Cutting of DNA

Correct answer: A. To keep transparent and reliable record

The direct answer is A: save the raw gel image to keep a transparent and reliable record. A restriction digest cuts DNA at particular recognition sites, and gel electrophoresis shows the resulting DNA bands. The unedited image preserves the original evidence, including faint bands, extra bands, background, and loading differences. Step by step: the digest is performed, DNA is separated in the gel, the gel is photographed, and the original file is stored before any cropping, brightness change, or contrast adjustment. This allows another person to check whether the conclusion is supported. Option A is correct because raw data supports verification and honest reporting. Option B is wrong because saving an image cannot activate an enzyme; enzyme activity depends on conditions such as temperature, buffer, and time. Option C is wrong because a gel image cannot convert DNA into protein. Option D is irrelevant and scientifically meaningless. A useful exam cue is: raw data must be preserved before processing.

Correct answer: A. Relevant lanes and ladder context should not be removed

The governing principle is scientific transparency: image editing may improve presentation, but it must not remove information needed to interpret the experiment. When cropping a gel photograph, the relevant sample lanes, lane order, molecular-size ladder, and identifying labels or boundaries should remain visible whenever they are needed to understand the result. Removing the ladder can prevent size estimation, while removing lanes can hide controls or unexpected outcomes. Cropping must never involve inventing bands, hiding lane order, or changing contrast in a way that alters the apparent data. A clearly disclosed, non-misleading crop preserves both readability and evidential context, so option A is the correct precaution.

Correct answer: A. Tracking enzyme conditions and gel images becomes easier

Records help repeat experiments and troubleshooting. Digital images and notes can be kept together.

Correct answer: A. Standard protocol and controlled conditions

Repeating the same conditions makes results comparable. Documentation increases reproducibility.

Correct answer: A. Remember restriction map fragment pattern gel resolution and documentation

In this level pattern analysis after DNA cutting is important. Gel condition and records make the result reliable.

Correct answer: A. Fragments made by restriction cuts are understood using map and gel

In restriction analysis expected fragments are compared with observed gel bands. Proper ladder resolution and records are needed.

Correct answer: A. Because contaminated stock can spoil many reactions

Restriction enzyme stock is used in many experiments. Clean sterile tips and quick handling are necessary.

Correct answer: B. To reduce repeated freeze-thaw and contamination risk

Aliquots avoid repeated thawing of main stock. This preserves enzyme stability and cleanliness.

Correct answer: C. To protect enzyme from freezing damage

Glycerol helps keep enzyme liquid-like and stable in freezer. Glycerol should not be too high in reaction.

Correct answer: D. Star activity can increase

High glycerol can affect enzyme specificity. Therefore keep enzyme volume within recommended limit.

Correct answer: A. To get complete digestion while keeping glycerol low

Restriction digestion requires enough enzyme units to cleave all accessible recognition sites in the DNA, but adding a large volume of enzyme stock is not automatically better. Commercial enzyme preparations commonly contain glycerol and storage salts. Excessive carryover of glycerol, especially beyond the recommended final concentration, can promote nonspecific or star activity and may disturb the reaction conditions. Therefore the reaction is planned with a sufficient number of enzyme units in the smallest practical volume, while keeping the correct buffer, temperature, time, and DNA amount. This balances complete digestion with specificity. The purpose is not to make DNA into protein, dry the gel, or freeze the sample, so A is correct.

Correct answer: C. To verify activity of the new lot

Verifying performance of a new lot gives reliable results. Known substrate control is a good method.

Correct answer: B. Enzyme activity may be reduced

After its expiry date, or because of improper storage, a restriction enzyme may lose activity. It may therefore fail to cut DNA completely, producing partial or failed digestion. A DNA ladder is a separate molecular-size standard; it is not formed because an enzyme has expired. Exam tip: Reduced enzyme activity is a common cause of incomplete DNA digestion.

Correct answer: A. Enzyme stability can decrease

Restriction enzymes are proteins and can be heat sensitive. Keep them on ice and return quickly to freezer.

Correct answer: C. To reduce contamination risk

Repeated entry increases risk of contamination and enzyme loss. Use fresh sterile tip and careful pipetting.

Correct answer: D. Mix or warm according to manufacturer instruction

Buffer precipitate can indicate salt imbalance. Correct buffer condition is needed for enzyme activity.

Correct answer: A. To keep salt and cofactors at uniform concentration

If buffer components are uneven enzyme activity can be affected. Gentle mixing increases reaction reliability.

Correct answer: B. manufacturer recommended working concentration / Manufacturer recommended working concentration

Recommended working buffer concentration is optimized for enzyme activity. Too dilute or too concentrated buffer can spoil result.

Correct answer: C. Enzyme activity can decrease

The direct answer is C: enzyme activity can decrease. A restriction enzyme needs a suitable chemical environment, including an appropriate pH, salt concentration, and sometimes a required cofactor such as magnesium ions. An incompatible buffer can disturb these conditions, so the enzyme may cut slowly, partly, or not at all. Option A is wrong because incompatible conditions do not guarantee complete digestion; the opposite may occur. Option B is wrong because buffer choice does not improve DNA color. Option C is correct because activity depends strongly on reaction conditions. Option D is wrong because a DNA ladder does not always vanish, and its appearance is unrelated to enzyme-buffer compatibility. Use the manufacturer-recommended buffer and check incubation conditions.

Correct answer: A. To know how active both enzymes are in one buffer

Both enzymes need suitable conditions in double digest. Compatibility table decides one-pot or sequential plan.

Correct answer: A. sequential digestion / Sequential digestion

In sequential digestion enzymes are used under separate conditions. This gives both enzymes suitable environment.

Correct answer: A. When second buffer is incompatible with first

Salts or enzyme from first reaction can affect the second reaction. Cleanup or buffer exchange can help.

Correct answer: A. Use cleanup or enzyme removal step

The direct answer is A: use a cleanup or enzyme-removal step. In a double digest, two restriction enzymes are used on the same DNA sample. If one enzyme is not heat-inactivated after the reaction, it may remain active and cut DNA during a later step, especially if the next buffer or temperature is suitable. The reasoning is: digestion ends, the unwanted active enzyme is removed or separated, and the cleaned DNA is then used for ligation or another procedure. Cleanup may use a column, magnetic beads, or another validated enzyme-removal method; the exact protocol depends on the enzymes and experiment. Option A is correct because it prevents unwanted continued cutting. Option B is absurd and unsafe; a reaction must never be eaten. Option C is wrong because restriction digestion does not make DNA become protein. Option D is wrong because removing the gel is not automatically the solution and is unrelated if the issue is an active enzyme in the reaction. Exam cue: when enzyme inactivation fails, remove the enzyme rather than guessing.

Correct answer: A. It can cut ligated DNA again

Active restriction enzyme can cut newly ligated DNA. Therefore use inactivation or purification.

Correct answer: A. In sensitive DNA recovery

Low-melt agarose can make DNA recovery easier and gentler. It can be useful for cloning fragments.