Question 1

What is the polymerase chain reaction and why is it used in molecular biology?

Accepted Answer

PCR is a molecular technique that amplifies specific DNA sequences exponentially through repeated cycles of heating and cooling. It enables researchers to generate millions of copies from minimal DNA samples, making it essential for genetic analysis, disease diagnosis, and research applications where DNA quantity or purity is limited.

Question 2

How does PCR amplify DNA through thermal cycling?

Accepted Answer

PCR uses three temperature-dependent steps repeated 25-35 cycles: denaturation separates DNA strands at 94-95°C, annealing allows primers to bind at 50-65°C, and extension synthesizes new DNA at 72°C using DNA polymerase. Each cycle doubles the target DNA, producing exponential amplification.

Question 3

What are the key components required to perform a PCR reaction?

Accepted Answer

PCR requires template DNA containing the target sequence, two primers flanking the region of interest, heat-stable DNA polymerase, deoxynucleotides, buffer solution, and magnesium ions. These components work together in a thermal cycler to enable efficient DNA amplification and synthesis of new strands.

Question 4

How are PCR products typically analyzed after amplification?

Accepted Answer

PCR products are commonly analyzed using agarose gel electrophoresis for separation of DNA fragments, which separates amplified DNA by size and allows visualization under ultraviolet light. This confirms successful amplification and verifies that the correct target sequence was produced during the reaction.

Question 5

What is the difference between conventional PCR and real-time PCR?

Accepted Answer

Conventional PCR amplifies DNA over fixed cycles and detects products at the end, while real-time PCR monitors amplification during each cycle using fluorescent signals. Real-time PCR enables quantification of DNA and provides real time PCR principle quantifying gene expression in research and diagnostics.

Question 6

What factors affect PCR efficiency and specificity?

Accepted Answer

PCR efficiency depends on primer design, template DNA quality and concentration, polymerase activity, and optimal magnesium ion concentration. Specificity is influenced by annealing temperature and primer length; higher temperatures and longer primers increase specificity by reducing non-specific binding and improving target selectivity.

Question 7

How does PCR compare to other DNA amplification and sequencing methods?

Accepted Answer

PCR rapidly amplifies specific DNA sequences in vitro, while methods like sangerchain termination sequencing using dideoxynucleotides determine DNA sequence after amplification. PCR is faster and more selective for targeted regions, whereas sequencing provides complete sequence information for comprehensive genetic analysis.

PCR - תגובת שרשרת פולימראז

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