U01.01.034 Polymerase chain reaction

Polymerase Chain Reaction (PCR) is a molecular biology laboratory technique used to amplify a specific fragment of DNA. It is widely used in diagnostics, such as neonatal HIV testing or herpes encephalitis detection. The process involves repeated heating and cooling cycles, with the use of heat-stable DNA polymerase and primers, to amplify the desired DNA sequence.

PCR Process Overview

The PCR process follows three key steps in each cycle:

Denaturation: DNA is heated to ~95°C to separate the two strands.
Annealing: The temperature is lowered to ~55°C. DNA primers, heat-stable polymerase (Taq), and dNTPs are added. The primers bind to the specific sequences at the ends of the DNA fragment to be amplified.
Elongation: The temperature is raised to ~72°C. DNA polymerase attaches the complementary deoxynucleotide triphosphates (dNTPs) to the strand, extending the DNA fragment.

These cycles are repeated until a sufficient quantity of the target DNA is produced.

PCR Process Steps

Step	Description	Temperature
Denaturation	DNA is heated to separate the double strands.	~95°C
Annealing	Primers bind to the target DNA sequence. Taq polymerase and dNTPs are added.	~55°C
Elongation	Taq polymerase extends the DNA strand by adding dNTPs complementary to the template strand.	~72°C

Key Components of PCR

DNA Template: The sample DNA containing the target sequence to be amplified.
DNA Primers: Short single-stranded DNA sequences that initiate replication of the desired fragment.
Heat-Stable DNA Polymerase (Taq Polymerase): The enzyme responsible for adding nucleotides to the growing DNA strand.
Deoxynucleotide Triphosphates (dNTPs): The building blocks (adenine, thymine, cytosine, and guanine) used for synthesizing the DNA strand.

Points to Remember for USMLE Step 1

PCR is used for DNA amplification in diagnostics and research.
The three main steps of PCR are denaturation, annealing, and elongation.
Taq polymerase is a key enzyme in PCR due to its stability at high temperatures (required for denaturation).
PCR is commonly used in genetic diagnostics, such as detecting neonatal HIV or herpes encephalitis.
Heating and cooling cycles in PCR are essential for proper DNA amplification.

USMLE Step 1 Review Series

Curriculum

U01.01.034 Polymerase chain reaction

PCR Process Overview

PCR Process Steps

Key Components of PCR

Points to Remember for USMLE Step 1

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