You decide to repeat the Meselson-Stahl experiment, except this time you plan to grow the E. coli cells on light 14N medium for many generations and then transfer them to heavy 15N medium and allow them to grow for 2 additional generations (2 rounds of DNA replication). If the conservative model of DNA replication was correct, what is the expected distribution of DNA in the density gradient after two rounds of replication?

Question

A.  One band of intermediate density.
B.  One band of light density.
C.  One band of heavy density.
D.  One band of light density and one band of heavy density.
E.  One band of light density and one band of intermediate density.

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 the Meselson-Stahl experiment?

  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.  One band of light density and one band of heavy density.

Clarify Question
  · What is the key concept addressed by the question?
    o This question addresses the Meselson-Stahl experiment.
  · What type of thinking is required?
    oYou need to Analyze the information given, using logic, to dissect the problem and determine the answer.
  · What key words does the question contain?
    o The Meselson-Stahl experiment was a classic experiment that demonstrated that DNA replication is semiconservative. The medium is the nutrient broth the cells are grown in – when grown with 14N, the DNA is light, when grown with 15N, the DNA is heavy.

  Gather Content
  · What do you already know about the Meselson-Stahl experiment?
    o The Meselson-Stahl experiment was a clever, classic experiment that tested whether DNA replication was conservative, semiconservative, or dispersive.
    o The bands on the density gradient indicated how much of the 15N was incorporated into the new strands of DNA. Heavy bands were all 15N, light bands were all 14N, and intermediate bands had a mixture.

  Consider Possibilities
  · What other information is related to the question? Which information is most useful?
    o The results of the real experiment showed that in each round of replication, the new DNA has one old strand and one newly-synthesized strand. This supported the semiconservative model.
    o The template strand was “conserved”, but each new DNA double helix contained one fresh strand.

  Choose Answer   · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
    o A diagram can help to answer this question. You can use two colors to indicate light 14N strands and heavy 15N strands.
    o Remember, the question asks you to assume that replication is semiconservative – that would mean that each new double helix has one strand of new DNA, and one strand of old DNA.
    o We start out with all light DNA. In the first round, there would result two new pieces of DNA – each a hybrid of heavy and light.
    o In the second round, there would be two hybrid heavy/light molecules, plus two all heavy molecules.
    o Thus, a heavy band and an intermediate density band.
   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 Analyze the information given, using logic, to dissect the problem and determine the answer.
    o Did you remember the meaning of the semiconservative model?
    o Did you recognize what types of DNA would result from this experiment?
    o Did you try drawing a diagram to help answer the question?