GRE Subject Test: Biology : Understanding Cloning and PCR

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

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Example Question #41 : Lab Techniques

Which step of PCR thermocycling is meant to allow the DNA polymerase to add nucleotides to the complementary strand?

Possible Answers:

Denaturation

Hot start

Annealing

Final hold

Extension

Correct answer:

Extension

Explanation:

The extension or elongation step of thermocycling is used to allow the DNA polymerase to create the new complementary strand to the template DNA strand. Temperature depends on the DNA polymerase being used but generally ranges from 75-80°C.

Denaturation is done at a high temperature generally around 94-98°C to break apart the hydrogen bonds of double stranded DNA into single strands. 

Annealing is the second step of thermocycling completed at 50-65°C.  This step must be done at a temperature low enough allowing primers to attach to the single strand, but high enough that hybridization is specific. 

Prior to the stages of thermocycling there is a often a hot start step to activate the polymerase if necessary. Following the stages of thermocycling is a final extension stage and final hold stage.

Example Question #452 : Gre Subject Test: Biology

The goal of a PCR reaction is to __________.

Possible Answers:

determine a DNA profile 

None of these answers

sequence a DNA template region(s)

separate DNA based on fragment size

produce copies of a targeted DNA region(s)

Correct answer:

produce copies of a targeted DNA region(s)

Explanation:

The purpose of a PCR reaction is to amplify a targeted DNA region into millions of copies.  PCR helps to yield several copies of DNA in a few hours allowing analysis. Each PCR cycles approximately doubles the amount of DNA present in the reaction.

Example Question #52 : Lab Techniques

rtPCR quantification method is a technique that __________.

Possible Answers:

determines the melting point of the DNA fragments during each cycle of PCR

measures the amount of fluorescence produced which is proportional to DNA concentration with each cycle of PCR

determines if the primers used in the PCR reaction are annealing

measures the amount of non-specific amplification products 

separates DNA fragments based on their size

Correct answer:

measures the amount of fluorescence produced which is proportional to DNA concentration with each cycle of PCR

Explanation:

Real time PCR is a technique that measures the amount of fluorescence that occurs with each cycle of PCR. The amount of fluorescence is directly proportional to the DNA concentration.  The assay can also provide info such as the possible presence of inhibitors. 

It is important to know how much DNA is present in a sample to optimize downstream processing.

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