Basic Laboratory Skills

Learning Objectives

By the end of this page you should be able to:

• Perform correct pipetting technique and explain why poor technique introduces measurement error
• Compare autoclaving, dry heat, chemical sterilization, and UV disinfection and choose the right one for a given item
• Describe the steps of basic slide preparation and microscope use
• Explain the operating logic of a centrifuge, spectrophotometer, PCR machine, and gel electrophoresis unit
• Distinguish descriptive experimental design elements (hypothesis, variables, controls, replication) and apply them to a simple experiment

Quick Answer

Basic laboratory skills are the hands-on habits that determine whether an experiment's result actually reflects the biology you're studying, rather than technique error. Pipetting delivers precise, reproducible volumes — the single most common source of experimental noise is inconsistent pipetting. Sterilization (autoclaving, dry heat, chemical, UV) prevents contamination from ruining sensitive cultures and reactions. Microscopy lets you directly observe cells and structures too small to see otherwise. And experimental design — a clear hypothesis, defined variables, a proper control group, and enough replicates — is what turns a single measurement into a trustworthy scientific conclusion. Mastering these fundamentals matters because even the most sophisticated instrument produces meaningless data if the sample feeding it was prepared carelessly.

Core Content

Pipetting: The Skill Behind Every Volume

A micropipette delivers small, precise liquid volumes (typically 0.1 μL to 1000 μL) using a spring-loaded piston that displaces air, drawing liquid up into a disposable tip. Correct technique matters because pipetting error compounds: if every reagent in a PCR reaction is off by 5%, the final reaction can fail entirely or give misleading results. Good technique means selecting the correct tip size for the volume range (using a pipette far outside its rated range is inaccurate), holding the pipette vertically, depressing the plunger smoothly to the first stop before drawing up liquid, pausing briefly to let liquid stabilize, and dispensing by pressing to the second stop to expel any residual liquid in the tip. A common beginner error is "double-dipping" — releasing the plunger too fast, which draws air into the tip and creates bubbles that throw off the actual volume delivered.

Sterilization: Matching Method to Material

Sterility prevents unwanted microorganisms from contaminating a culture, reagent, or reaction — a single stray bacterium in a "pure" cell culture can outcompete and destroy weeks of work within days. Different materials need different sterilization approaches:

• Autoclaving: uses pressurized steam (typically 121°C at 15 psi for 15-20 minutes) to kill essentially all microorganisms, including heat-resistant spores. Best for liquids, media, and heat-tolerant glassware/plasticware.
• Dry heat sterilization: uses a hot-air oven (160-180°C for 1-2+ hours) — gentler on moisture-sensitive items like glass pipettes and metal instruments, since no steam is involved, but requires longer exposure because dry heat penetrates less effectively than steam.
• Chemical sterilization: uses agents like ethylene oxide gas or liquid disinfectants (70% ethanol, bleach) for heat-sensitive plastics and equipment that would melt or warp in an autoclave.
• UV light disinfection: damages microbial DNA directly, commonly used to sterilize the air and surfaces inside a laminar flow hood or biosafety cabinet between uses — but UV only works on exposed surfaces, not inside sealed containers or liquids.

Microscopy: Preparing and Observing a Sample

Basic slide preparation follows a consistent sequence regardless of sample type: clean the slide and stage, apply a small drop of mounting medium or sample, gently lower a coverslip at an angle to avoid trapping air bubbles, and press out any remaining bubbles before viewing. Under the scope, you focus first at low magnification to locate your sample, then increase magnification and fine-tune focus, adjusting contrast (via the condenser or light intensity) to make transparent structures like unstained cells visible. A common beginner mistake is jumping straight to high magnification, which produces a tiny field of view that makes it nearly impossible to find your sample.

Correct Operation of Core Instruments

Knowing an instrument's principle (covered in the Introduction chapter) is different from knowing how to run it correctly.

• Spectrophotometer: always calibrate first with a "blank" (the solvent/buffer alone, with no analyte), because the instrument needs a zero-absorbance reference to subtract background absorbance from your actual sample readings. Skipping the blank step silently shifts every subsequent reading.
• Centrifuge: must always be balanced — a tube of matched (or counterbalanced) weight must be placed directly opposite the sample tube in the rotor, or the imbalance can damage the rotor and even cause the centrifuge to shake violently or fail. Speed and time are set according to the protocol; higher speeds pellet smaller/lighter particles.
• PCR machine (thermocycler): reactions are assembled on ice to prevent premature enzyme activity, then the program is run through denaturation (94-98°C), annealing (50-65°C, primer-dependent), and extension (72°C) for 25-35 cycles, before results are checked by gel electrophoresis.
• Gel electrophoresis unit: the gel is cast with combs to form sample wells, submerged in running buffer, loaded with samples (plus a DNA ladder for size reference), and run at a set voltage — DNA migrates toward the positive electrode because its phosphate backbone is negatively charged.

