23. What tools are necessary for studying viruses? Why couldn’t van Leeuvenhoek

Question

23. What tools are necessary for studying viruses? Why couldn’t van

Leeuvenhoek observe them in his studies? How can we study viruses today?

24. Describe how an animal virus infects a host cell (in general)

25. Describe how a bacteriophage infects a host cell. Compare and contrast lytic and lysogenic mechanisms of replication. Why is it an advantage for a virus to be able to have both of these mechanisms of replication?

27. What component of the flu virus changes every year? How is this

advantageous for the virus? How does it affect us?

29.What components of a host cell are used by viruses during viral replication?

30. Describe 3 different explanations for the origin of viruses.

Explanation / Answer

23. To study viruses we need following tools : isolation from sources, tissue culture methods, physical methods, serological methods, immunological methods, and others and molecular biology. Most robust technique to study viruses is plaque assay ( used to determine virus concentration in terms of infectious dose).

van Leeuvenhoek work involved studies on living micro-organisms with a defined cellular boundry. Viruses lack these characteristic features of micro-organisms.

24.

Infection of host cell by animal virus involves following steps:

Attachment: Some specialized proteins in capsid attach to a specific receptor site on the host cell membrane which is followed by fusion of viral envelope protein and cellular membranes.

Penetration : Virions enter the host cell through receptor-mediated endocytosis or membrane fusion;  viral capsid is removed; and the nucleic acid of bacteriophages enters the host cell naked, leaving the capsid outside the cell.

Replication annd assembly: Nucleic acid e.g. DNA is transcribed to mRNA from "early" genes which directs the host cell to synthesize viral enzymes and capsid proteins, and to assemble new virions. This may be followed, for complex viruses with larger genomes, by one or more further rounds of mRNA synthesis: "late" gene expression is, in general, of structural or virion proteins.

Release: The last stage of viral replication is the release of the new virions produced in the host organism. They are then able to infect adjacent cells and repeat the replication cycle.

  Some viruses undergo a lysogenic cycle where the viral genome is incorporated into a specific place in the host's chromosome. The viral genome is then known as a "provirus" and at some point, the provirus or prophage may give rise to active virus, which may lyse the host cells.

25.

After encountering a bacterial host bacteriophage attach to specific receptors on its surface like including lipopolysaccharides, teichoic acids,proteins, or even flagella.After making contact with the appropriate receptor, tail fibres attach to cell surface followed by injection of genetic material through the bacterial membrane.Within minutes, bacterial ribosomes start translating viral mRNA into protein.

Bacteriophages may have a lytic cycle or a lysogenic cycle. In lytic, phase bacterial cells are broken open (lysed) / destroyed after immediate replication of the virion and the phage progeny can find new hosts to infect. Lytic phages are more suitable for phage therapy. In contrast, the lysogenic cycle does not lyse the host cell. Instead, viral genomel integrates with host DNA and replicate along with it. At some point, however, the viral nucleic acid becomes active. It separates itself from the host cell’s genetic material, takes over the functions of the cell to produce new viruses, and destroys the host cell as the new viruses are released.

In other words, we can say that in the lytic cycle, the genome remains in the cytoplasm while in the lysogenic cycle the genome enters the nucleus and inserts into the host genome. However, in both cases the virus is dependent on the host replication machinery to make viral proteins and genome (new virions).

Advantage of having both these mechanisms of replication are

a) replication of new virus is fast (lytic cycle)

b) many more viruses can be made because viral genome is passed onto future generation of host cells (lysogenic cycle)

27.

The answer is viral envelope. The mutations in this component protects the virus from being recognized by host immune system. Glycoproteins on the surface of the envelope serve to identify and bind to receptor sites on the host's membrane. The presence of mutation introduces a new strain of virus and thus our programmed immune system (after vaccination) will not be able to produce soluble mediators which neutralize the effect of viruses and as such we will get prone to infection.   

29.

Enymes like polymerase ; ribosomes mainly

30.

a) The Progressive Hypothesis: Mobile genetic elements, pieces of genetic material capable of moving within a genome, gained the ability to exit one cell and enter another e.g replication of retroviruses.

b) The Regressive Hypothesis : certain bacteria that are obligate intracellular parasites, like chlamydia and rickettsia species, evolved from free-living ancestors. The nucleocytoplasmic large DNA viruses (NCLDVs), best illustrate this hypothesis.

c) The Virus-First Hypothesis: viruses may have been the first replicating entities. Over time these units became more organized and more complex. Eventually, enzymes for the synthesis of membranes and cell walls evolved, resulting in the formation of cells. Viruses, then, may have existed before bacteria, archaea, or eukaryotes

Related Questions

Navigate

• Browse (All)
• Browse 2
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.