pset6S - Solution key - 2010 7.012 Problem Set 6 Question 1...

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1 Solution key - 2010 7.012 Problem Set 6 Question 1 Working as a UROP, you are asked to complete some experiments that involve several different viruses, including R ous s arcoma v irus retrovirus (RSV) . After a long day, you place your viral samples in the refrigerator, but you forget to label them. As a 7.012 student, you have recently heard Prof. Weinberg’s lecture on cancer viruses, so you know you can identify the tubes. a) The type of genomes found in viruses can vary. Some viruses have single (ss) or double-stranded (ds) DNA genomes; others have single (ss) or double-stranded (ds) RNA genomes. Given the data below, identify all possible types of genomes for each virus. Virus % A % T % U % C % G Type of genome, i.e., ds-DNA 1 18 18 0 32 32 ds-DNA (ss-DNA also possible) 2 25 0 25 25 25 ds-RNA (ss-RNA also possible) 3 35 15 0 15 35 ss-DNA The experiment in part (a) allows you to identify all of the unlabeled viruses except 2. The remaining 2 viruses are both single-stranded RNA viruses. You know that one virus is the Norwalk virus, and the other is RSV. The Norwalk virus causes intestinal illness. Norwalk and Norwalk-like viruses are increasingly being recognized as leading causes of food borne disease in the United States. The Norwalk virus is a + strand RNA virus that is surrounded by an icosahedron-shaped capsid composed of two different polypeptides. RSV is a retrovirus that also has a + strand RNA genome that encodes four genes; gag (encodes the capsid protein), pol (encodes the reverse transcriptase), env (encodes the envelop glyco-protein) and src (encodes a tyrosine kinase enzyme). b) Given the information above… i) Do you expect that the Norwalk virus would encode its own RNA polymerase? Explain why or why not. Yes, this virus must encode its own polymerase as the host cell will not have an RNA dependent RNA polymerase. However, it does not have to package and deliver this polymerase to the host cell because it has the same polarity as an mRNA and the host cell translation machinery can recognize the viral RNA and translate it. The viral polymerase is then used during replication of the viral genome, which proceeds through the (+)RNA --> (-)RNA --> (+)RNA process. ii) Reverse transcriptase is considered which of the following? A DNA directed DNA polymerase A DNA directed RNA polymerase A RNA directed DNA polymerase A RNA directed RNA polymerase iii) Why is it essential that the RSV encodes Reverse transcriptase? The enzyme needed to copy the viral genome must be able to copy RNA into a cDNA before it can integrate into the genome of the host cell. The host cell will not have an enzyme that can carry out these functions. c) You have a cell line that can be infected by these two viruses. What is a cell line? A cell line is an immortalized cell, which continuously grows in culture. All the cells in the culture are
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pset6S - Solution key - 2010 7.012 Problem Set 6 Question 1...

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