Experimental Design: Turning a Measurement into Evidence

A single data point proves nothing on its own — good experimental design is what makes a result trustworthy. It starts with a clear, testable hypothesis (e.g., "environmental stress alters gene expression in Arabidopsis"), identifying the independent variable you deliberately change, the dependent variable you measure, and confounding variables you must control for (temperature, humidity, time of day). A control group — treated identically except for the variable being tested — is what lets you attribute any difference to your variable rather than to chance or an uncontrolled factor. Finally, replication (running the experiment multiple times, ideally with a statistically justified sample size) distinguishes a real effect from random noise, and is why a single successful PCR reaction is a start, not proof.

Real-World Example

A student measuring bacterial growth in response to a new antibiotic candidate pipettes precise volumes of bacterial culture into several tubes, sterilizes the tubes and media beforehand to avoid contaminating results with unintended microbes, includes a control tube with no antibiotic added, incubates all tubes identically, and reads optical density (OD600) on a spectrophotometer at several time points. Only because the control tube was treated identically except for the antibiotic can the student conclude that a difference in growth is due to the antibiotic itself and not to some other variable, like a batch difference in the growth medium.

Key Terms

Term	Definition	Context/Related
Pipetting	Using a micropipette to transfer precise liquid volumes	Most common source of technique-driven experimental error
Autoclaving	Sterilization using pressurized steam (typically 121°C, 15 psi)	Kills spores; used for media, liquids, heat-tolerant items
Blank	A reference reading (solvent only, no analyte) used to zero a spectrophotometer	Subtracted from sample absorbance readings
Independent variable	The factor deliberately changed by the experimenter	Basis of experimental comparison
Dependent variable	The factor measured as an outcome	What the experiment records
Control group	A group treated identically except for the variable being tested	Isolates cause from coincidence
Confounding variable	An uncontrolled factor that could also explain the result	Must be minimized or accounted for
Replication	Repeating an experiment or measurement multiple times	Distinguishes real effects from random noise

Common Mistakes

Misconception 1: "As long as I get liquid into the tip, my pipetted volume is accurate." Why it's wrong: Volume accuracy depends on consistent technique — plunger speed, angle, tip seating, and using a tip size matched to the volume range all affect the true delivered volume, even if liquid visibly enters and leaves the tip. Correct explanation: Correct technique (vertical pipette, smooth plunger action to the first and second stop, appropriate tip size) is required for accuracy; inconsistent technique introduces random error that can derail downstream reactions like PCR.

Misconception 2: "UV light sterilizes anything it's near, including inside closed tubes." Why it's wrong: UV disinfection only damages microorganisms on directly exposed surfaces — it cannot penetrate plastic, glass, or liquid to reach organisms inside a sealed container. Correct explanation: UV is effective for surfaces and open-air spaces (like inside a laminar flow hood between uses) but liquids, media, and anything inside closed containers require autoclaving or chemical sterilization instead.

Misconception 3: "A control group is just an extra sample to have more data." Why it's wrong: The point of a control isn't to add more measurements — it's to isolate the effect of the variable being tested by holding every other condition identical. Correct explanation: Without a properly matched control, you cannot rule out that an observed difference was caused by something other than your intended variable (a confounding factor), so the experiment can't support a causal conclusion.

Comparison and Connections

Sterilization Method	Mechanism	Best For	Limitation
Autoclaving	Pressurized steam heat	Media, liquids, heat-tolerant glass/plastic	Not for heat-sensitive plastics
Dry heat	Hot air, no moisture	Metal instruments, glass pipettes	Requires longer exposure time
Chemical	Gas or liquid disinfectant	Heat-sensitive equipment	Can leave residue; ventilation needed
UV disinfection	DNA-damaging radiation	Surfaces, hood interiors, air	No penetration into liquids or closed containers

Experimental Design Element	Purpose	Example
Hypothesis	States the predicted relationship being tested	"Stress alters gene expression"
Independent variable	What you deliberately change	Presence/absence of stress treatment
Dependent variable	What you measure as the outcome	Gene expression level
Control group	Isolates the variable's effect	Untreated plants under identical conditions

Practice Questions

Recall

1. List the four common sterilization methods and name one item best suited to each. Answer guidance: Autoclaving (media/liquids), dry heat (metal instruments/glass pipettes), chemical (heat-sensitive plastics), UV (hood surfaces/air).
2. What is a "blank" in spectrophotometry, and why is it measured before the actual sample? Answer guidance: A blank is a reading of the solvent/buffer alone with no analyte; it's measured first to zero the instrument so background absorbance is subtracted from later sample readings.

