Cutting of DNA

Correct answer: A. The enzyme cut at one site in plasmid

The direct answer is A: the enzyme may have cut the circular plasmid at one site. A circular plasmid cut exactly once becomes one linear DNA molecule. During gel electrophoresis, this can appear as a band near the plasmid’s expected total size, although migration can differ from uncut circular forms because DNA shape affects movement. Thus, a linearized band supports the idea of a single restriction site for that enzyme, provided the band pattern and controls agree. Option A is correct but should be interpreted with the expected size and controls. Option B is wrong because DNA does not become protein during restriction digestion. Option C is wrong because a band shape or linearization does not prove that the gel is always bad. Option D is wrong because a linearized product suggests enzyme activity, not enzyme absence. Remember that one cut in a circle opens it into a line; multiple cuts would produce multiple fragments.

Correct answer: A. Restriction digestion and uncut control comparison

Uncut control shows plasmid conformations. Comparison with cut sample makes interpretation easier.

Correct answer: A. By agarose concentration

Agarose gel acts as a molecular sieve, and its pore size is mainly controlled by the concentration of agarose used during casting. A higher agarose percentage generally forms a tighter network with smaller pores, while a lower percentage produces larger pores. This changes how DNA fragments move during electrophoresis: smaller pores improve separation of relatively small fragments, whereas larger pores allow large DNA molecules to migrate more effectively. The restriction enzyme determines where DNA is cut, but its name does not set the physical pore size. DNA methylation may affect cutting by some enzymes, but it is not the principal factor controlling gel pores, and the tube cap has no such role.

Correct answer: B. lower concentration / Lower concentration

Agarose gel contains a network of pores through which DNA fragments move under an electric field. Lower agarose concentration produces a looser gel with larger pores. Large DNA fragments can pass through such a network more easily, so their migration and separation are generally improved. Higher agarose concentrations make smaller pores and are more useful for resolving relatively small fragments.

Therefore option B is correct for large fragments. The word “generally” matters because the best concentration also depends on fragment size, gel length, voltage, and running conditions; lower concentration is not the only consideration in every experiment. No agarose would provide no normal agarose-gel matrix, and a protein gel is designed for proteins rather than DNA. The principle is pore size matched to fragment size.

Correct answer: C. Use higher percentage agarose gel

Higher agarose concentration can separate small fragments better. Gel percentage depends on expected size.

Correct answer: D. To separate very large DNA fragments

PFGE is useful for separation of large DNA fragments. Treat it as a different application from normal agarose gel.

Correct answer: A. To make DNA bands visible

After digestion, DNA fragments separate as bands in the gel, but they are usually not visible directly. A DNA stain, such as ethidium bromide or a safer fluorescent dye, binds to DNA and makes the bands visible under UV or blue light. Restriction enzymes cut DNA; a stain does not cut enzymes or neutralize the buffer. Exam tip: in gel electrophoresis, the main role of a DNA stain is visualization of DNA bands.

Correct answer: A. Some DNA stains can be mutagenic or hazardous

Some stains used to visualize DNA, such as ethidium bromide, can be mutagenic and may pose health or environmental risks. Therefore, toxicity, safe handling, PPE, and proper disposal must be considered when selecting a stain. Option D is incorrect because not all stains are non-toxic. Exam tip: For DNA-stain safety questions, remember mutagenicity and waste disposal.

Correct answer: A. Bands become saturated, making intensity comparison unreliable

With excessive exposure, very bright DNA bands can reach the detector's limit and appear saturated. Their actual intensities can then no longer be compared quantitatively and reliably. Image exposure does not change enzyme activity, DNA fragment size, or migration through the gel. Exam tip: associate overexposure mainly with poor image quality and unreliable band quantification.

Correct answer: A. Faint bands can be missed

Direct answer: Option A, an underexposed gel image can cause faint DNA bands to be missed. Exposure controls how much signal the imaging system records. If exposure is too short or the signal is too weak, real low-intensity bands may look invisible, making the digest appear to have fewer fragments than it actually has. Proper imaging, while avoiding overexposure and background, is needed for reliable interpretation. Option A is correct. Option B is wrong because an image setting does not make DNA undergo more cutting; digestion happened earlier in the tube. Option C is wrong because underexposure does not chemically inactivate the enzyme. Option D is wrong because a gel does not become protein. The image problem affects observation, not the molecular reaction. Memory cue: underexposure hides bands; it does not remove DNA.

Correct answer: A. Bands can appear diffuse or smeared

When too much DNA is loaded into one gel lane, the DNA fragments do not separate cleanly and the bands may appear broad, diffuse, or smeared. Complete absence of DNA usually indicates a sample or loading failure, not the direct effect of lane overloading. Exam tip: Load the recommended amount of DNA per lane to obtain sharp, well-resolved bands.

Correct answer: A. The sample may leak or may not load properly

A damaged sample well may not retain the sample at the start of the gel. The sample can leak out, load unevenly, or spread into a nearby lane, producing faint, smeared, or irregular bands. Damage to a well does not change the restriction enzyme’s recognition or cutting sites. Exam tip: associate damaged wells mainly with faulty loading and poor band patterns.

Correct answer: A. Helping the sample settle into the gel well

Loading dye contains a dense component such as glycerol or sucrose. It increases the density of the sample so that the sample settles at the bottom of the gel well instead of dispersing into the buffer. DNA is cut by restriction endonucleases, not by loading dye. Exam tip: the coloured tracking dyes indicate the progress of electrophoresis; they do not show the position of DNA itself.

