Acyclovir is an antiviral chemical that resembles a nucleotide but lacks a 3' OH group and cannot form phosphodiester bonds. Why is Acyclovir useful in treating some viral infections?

Question

A.  Acyclovir stops viral protein synthesis.
B.  Acyclovir halts viral DNA replication.
C.  Acyclovir blocks the assembly of viral cell capsules.
D.  Acyclovir hydrolyzes viral polysaccharides.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
· What key words does the question contain and what do they mean?

Gather Content
· What do you know about the synthesis of DNA? How does it relate to the question?

Consider Possibilities  
· What other information is related to the question? Which information is most useful?

  Choose Answer
  · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
 
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

B.  Acyclovir halts viral DNA replication.

Clarify Question
· What is the key concept addressed by the question?
        o The question asks about the role a 3’ OH group in the formation of phosphodiester bonds.
· What type of thinking is required?
        o You are being asked to analyze statements about why a nucleotide lacking a 3’ OH group would serve as an antiviral drug.
· What key words does the question contain and what do they mean?
        o Antiviral chemical – a drug that kills viruses.
        o 3’ OH group – this is a hydroxyl group on the deoxyribose of a nucleotide.  It is used to form phosphodiester bonds between adjacent nucleotides on a strand of DNA.

Gather Content
· What do you know about the synthesis of DNA? How does it relate to the question?
        o In the synthesis of DNA, the new strand grows from the 3’ end. The free 3’ OH group attacks the phosphate on the next nucleotide to be added, forming a phosphodiester bond.  If the growing end lacks a 3’ OH group then no new nucleotides can be added.

Consider Possibilities  
· What other information is related to the question? Which information is most useful?
        o Proteins and carbohydrates do not contain phosphodiester bonds, so these would not be affected by Acyclovir.

  Choose Answer
  · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o When Acyclovir is incorporated into a strand of viral DNA that strand now lacks a 3’ OH group.  As a result it cannot add the next nucleotide because it cannot form a phosphodiester bond.  Viral DNA replication ends where the Acyclovir was introduced into the viral DNA and the infection stops.
 
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question asked you to analyze explanations for how a nucleotide lacking a 3’ OH group would function as an antiviral drug. If you got the correct answer, great job! If you got an incorrect answer, where did the process break down? Did you realize the role of the 3’ OH group in the formation of a phosphodiester bond?  Did you know that phosphodiester bonds are found in nucleic acids like DNA but not in proteins or carbohydrates?  Did you predict that the lack of a 3’ OH group would inhibit viral replication?