Understanding

3. Explain why inconsistent pipetting technique can cause a PCR reaction to fail even if all the correct reagents were added. Answer guidance: PCR requires precise ratios of template, primers, dNTPs, and polymerase; inconsistent plunger speed or air bubbles from poor technique change the actual delivered volumes, throwing off these ratios enough to inhibit amplification or produce nonspecific products.
4. Why must a centrifuge rotor always be balanced before running? Answer guidance: An unbalanced rotor creates uneven centrifugal force distribution, which can cause severe vibration, damage the rotor or motor, and in extreme cases cause the centrifuge to fail catastrophically during a high-speed spin.

Application

5. A student wants to sterilize a batch of liquid growth medium before starting a bacterial culture. Which sterilization method should they use and why not the alternatives? Answer guidance: Autoclaving — it directly sterilizes liquids using moist heat; dry heat would take far too long and risks boiling/evaporating the liquid, UV cannot penetrate the liquid volume, and chemical sterilants would contaminate the medium itself.
6. A student is testing whether a new fertilizer increases plant growth. Describe the control group they should include and what would happen to their conclusion without it. Answer guidance: The control should be identical plants grown under the same conditions but without the fertilizer. Without this control, any growth difference could be due to other factors like watering schedule, light exposure, or natural variation, so no causal claim about the fertilizer could be made.

Analysis

7. Two students get very different absorbance readings for what should be identical samples on a spectrophotometer. List two possible technique-related causes and how you'd investigate each. Answer guidance: (1) Failure to blank the instrument correctly — check by re-running the blank and re-zeroing; (2) inconsistent pipetting of sample or dilution volumes — check by re-pipetting from the same stock with careful technique and comparing. Cuvette cleanliness/orientation is another possible cause.
8. A student runs a gel electrophoresis and gets no visible bands for any sample, including a positive control that has worked before. What does the failed positive control tell you, and where should troubleshooting start? Answer guidance: A failed positive control indicates the problem lies in the gel run itself (buffer, voltage, staining, loading) rather than in the individual samples, since the previously successful control should have produced a band regardless of sample quality. Troubleshooting should start with the electrophoresis setup — checking buffer, gel polymerization, power supply connections, and staining/visualization steps — before questioning the samples.

FAQ

Q: Why do micropipettes come in different volume ranges instead of one universal pipette? A: Accuracy degrades sharply outside a pipette's rated range — a pipette designed for 100-1000 μL is imprecise at 5 μL, so labs stock several pipettes (e.g., P2, P20, P200, P1000) to keep every measurement within its accurate range.

Q: Is autoclaving always the "best" sterilization method? A: No — it's the most broadly effective, but it destroys heat-sensitive plastics, degrades some chemicals, and isn't practical for surfaces or air, which is why labs use a combination of methods matched to the material.

Q: Why do you focus a microscope at low magnification first? A: Low magnification gives a wide field of view, making it much easier to locate your sample; increasing magnification narrows the field dramatically, so starting there makes finding anything nearly impossible.

Q: What happens if I forget to include a control group in my experiment? A: You can still make a measurement, but you lose the ability to attribute any observed effect specifically to your variable of interest — the result becomes suggestive at best, not conclusive.

Q: Why does a PCR reaction need to be assembled on ice? A: Keeping reagents cold slows enzyme activity and non-specific primer binding before the controlled thermal cycling begins, preventing premature or off-target amplification that would show up as noise or artifact bands on the gel.

Quick Revision

• Pipetting error compounds through downstream reactions — always match tip size to volume and use consistent plunger technique.
• Autoclaving = steam heat for liquids/media; dry heat = hot air for metal/glass; chemical = heat-sensitive items; UV = surfaces and air only.
• UV cannot penetrate liquids or sealed containers — it is a surface/air disinfection method only.
• Always blank a spectrophotometer before reading samples.
• Centrifuge rotors must be balanced to avoid damage and safety hazards.
• Microscope slide prep: clean slide, apply sample, lower coverslip at an angle, remove air bubbles.
• Focus at low magnification first to locate the sample, then increase magnification.
• PCR is assembled on ice, then cycled through denaturation, annealing, and extension.
• A hypothesis, independent/dependent variables, a matched control group, and replication together make a result scientifically trustworthy.
• A single result without a control cannot support a causal conclusion.
• Gel electrophoresis separates by size because DNA/RNA's negative charge drives migration toward the positive electrode.

Related Topics

Prerequisites

• Introduction to Laboratory Techniques and Instrumentation
• Basic chemistry and units (molarity, dilution)

Related Topics

• Instrumentation in Biotechnology
• Molecular Techniques (PCR and gel electrophoresis in depth)

Next Topics

• Cell Culture Techniques
• Analytical Techniques (chromatography, mass spectrometry)