PCR in a teaching laboratory

A teaching PCR has to survive shared cyclers, many pairs of gloves, and a room that is often warmer and more humid than the protocol assumed. The decision this guide supports is how to design that practical so students see the method, the controls stay interpretable, and the work stays inside the institution's biosafety rules. The cycle itself is explained in how polymerase chain reaction works. How amplicons travel on a shared bench is the note on PCR controls and contamination control.

The photograph shows a modern research laboratory at dusk, with researchers at benches and a city skyline beyond the glass. A teaching session is usually more crowded than that frame, and the crowding is the hazard. Order enzymes and vessels from the molecular biology catalogue against a written class specification, and use the quote request for the quantities the timetable actually needs.

Start from what students may legally handle

Institutional biosafety decides the material. This article does not. For a first practical, design a short amplicon from a safe research plasmid: a non-pathogenic cloning plasmid with a known sequence, not an unknown clinical specimen, not a swab from a student, and not an environmental sample whose hazard you have not assessed. The plasmid gives every group the same template. The short product, often a few hundred base pairs, finishes inside a class period and separates from primer-dimer on a gel. Extension time follows the enzyme. A classical Taq-like plan is often on the order of a minute per kilobase near 72 Celsius. A short amplicon makes that hold brief. Engineered enzymes publish different speeds. Follow the bottle the class will open.

Record the plasmid the way a sequence record deserves: a name, a map, and a public accession if one exists in a place such as NCBI GenBank. Students should see that the expected size comes from the map, not from a wish.

Shared cyclers and many students

One block will often serve several groups in a day. Book it, name the programme in the instrument library with the class and the date, and write the cycler identity in the demonstrator's book. Two machines that display the same annealing temperature can still heat a plate differently if the plastic, the lid and the service history differ. Do not move a plate to "the other cycler" without naming that change in the record.

Prepare the master mix in a space that has not seen today's products. Aliquot it before student template tubes are open. Each group adds template to its own tubes, or the demonstrator adds it if the lesson is only the gel. The no-template control is closed first, while the plasmid is still shut. The positive, a modest amount of the same plasmid, is added last. Thirty students opening a concentrated plasmid over one rack is how the blank becomes a positive before the block starts.

Gloves and coats move the contamination. The gel bench is downstream. Coats that loaded a bright band do not return to the setup bench. Tip boxes do not travel backward. A pipette used to load the gel does not build the next mix. Say this out loud at the start, because the failure is social as much as chemical. The companion controls note describes why a few copies of yesterday's amplicon are enough.

Humidity in a hot teaching lab changes the setup even when the biosafety plan is sound. A master mix that waits on the bench through a long briefing is warmer than a protocol written for 22 Celsius assumed. Hot-start enzymes, blocked by antibody, chemistry or an aptamer and released by the initial heat, reduce extension during that wait. They do not fix a primer pair that dimerises once cycling starts, and they still follow the storage temperature on the label. Labels smear, films stick badly to fogged plates, and condensation on cold racks looks like a leak. Bring plastic to room condition before you seal, and look at liquid height after the run.

What to do when the power drops

Teaching buildings lose power. If the cycler stops, the demonstrator owns the interpretation, not the first student who sees a band. Read the instrument log. Note whether the programme finished, aborted, or restarted at cycle one, and whether the heated lid came back to the set-point in the programme. Extra cycles push a teaching reaction deeper into plateau and can grow non-specific products. A lid that stayed cold lets small student volumes evaporate. Verify the block with a known positive and a no-template control before any precious or assessed set is trusted. Do not invent a calibration offset. The calibration is the institution's service record.

The demonstrator's record

Write, before the class is dismissed: date, plasmid identity and why it was an allowed teaching template, primer names, expected amplicon length, master-mix lot references you actually used, cycler identity, programme including the heated-lid set-point, which groups shared a mix, the no-template result, the positive result, any power interruption, and the gel image with the marker visible. A student notebook that says "it worked" is not this record. The demonstrator's copy is what you will need when the next class inherits the same cycler and the same room.

If the no-template lane shows the assay band, the practical stops being a success story. Show the students that stop. Retire the aliquots. Do not send them home believing a contaminated blank is a positive sample. If the positive fails and the blank is clean, the lesson is that the reaction failed and the samples cannot be called negative. Both outcomes teach the method more honestly than a perfect gel.

Failure modes that look like a bad class

A blank that is positive after a morning practical is carryover until proved otherwise. A whole edge of a student plate that failed is often film, lid contact or a plate that does not match the block, not thirty biological negatives. A mix that sat through speeches in a hot room can throw primer-dimer even with a hot-start enzyme if the oligos were a poor pair. Change the pair, or show the dimer as the lesson, and do not call it the amplicon.

If groups disagree and they shared a mix, compare their caps and their volumes before you compare their skill. Evaporation changes salt. A loose cap is a chemistry problem.

Safety

Hot blocks, ultraviolet gel imagers and nucleic-acid stains are the everyday hazards. Students do not look into a bare ultraviolet box. Follow the stain's safety data. The plasmid may be harmless by the institution's assessment and the product is still DNA that will contaminate next week's mix. Containment, waste and who may supervise are institutional decisions. The WHO laboratory biosafety manual is background for that committee, not a permission slip from this page.

What to put in an enquiry

State that the material is for a teaching practical, the enzyme class, the approximate amplicon length, the number of reactions in a session, endpoint readout, and the control set (no-template and a plasmid positive). Ask for a hot-start class if the briefing is long and the room runs warm. The nucleic acid analysis pathway places the practical in a wider sample-to-result path if you later add extraction. The multiplex PCR enquiry reference is only a prompt if a later class truly needs more than one product. It is not a statement that a multiplex assay is running. Ask whether a quotation is possible. Leave clinical language out of the note.