Correct answer: A. Immediately before loading the sample into the gel

Loading dye is added after restriction digestion, immediately before the sample is loaded into an agarose gel. Components such as glycerol or sucrose help the sample settle into the well, while tracking dyes show the progress of electrophoresis. It is not added before enzyme storage because its purpose is limited to gel electrophoresis. Exam tip: remember that loading dye makes the sample dense and helps track the dye front.

Correct answer: A. Uneven heating or gel running issue

The direct answer is A: uneven heating or another gel-running problem can cause curved bands. During electrophoresis, excessive voltage can generate heat. If the gel, buffer depth, electrical field, or temperature is uneven, DNA may migrate at different speeds across the lane, producing bowed or smiling bands. Old or incorrectly prepared buffer and a poorly positioned gel can also contribute. Option A is correct because these are physical running conditions that distort migration. Option B is wrong because perfect digestion does not by itself create curved bands; digestion concerns cutting, whereas curvature concerns separation conditions. Option C is wrong because absent DNA would usually give no visible band, not a curved one. Option D is wrong because enzyme naming has no physical effect on migration. Use the recommended voltage, fresh appropriate buffer, level the gel, and avoid overheating. Memory cue: curved bands usually suggest a field, heat, or buffer problem.

Correct answer: A. Uneven heating or high voltage

Direct answer: Option A, smiling bands can indicate uneven heating or an overly high voltage during gel electrophoresis. In a properly run gel, bands travel in reasonably straight, even paths. Excessive voltage can heat the buffer, and uneven temperature or conductivity can make migration faster near the edges than in the centre, bending bands upward like a smile. Check voltage, buffer volume and concentration, gel setup, and temperature; also ensure the gel is properly cast. Option A is correct. Option B is too narrow and unrelated: an enzyme-name error does not produce the characteristic migration shape. Option C is wrong because smiling is not evidence only of DNA methylation. Option D is wrong because no buffer is not the only explanation and a gel cannot run normally without buffer. Memory cue: smiling bands suggest a running-condition problem.

Correct answer: A. Poor conductivity and weak band resolution

In gel electrophoresis, repeatedly used running buffer can become depleted of ions and its pH may change. This lowers conductivity and can produce faint or poorly resolved DNA bands. Reusing buffer does not increase the amount of restriction enzyme. Exam tip: use fresh buffer of the correct concentration for clear bands and proper migration.

Correct answer: A. For consistent ionic condition

The governing concept is that electrophoresis depends on a stable electrical and chemical environment. The buffer used to cast the agarose gel and the buffer placed in the electrophoresis tank should normally be the same compatible type and concentration. This maintains consistent ions, pH, and conductivity throughout the gel system, allowing DNA fragments to migrate predictably and bands to resolve properly. A mismatch can alter current flow, heating, migration rate, or band sharpness, making interpretation less reliable. The buffer is not used to color the enzyme, methylate DNA, or make the gel edible. Thus option A correctly identifies the need for consistent ionic conditions.

Correct answer: A. DNA fragment sizes may be estimated incorrectly

A DNA ladder contains fragments of known sizes and is used as a reference to estimate the sizes of sample bands. If the ladder is loaded in the wrong lane, or its lane is identified incorrectly, sample bands may be compared with the wrong marker and their fragment sizes may be estimated incorrectly. Restriction enzyme activity depends on digestion conditions, not on the lane containing the ladder. Exam tip: always record the loading order and prepare a clear lane map for a gel.

Correct answer: A. For accurate size estimation

If ladder does not cover expected size range size estimate will be weak. Choose the correct ladder.

Correct answer: A. Longer run and suitable agarose percentage

The direct answer is A: use a longer run and a suitable agarose percentage. DNA fragments that differ only slightly in size may travel almost together under ordinary conditions. Gel resolution can improve when the agarose concentration is selected for the expected size range and electrophoresis is allowed to run sufficiently long at suitable conditions. Option A is correct because both gel percentage and run time affect separation and band resolution. Option B is wrong because boiling can denature or destroy the restriction enzyme and will not separate bands. Option C is wrong because DNA does not become protein during electrophoresis. Option D is wrong because a ladder is needed to estimate fragment sizes; removing it does not improve the actual separation. Also avoid overloading the sample.

Correct answer: A. For high resolution fragment sizing

Capillary electrophoresis can give precise sizing of fragments. It is useful in advanced analysis.

Correct answer: A. Estimate of DNA amount in band

Band brightness is related to DNA mass. It gives a rough estimate but not exact quantification.

Correct answer: A. Due to stain binding and imaging conditions

Band brightness is an observation produced by staining and imaging, not a direct and perfect measurement of DNA mass. Even fragments with equal mass can appear different because dye binding may vary with fragment structure, local conditions, staining time, destaining, gel thickness, and the position of the band. The imaging system also contributes through exposure, focus, background fluorescence, and detector response. In addition, a band’s apparent intensity can be affected by overlap or diffusion. Therefore brightness should be interpreted together with the molecular-size ladder and experimental controls rather than treated as an exact mass scale. DNA is not protein, enzyme colour is irrelevant, and water volume cannot explain the stated general effect.

Correct answer: A. To show what is in each lane

A lane legend is a documentation record that identifies the material loaded in each lane of a gel image. It may specify the DNA ladder, untreated control, restriction digest, different samples, and relevant experimental conditions. This information allows another person to connect each visible band with the correct sample and judge whether the observed pattern supports the expected digestion. Without a legend, lane positions can be forgotten or confused, so even a technically good image becomes difficult to interpret or reproduce. A legend does not chemically modify the enzyme or DNA, dry the gel, or remove DNA. Thus option A is correct because it preserves the meaning and traceability of the image.