Terminating PCR: How to Control DNA Amplification Length

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Discussion Overview

The discussion revolves around the termination of PCR (Polymerase Chain Reaction) and how to control the amplification length of DNA, specifically focusing on ensuring that only a desired 2kb region is amplified rather than a larger segment. Participants explore the mechanisms involved in the PCR process, including the roles of primers and polymerase during the elongation phase.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions how PCR is terminated and how to ensure that only a specific 2kb region is amplified, expressing confusion about the elongation phase.
  • Another participant asks about the role of primers in the PCR process.
  • A different participant asserts that primers determine the 5' start of the new strand but questions whether they also determine the 3' end.
  • One participant discusses the use of one or two primers and suggests considering the length of copies from subsequent DNA generations, noting that earlier generations are discarded before starting the next cycle.
  • A later reply reflects a realization that while polymerase may extend beyond the region of interest, the primers will only bind to the start of the region of interest in subsequent cycles, leading to amplification of just that region over time.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the termination of PCR and the role of primers, with some confusion remaining about the specifics of how amplification length is controlled. There is no consensus on the details of the mechanisms involved.

Contextual Notes

Some assumptions about the behavior of polymerase and primers may not be fully articulated, and the discussion does not resolve the complexities of PCR termination and amplification length control.

nokia8650
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I have a question regarding PCR which I was unable to find the answer to (maybe because its obvious and I am just not thinking of the reason!) How is PCR terminated? For instance, say you wish to amplify a 2kb length of dna, how does one ensure that just this 2kb region is amplified as opposed to a 100kb region for instance - how do you make sure that the polymerase doesn't just keep copying the template strand in a given cycle. I know that there are the denaturation, annealing and elongation phases, but I just don't understand how the elongation phase is terminated at the right point.

Thanks
 
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What role primers play?
 
primers determine the 5' start of the new strand... not the 3' end?
 
Do you use, or two primers? Why?

Think what is length of a copies from the second & third DNA generation. To simplify things imagine you are throwing away all earlier generations before starting next cycle.

That is not exactly the case, but it doesn't matter much.
 
I started to reply about how i still didnt understand, but then as I tried explaining why I saw the reason! Although the polymerase may go beyond the region of interest, on the next cycle, the primer will only bind to the start of the region of interest, and so after a few cycles, youll be left with just the region of interest.

Thanks so much!
 

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