Starting a lab job and facing your first reaction tube? These PCR basics help: PCR copies a chosen stretch of DNA by cycling heat, primers and an enzyme until billions of copies exist.
This guide is part of our wider series on lab skills, written for people building a career in biotech.
| Item | Value |
|---|---|
| Enzyme that builds new DNA | Taq polymerase |
| Machine that controls temperature | Thermocycler |
| Typical number of cycles | As many as 30 or 40 |
| Result of those cycles | More than one billion copies |
| Nobel Prize in Chemistry to PCR’s creator, Kary B. Mullis | 1993 |
These figures come from the NHGRI PCR fact sheet. Figures can change, so check the source before you rely on them.
What PCR does and why labs use it
The polymerase chain reaction is a way to make many copies of a small piece of DNA. Polymerase errors can introduce sequence changes. Instead of hunting for a rare sequence in a sample, you make that sequence abundant enough to detect, measure or study.
The NHGRI fact sheet notes that PCR is often heralded as one of the most important advances in molecular biology. Its creator, Kary B. Mullis, was awarded the Nobel Prize for Chemistry in 1993.
In practice, that means PCR sits behind a huge share of everyday lab work. Typical uses include:
- Checking whether a particular gene or sequence is present in a sample
- Preparing DNA fragments for cloning or sequencing
- Confirming that a genetic edit worked in cells or organisms
- Detecting the genetic material of a microbe or virus
Because it appears in so many job descriptions, comfort with PCR is one of the most transferable bench skills you can show an employer.
The ingredients in a PCR tube
A reaction needs a handful of components, and each one has a clear job. Knowing the role of each makes troubleshooting far easier later.
- Template DNA: the sample that contains the segment you want to copy.
- Primers: short pieces of DNA that match the two ends of your target and tell the enzyme where to start.
- DNA polymerase: the enzyme that builds new strands. The NHGRI describes an enzyme called Taq polymerase that synthesizes two new strands using the original strands as templates.
- Nucleotides: the building blocks the polymerase assembles into new DNA.
- Buffer: the salt solution that keeps conditions right for the enzyme.
Kits usually bundle several of these into a premixed solution, so you often add only template, primers and water. Always follow the protocol that your lab or kit manufacturer gives you.
How the cycle works, step by step
PCR is driven by a machine called a thermocycler. The NHGRI explains that it is programmed to alter the temperature of the reaction every few minutes to allow DNA denaturing and synthesis.
Each cycle follows the same pattern:
- Denaturation. The sample is heated so the DNA denatures, meaning it separates into two pieces of single-stranded DNA.
- Annealing. The mixture cools so primers can bind to the matching sequences on each single strand.
- Extension. The polymerase builds a new strand from each primer, using the original strand as its template.
Exact temperatures and hold times depend on the enzyme and the primers, so take them from your protocol rather than from memory.
Why the copies add up so fast
After a cycle, every original strand has a partner copy. Each template strand directs synthesis of one complementary strand, ideally doubling the number of double-stranded DNA molecules per cycle.
Because the amount roughly doubles each round, growth is exponential rather than steady. The cycle of denaturing and synthesizing new DNA is repeated as many as 30 or 40 times, leading to more than one billion copies of the original DNA segment.
That exponential growth is also why PCR is so sensitive. A tiny amount of stray DNA can be amplified just like your sample, which leads to the next point.
Good habits that protect your results
Setup mistakes can cause failed reactions. A few habits prevent the common ones.
- Include a negative control with water in place of template. A product may indicate contamination or primer-dimer formation.
- Use a positive control with a template known to work, so you can tell a failed reaction from a true negative.
- Use filter tips and keep pre-PCR and post-PCR work in separate areas where possible.
- Label tubes clearly and keep the polymerase cold while you pipette.
Write down every reagent lot, primer name and program setting as you go. Our guide to lab notebook best practices shows how to record this so that you or a colleague can repeat the run.
If your work starts from living samples, the cell culture techniques guide covers how cells are grown before DNA is extracted for PCR.
Reading your results and fixing common problems
Most labs check a finished reaction by running it on a gel, which separates DNA fragments by size. A clean band at the expected size suggests that the primers found their target and the polymerase copied it.
When the result looks wrong, work through the likely causes in order. Change one thing at a time so you can tell what fixed it.
- No band at all: the template may be missing or damaged, a reagent may have been left out, or the program may not have run as intended.
- Several bands: primers may be binding to more than one place. A stricter annealing condition or a redesigned primer pair often helps.
- A faint smear: there may be too much template, degraded DNA or too many cycles for the amount of starting material.
- A band in the negative control: treat the whole run as suspect, clean your workspace and repeat with fresh aliquots.
Ask a more experienced colleague to look at your gel image before you change a protocol. A second pair of eyes catches pipetting and labelling slips quickly.
Building PCR skills for a biotech career
Employers rarely expect a newcomer to design assays from scratch. They do expect you to set up reactions accurately, keep clean technique and describe what each step does.
You can build that confidence in a few practical ways:
- Practise accurate pipetting of small volumes until your replicates agree.
- Learn the reasoning behind each component so you can explain it in an interview.
- Keep a tidy record of every run, including the failures, and note what you changed.
- Ask to shadow a colleague who runs PCR daily, and watch how they handle controls.
Variations of the technique, such as reactions that measure how much DNA is present as it is copied, build on the same core idea. Once the basic cycle makes sense, those methods are far easier to pick up.
Frequently asked questions
What does PCR stand for?
Show answer
PCR stands for polymerase chain reaction. It is a method that amplifies a small segment of DNA into many copies.
Why is a thermocycler needed?
Show answer
The reaction depends on repeated temperature changes. According to the NHGRI, the thermocycler is programmed to alter the temperature every few minutes so DNA can denature and be synthesized.
How many cycles does a reaction run?
Show answer
The NHGRI says the cycle is repeated as many as 30 or 40 times. Your protocol sets the exact number for your target and sample.
What is Taq polymerase?
Show answer
It is the enzyme the NHGRI names as the one that builds two new strands of DNA from the original strands as templates.
