If you were to completely digest this fragment of DNA with PstI, how many pieces of DNA would you obtain?

Question

A 6.85 kb EcoRI fragment of DNA is shown below. The location of several restriction sites is indicated. Scale is approximate.





A.  1

B.  2

C.  3

D.  4



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 already know about restriction enzymes?



  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

C.  3

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses restriction enzymes.
· What type of thinking is required?
        o This question is asking you to analyze the information given, using logic, to dissect the problem and determine the answer.
· What key words does the question contain and what do they mean?
        o A 6.85 kb EcoRI fragment of DNA is a piece of DNA 6850 base pairs long, that was already cut with the enzyme EcoRI.
        o Restriction sites are recognition sites for different restriction enzymes.
        o Digesting the DNA with PstI means to treat the DNA with the enzyme PstI, cleaving it.

  Gather Content
· What do you already know about restriction enzymes?
        o Restriction enzymes recognize a specific DNA sequence, ranging from 4 bases to 12 bases, and cleave the DNA at a specific base within this sequence.
        o The recognition sites for most type II enzymes are palindromes. The palindromic DNA sequence reads the same from 5¢ to 3¢ on one strand as it does on the complementary strand.
        o Enzymes can cut in more than one spot on the same DNA fragment, as long as the treatment is given enough time.

  Consider Possibilities
· What other information is related to the question? Which information is most useful?
        o Where will the enzyme cut? Only at the sites marked PstI.
        o Assume that the reaction is set up so that the enzyme cuts each DNA completely.

  Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o Since there are two PstI sites, the fragment will be cut twice, leaving three pieces of DNA.
        o Remember this was already a DNA fragment, so the ends are free.

  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 how many fragments would result for the DNA with a PstI digestion.
        o The question required you to analyze the information given, using logic, to dissect the problem and determine the answer.
        o Did you recognize that by cutting in two spots, the DNA is broken into three fragments?
        o Did you read carefully enough to understand that the DNA had already been cut with EcoRI, and now you are cutting with PstI?
        o If the original fragment was from an EcoRI digestion, how did they get it? Do you think they let that digestion go to completion?