Some people are naturally resistant to infection by parvovirus, what cellualar s
ID: 85791 • Letter: S
Question
Some people are naturally resistant to infection by parvovirus, what cellualar structure is different in these virus resistant individual. What type of enzymes must viruses bring either coded in their genetic material or as proteins into the cell for synthesis? Why are these enzymes not found in the host cell? Why do dsDNA viral genomes migrate into nucleus, but ssRNA genomes stay in the cytoplasm? Some people are naturally resistant to infection by parvovirus, what cellualar structure is different in these virus resistant individual. What type of enzymes must viruses bring either coded in their genetic material or as proteins into the cell for synthesis? Why are these enzymes not found in the host cell? Why do dsDNA viral genomes migrate into nucleus, but ssRNA genomes stay in the cytoplasm? What type of enzymes must viruses bring either coded in their genetic material or as proteins into the cell for synthesis? Why are these enzymes not found in the host cell? Why do dsDNA viral genomes migrate into nucleus, but ssRNA genomes stay in the cytoplasm?Explanation / Answer
Parvoviruses are unique among all known viruses in having singlestranded DNA genomes which are linear. Virions are non-enveloped, containing a single copy of the small (4-6 kb) viral chromosome encapsidated in a rugged icosahedral protein capsid 18-26 nm in diameter. Although lacking associated enzymes or nucleosomal proteins, the particles have been shown, in some cases, to contain polyamines such as spermidine, spermine, and putrescine. The family Parvoviridae contains a broad spectrum of physically similar viruses that replicate in the nuclei of both invertebrate and vertebrate hosts. The icosahedral parvovirus particles contain just protein and DNA genome. The three capsid proteins are VP1, VP2, and VP3. Together, the capsid confers much stability upon the virus particles, allowing for resistance to inactivation by pH, solvents, or temperatures up to 50 degrees C. Parvoviruses are thus among the most resistant viruses known.
The presence of a specific cellular receptor is thought to be necessary for susceptibility to viral infection. A specific receptor for B19V, present on the plasma membrane of erythroid progenitor cells as well as erythrocytes, was then identified in the glycolipid globoside. Globoside (Globotetraosylceramide, Gb4Cer) is an antigenic determinant within the P blood group system, for which a limited degree of polymorphism is also present in the population. B19V binds to globoside, as measured by thin-layer chromatography. Purified globoside blocks the binding of the virus to erythroid cells and the infectivity of the virus in a hematopoietic colony assay, while target cells can be protected from infection by preincubation with monoclonal antibody to globoside. Furthermore, erythrocytes lacking P antigen cannot be hemagglutinated by B19V. The rare persons with the p phenotype, lacking globoside on the plasma membrane of erythrocytes and erythroid progenitor cells, were naturally resistant to B19V infection.
The replication mechanism depends on the viral genome. DNA viruses usually use host cell proteins and enzymes to make additional DNA that is transcribed to messenger RNA (mRNA), which is then used to direct protein synthesis. RNA viruses usually use the RNA core as a template for synthesis of viral genomic RNA and mRNA. The viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins, and to assemble new virions. Of course, there are exceptions to this pattern. If a host cell does not provide the enzymes necessary for viral replication, viral genes supply the information to direct synthesis of the missing proteins. Retroviruses, such as HIV, have an RNA genome that must be reverse transcribed into DNA, which then is incorporated into the host cell genome. To convert RNA into DNA, retroviruses must contain genes that encode the virus-specific enzyme reverse transcriptase, which transcribes an RNA template to DNA. Reverse transcription never occurs in uninfected host cells; the needed enzyme, reverse transcriptase, is only derived from the expression of viral genes within the infected host cells. Generally in uninfected host cells process goes from DNA to RNA conversion so it does not require reverse transcriptase enzyme for that.
Once the nucleocapsid of DNA virus enters the host cell, it proceeds to the nucleus where it mimics the genome of the host cell. Usually, the viral genome is replicated using the host cell DNA polymerase, and the viral genome is transcribed by the host cell RNA polymerase.
There are exceptions, notably the smallpox DNA virus encodes its own DNA replication machinery so it replicates in the cytoplasm. Host cell does not have a mechanism to replicate RNA (there is no host enzyme that uses RNA as a template for nucleic acid synthesis). So the genome of RNA virus must encode a viral enzyme that can replicate viral RNA.
As the enzymes used to replicate viral RNA are virally encoded, most RNA viruses replicate in the cytoplasm.In case of Retroviruses (+ SS RNA) it replicates forming RNA: DNA hybrid double helix. The copying of RNA into DNA is carried out by viral enzyme reverse transcriptase and occurs in cytoplasm.Reverse transcriptase also degrade RNA portion and copies remaining DNA strand into dsDNA. Once the ds viral DNA is synthesized, it is transported into the nucleus and is inserted and covalently linked to the host chromosomal DNA.
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