susceptible hosts by means of attachment of their tail fibers to specific cell wall receptors of the bacteria. Bacteriophage lysozyme digests part of the cell wall, and the tail penetrates into the host-cell cytoplasm. Injection of bacteriophage DNA, together with a small amount of internal protein, initiates replication. First of all, enzymes necessary for the produc-tion of DNA are produced; then both DNA and structural protein make their appearance in increasing amounts. Condensation of DNA and assembly of heads and tails leads to the production and accumulation of new progeny particles. The accumulation of lysozyme within the infected cell leads to its ultimate lysis and the release of up to several hundred newly formed virus particles. C. Lysogeny Bordet and Ciuca (1921) found that many apparently healthy cultures of E. coli contained bacteriophage. The presence of the virus was only detectable when the carrier strain was placed in contact with other strains (indicator strains) of E. coli. Cultures of bacteria which always have virus associated with them are described as lysogenic bacteria. Treatment of such bacteria with antiphage serum does not block their capacity to produce virus, and artificial lysis of lysogenic bacteria reveals that virus is present in less than 0.1% of the organisms. LwofT and Gutmann (1950) grew lysogenic bacteria for nineteen genera-tions without virus appearing in the medium. Occasionally, a cell was seen to lyse, and virus then appeared free in the medium.
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