You are asked to give a classroom demonstration of DNA supercoiling. To do so, you take two different colored pieces of rubber tubing, twist them very tightly and excessively around each other and join the ends of the tubing appropriately. Next, you relieve the supercoiling by breaking one strand and unwinding it. In this demonstration, the breaking and unwinding to relieve the supercoiling simulates the action of

Question

A.  exonucleases.
B.  polymerases.
C.  ligases.
D.  topoisomerases.
E.  primases.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
· What key words does the question contain?

  Gather Content
· What do you already know about supercoiling?

  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

D.  topoisomerases.

Clarify Question
· What is the key concept addressed by the question?
    o This question addresses supercoiling and the function of different enzymes.
· What type of thinking is required?
    o Although you may not have seen this example before, you know enough about the concept to Apply your knowledge and understanding to this unfamiliar situation.
· What key words does the question contain?
    o Supercoiling is the twisting that can occur in DNA, particularly after helicase has unwound part of the double-stranded portion for replication.

  Gather Content
· What do you already know about supercoiling?
    o DNA can become highly twisted, or supercoiled. These twisted loops interfere with replication, and the torsion is relieved by enzymes.
  Consider Possibilities
· What other information is related to the question? Which information is most useful?
    o Look at the answer options. Each is the name of an enzyme.
    o An enzyme often ends in “-ase”. The beginning of the name is usually a prefix that hints at the enzymes job. The name can help us remember what the enzyme does.

  Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
    o You are being asked to name the type of enzyme that relieves supercoiling by cutting, unwinding, and reconnecting the DNA.
    o You may remember that DNA gyrase does this, but that option isn’t here. What is the name of the class of enzymes to which DNA gyrase belongs?
    o If you don’t remember the name of the enzyme, go through each of the options and figure out their names:
    o Exonuclease: An enzyme that cuts nucleotides (“nucle”) from the outside (the end, “exo”).
    o Ligase: An enzyme that ties (“ligates”) DNA together.
    o Polymerase: An enzyme that makes “polymers”, or strings of multiple units.
    o Primase: An enzyme that “primes”, or starts the process of synthesis by making a primer.
    o Topoisomerase: An enzyme that changes the shape (“topology”) of the DNA into another version (“isomer”). That’s it!
 
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 required you to Apply your knowledge and understanding to this unfamiliar situation.
    o Did you remember that topoisomerases are the enzymes that relieve supercoiling?
    o If not, did you try analyzing the enzyme names to infer their functions?