You take a sample of bacteria with DNA that contains only 14N and allow them to replicate in a medium that contains only 15N. After 3 rounds of replication, what percent of the DNA molecules will contain one strand with only 14N and one strand with only 15N? Enter your answer as a whole number without the percent sign. For example, 5% would be entered as 5 and 37% would be entered as 37.

Question

Fill in the blank(s) with the appropriate word(s).

Answer 1

25

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? 
    o You need to analyze the information given, using logic, to dissect the problem and determine the answer.   
• What key words does the question contain? 
    o This question describes something similar to the Meselson-Stahl experiment-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, DNA replication is semiconservative-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 In the third round, there would be two hybrid heavy/light molecules, plus six all heavy molecules. 
    o Thus, 2/8 or ¼ or 25% of the DNA molecules would be a hybrid of intermediate density.   

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 that DNA replication is semiconservative? 
    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?