Neither temperature is automatically right for every PCR workflow. Use the temperature specified for the exact step in the current manual for your kit and assay—not a number lifted from a different protocol or a general textbook explanation. A single kit program can even include both temperatures in sequence.
Why can both 68°C and 72°C appear in PCR instructions?
Temperature only makes sense alongside the step it belongs to. Instructions may refer to different stages, assay designs, enzymes, primers, or protocol configurations; the numbers are not necessarily competing settings.
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For example, Takara Bio USA’s 2021 SMART-Seq v4 PLUS Kit User Manual specifies a cDNA amplification program with a 68°C step lasting 3 minutes, followed by a 72°C step lasting 10 minutes. In that program, both temperatures are used, each at its own stage. They are not alternatives from which the user should choose one.
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A separate kit can offer different cycle configurations and instructions. Takara Bio’s 2015 PrimeScript One Step RT-PCR Kit Ver.2 Instruction Manual describes both three-step and two-step PCR configurations and says to determine the optimal annealing temperature for sequence-specific primers.
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That does not establish a universal preference for either 68°C or 72°C. The manuals document distinct protocol contexts, not a head-to-head comparison or a general rule about which temperature performs better.
How to determine which instruction applies at your bench
- Identify the exact kit. Record its full name and catalog number; do not rely on a generic description such as “PCR kit.”
- Check the protocol revision. Find the current manufacturer’s manual for that kit and confirm its revision or date. The SMART-Seq example is from a 2021 manual, and the PrimeScript example is from a 2015 manual, so check current documentation before using either in a live workflow.
- Match the named step. Locate the program step and its label—such as annealing, extension, denaturation, or terminal extension. Compare temperatures only when they refer to the same step in a relevant protocol.
- Confirm the assay context. Check that the instructions apply to your assay, target, and primers, as well as to the cycle configuration you are running.
- Resolve ambiguity before changing the program. If the manual is unclear or you are considering a modification, consult the manufacturer or your lab’s validated standard operating procedure. Do not infer a setting from the two examples here.
Why not simply substitute one temperature for the other?
A kit’s instructions are designed around its components and protocol. The SMART-Seq manual says its kit components are designed to work together and optimized for that protocol, and advises against substitutions. It warns that substitutions or protocol modifications may lead to unexpected results. A textbook’s general description cannot, by itself, justify changing a kit-specific program.
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What to check on the instrument
The PrimeScript manual lists a DNA thermal cycler among the required equipment, but it does not establish compatibility with a particular make or model. Use the instrument program required by the applicable manual or validated lab SOP, and verify that the selected steps, temperatures, durations, and cycle structure match it.
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