Suppose a new species of bacterium is discovered. It contains a form of DNA pol III that can add new nucleotides to either the 5' end or the 3' end of an existing strand. All other properties of the enzyme remain unchanged. How would DNA replication in these bacteria differ from normal replication?

Question

A.  Both new strands could be synthesized continuously like the leading strand.
B.  The same polymerase molecule could synthesize both strands at the same time.
C.  Helicase would no longer be required for replication.
D.  DNA gyrase would no longer be required for replication.

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 DNA replication?

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

A.  Both new strands could be synthesized continuously like the leading strand.

Clarify Question
· What is the key concept addressed by the question?
    o This question addresses DNA replication.
· 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 DNA Pol III is the polymerase that synthesizes DNA during replication. 5’ and 3’ are directions of the DNA backbone.
 
Gather Content
· What do you already know about DNA replication?
    o Normally, DNA replication proceeds differently on each strand, due to the directionality of the DNA pol III enzyme.
    o On the leading strand, DNA pol III can add nucleotides continuously after a single primer starts the process.
    o On the lagging strand, DNA pol III has to add DNA in short segments called Okazaki fragments, each proceeding from a primer.

Consider Possibilities
· What other information is related to the question? Which information is most useful?
    o The question suggests that the hypothetical enzyme is different from normal DNA pol III – it can add nucleotides in either direction.
    o What would this mean for the leading strand? Not much – synthesis on the leading strand can already go quickly and continuously, following the replication fork.
    o So this would most affect the lagging strand.
 
Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
    o What would occur on the lagging strand in this scenario? After primase adds the first primer, the mutant DNA pol III could add nucleotides normally in the 5’ to 3’ direction. But it could ALSO add nucleotides starting from the other side of the RNA primer – in the 3’ to 5’ direction.
    o Thus, in this scenario, even the “lagging strand” could proceed with a single, continuous activity of the DNA polymerase following the unwinding of the replication fork. So it would be continuous, just like the leading strand.
    o Helicase and gyrase would still be required – the replcation fork would still need the DNA to be separated, unwound, and have the supercoiling relieved.
    o Although each strand of DNA could be synthesized continuously, they would still require separate enzymes to do so. Remember, even in a circular bacterial genome, the two strands are two separate strands – their backbones do not connect.
 
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 recognize that lagging strand synthesis is normally discontinuous because of the directionality of DNA pol III?
    o Did you realize that the functions of helicase and gyrase would not be changed?