Gel electrophoresis virtual lab: read bands, estimate DNA size, and connect PCR
Interpret a virtual DNA gel using migration direction, a ladder calibration, voltage and time, detection limits, and the distinction between PCR amplification and separation.
A band on a DNA gel combines evidence about fragment movement, amount, and detection. It is not a direct photograph of a base sequence. Scivra's PCR & Gel Electrophoresis connects an agarose-gel model, a size ladder, unknown lanes, and an amplification view so you can inspect those different parts of the measurement chain.
Start by asking what you want to infer: fragment size, a compatible band pattern, or the effect of a preparation and run condition. Each question needs a different comparison. A striking match between two lanes is useful evidence, but it does not automatically answer every question about the source of the DNA.
Why do the wells belong near the negative electrode?
The course explains DNA's negative charge through its phosphate backbone. Under the selected electric field, DNA moves toward the positive electrode. Placing the wells near the negative end lets the fragments travel into the gel under that setup.
Record the electrode orientation before starting. A polarity mistake changes the direction of migration; a missing band is then not necessarily evidence that the sample lacked DNA. Keep loading, field direction, and the later visible result as separate steps in your explanation.
Why do shorter fragments travel farther?
The agarose model gives smaller fragments greater mobility under the matched conditions. Compare lanes run in the same gel and retain voltage and elapsed time. A band position after a longer run cannot be compared directly with a position from a shorter run as if both represented identical fragment size.
A selected fragment's mobility and the gel conditions determine its movement. Apparent brightness supplies another kind of information about the represented amount and visibility. A dark-looking lane and a short migration distance therefore answer different questions; neither observation alone establishes an exact sequence.
How do you estimate the size of an unknown band?
Use the known ladder and measure distances from the loading reference. The course calibration relates the logarithm of fragment length to migration distance. Retain the ladder fragment identities, units, and the fitted relationship used for the unknown.
A distance axis is not a fragment-length axis, and equally spaced distance marks need not represent equal base-pair differences. Use standards from the matched run instead of applying a remembered calibration from another voltage, duration, or gel condition. Keep the raw measured distance with the estimated size so another student can reconstruct the inference.
How do voltage and time change the record?
The course reference follows a 1000 bp fragment reaching 6.0 cm. At 135 V, the specified model time is 2699.984 s; at 50 V, it is 7289.958 s. Those values belong to the course geometry and mobility model.
The comparison illustrates why voltage and time must remain attached to a distance result. It does not imply that every physical gel follows the same times or that extending a run indefinitely improves the evidence. Bands can move beyond the useful gel region or fail to remain separately interpretable.
Choose an interval that supports the question: enough separation for measurement while retaining the features needed for comparison.
What are the limits of a matching band pattern?
Fragments close in size can merge into one apparent band, and a small amount can fall below the detection condition. A visible-band count is therefore not necessarily the number of fragment types originally present.
The course's unknown-lane exercise asks which reference pattern is consistent with the observation. Keep that wording and the detection limits in the conclusion. One selected gel comparison does not establish a person's identity or justify a legal conclusion. More generally, compatible data and unique identification are different claims that require different evidence.
How does PCR fit into the workflow?
PCR amplifies selected genetic material before the gel comparison when the starting amount needs to become detectable. The course follows denaturation, annealing, and extension, with approximately doubling product in its idealized cycle account. The thermostable polymerase supports the repeated heating steps.
Amplification and separation are distinct operations. PCR does not make the gel's migration calibration unnecessary, and a visible product still needs a size or pattern interpretation. Follow DNA Extraction & Structure for the starting material, then From Gene to Protein to distinguish DNA sequence and expression questions from this fragment-size procedure.
Finish the virtual gel investigation with sample preparation, electrode direction, voltage, time, ladder calibration, estimated sizes, and the claim supported by the visible lanes. Retain uncertainty and a specific next comparison instead of filling an answer key from band appearance alone.