A Polymerase Chain Reaction (PCR) strip is a specialized laboratory plastic designed to facilitate DNA amplification. It serves as the reaction vessel for PCR, a technique that creates millions of copies of a specific DNA segment from a minute starting sample. The strip is engineered to hold the reaction mixture (DNA template, primers, nucleotides, and polymerase enzyme) in a format compatible with automated thermal cycling instruments. This container is necessary to control and execute the rapid, repeated temperature shifts required for successful DNA replication.
Physical Design and Materials
A PCR strip typically consists of eight or twelve small, individual tubes physically joined in a linear arrangement. This connected format allows multiple samples to be processed and handled simultaneously, improving laboratory efficiency. The tubes are small, often conical, and designed to accommodate minimal reaction volumes, commonly ranging from 10 to 200 microliters.
The material of choice is high-grade polypropylene, selected for its unique physical properties. Polypropylene is chemically inert, meaning it will not react with the sensitive components of the PCR mixture. It also possesses a high melting point, enabling it to withstand repeated heating cycles up to 95°C without deforming or leaching contaminants.
The tube walls are manufactured to be exceptionally thin and uniform, often less than a millimeter thick. This thinness ensures maximum surface area contact with the thermal cycler’s metal block. While most strips are clear for standard reactions, quantitative PCR (qPCR) often uses white or frosted strips. The opaque color reflects the fluorescent signal back to the detector, lowering background noise and improving real-time DNA quantification accuracy.
Function in Thermal Cycling
The strip’s physical design is linked to its function within the thermal cycler, the instrument that rapidly heats and cools the samples to drive the PCR process. The thin, uniform polypropylene walls have low thermal mass, allowing for fast and accurate heat transfer from the heating block to the liquid reaction mixture. This rapid temperature change, or ramp rate, is fundamental to the reaction’s efficiency and speed.
The thermal cycling process involves three distinct temperature phases executed with precision. First, the denaturation phase requires a brief, high temperature (around 94°C to 98°C) to separate the double-stranded DNA template. Next, the temperature is rapidly lowered to the annealing temperature (typically 50°C to 65°C), allowing primers to bind to the target DNA sequence.
Finally, the temperature is raised to the extension phase, usually around 72°C, which is optimal for the DNA polymerase enzyme to synthesize a new DNA strand. The strip’s connected format ensures all reaction mixtures experience these temperature shifts simultaneously and uniformly. This synchronization is paramount for experimental reproducibility, preventing variations in DNA amplification yield. The tight fit maximizes conductive heat exchange, enabling the entire cycle to complete quickly.
Practical Handling and Use
Proper handling of the PCR strip is necessary to ensure reaction integrity and reliable results. The most important step is effective sealing of the tubes before placing the strip into the thermal cycler. Sealing prevents sample evaporation, which causes a loss of reaction volume and an undesirable increase in component concentration.
Sealing is typically achieved using individual snap-on caps, which may be pre-attached, or a dedicated adhesive sealing film. When using caps, press firmly until a distinct snap confirms a secure, airtight seal. Adhesive films require a flat tool, such as a roller, to apply even pressure and ensure the film adheres tightly to the rim of every tube.
A secure seal also prevents cross-contamination, a major concern in PCR that can lead to false positive results. The strip design aids in organization and high-throughput loading, often compatible with multi-channel pipettes. Before thermal cycling, it is good practice to briefly spin the sealed strip in a centrifuge. This consolidates the reaction mixture, eliminates air bubbles, and ensures the sample contacts the strip’s heated surface.