Cutting of DNA

Correct answer: A. Plan sequential digestion

Sequential digestion gives enzymes suitable conditions in separate steps. This improves digestion success.

Correct answer: A. Using two enzymes together in one tube

One-pot double digestion is good only when both enzymes work in compatible buffer and temperature.

Correct answer: A. A single-digest-like pattern can appear

One enzyme failure will not give expected double digest pattern. Controls help identify which enzyme failed.

Correct answer: A. Storage glycerol can become high and increase star activity

The direct answer is option A: too much enzyme volume can add too much glycerol from the enzyme’s storage buffer and may increase star activity. Restriction enzymes are usually stored in a glycerol-containing solution so they remain stable. If a very large volume is added to the reaction, the final glycerol concentration can become too high. Under unsuitable conditions, the enzyme may cut DNA at sites similar to, but not exactly the same as, its normal recognition site. This relaxed, unwanted cutting is called star activity and reduces specificity. Option A is correct. Option B is wrong because more enzyme does not always improve DNA; excessive enzyme or unsuitable conditions may damage the intended pattern. Option C is meaningless because the enzyme is already a protein; it does not need to “become” one. Option D is too absolute: too much enzyme does not always cause no bands, although it can produce unexpected digestion patterns. The memory cue is: respect the recommended enzyme-to-reaction volume and glycerol limit.

Correct answer: A. Unwanted further cutting can occur

The direct answer is A: residual active restriction enzyme may cut the DNA again during the next cloning step. Digestion is intended to create particular DNA ends, after which ligase should join compatible ends. If active enzyme remains in the mixture, it may recognize the restriction site again and cut those ends, including newly ligated molecules if the site is restored. Step by step: digestion produces DNA ends; the mixture is transferred or used for ligation; leftover enzyme remains active under compatible conditions; it can make unwanted cuts; cloning efficiency or product integrity may fall. A is correct. B is wrong because DNA is not guaranteed to remain stable when an active nuclease is present. C is wrong because ligation does not automatically become perfect; it may be disrupted by further cutting. D is wrong because restriction digestion does not make protein. Cleanup, heat inactivation when appropriate, or a suitable reaction design can reduce the risk. Memory cue: “digest, then stop the cutter before joining.”

Correct answer: A. It affects ligation product formation

Correct insert and vector ratio can increase chances of recombinant clones. Incorrect ratio can give low cloning efficiency.

Correct answer: A. Reducing vector self-ligation

Removing phosphate reduces the chance of vector ends joining themselves. This can increase recombinant clone proportion.

Correct answer: A. To remove phosphatase that can affect ligation

Residual phosphatase can keep removing phosphate in ligation setup. Therefore cleanup is useful.

Correct answer: A. Single enzyme sticky ends are compatible

Same compatible ends can let vector ligate to itself. Dephosphorylation or directional cloning can reduce this.

Correct answer: A. By creating two different incompatible vector ends

Different ends can prevent the vector from closing in the same orientation. Insert can join in correct orientation.

Correct answer: A. Contaminants can inhibit ligase activity

Salt ethanol phenol or enzymes can interfere with ligation. Clean DNA ends give better joining.

Correct answer: A. Restriction enzyme can be inhibited by impurities

Phenol ethanol and salts can reduce enzyme activity. Therefore check DNA purity before digestion.

Correct answer: A. It can chelate \(Mg^{2+}\) and reduce restriction enzyme activity

EDTA is a chelating agent that binds divalent metal ions such as \(Mg^{2+}\). Most restriction enzymes require \(Mg^{2+}\) as a cofactor to cut DNA, so residual EDTA can reduce enzyme activity or prevent digestion. EDTA does not alter the DNA recognition sequence. Exam tip: if restriction digestion fails, consider EDTA contamination and check the availability of \(Mg^{2+}\) in the reaction buffer.

Correct answer: A. From DNA purification wash carryover

Leftover salts from purification can alter enzyme activity. DNA cleanup and proper drying are helpful.

Correct answer: A. Pipetting error effect can be larger

In very small volumes small pipetting errors can change concentrations. Accurate pipetting is necessary.

Correct answer: A. It helps keep common components the same in all reactions

The governing concept is standardization and reduction of pipetting variation. A master mix is prepared by combining the components shared by several reactions, such as buffer, water, and sometimes enzyme, before distributing equal portions into individual tubes. This makes the common volumes and concentrations more consistent from tube to tube. It also reduces the number of separate pipetting steps, which lowers the chance of calculation or transfer errors and improves reproducibility when comparing digests. The mix does not destroy the enzyme, methylate DNA, or stain an agarose gel. Those are unrelated actions. Therefore, keeping common reaction components uniform is the correct benefit.

Correct answer: A. To maintain accurate mixing and pipetting

The direct answer is option A: bubbles are avoided to maintain accurate pipetting and proper mixing. Restriction-digest reactions often use very small volumes, so even a small bubble can occupy part of the measured volume or prevent liquid from being drawn and delivered accurately. Bubbles can also keep reagents from contacting one another uniformly, producing incomplete or uneven digestion. Gentle pipetting and careful mixing help keep the reaction homogeneous without damaging the sample. Option A is correct because accurate volume and mixing are essential for reproducible enzyme activity. Option B is wrong: bubbles do not convert DNA into protein. Option C is wrong because bubbles do not stain an enzyme. Option D is wrong because bubbles do not freeze a gel; gel temperature is controlled by other methods. The word “always” should be avoided in laboratory reasoning: bubbles may not ruin every reaction, but avoiding them is good technique. Memory cue: small reaction, small error matters—pipette gently and remove visible bubbles.

Correct answer: A. To collect liquid at the bottom

Spin-down brings droplets from tube wall to bottom. This helps reaction components mix properly.

Correct answer: A. To keep reaction concentration stable

Evaporation can change salt and enzyme concentrations. Proper tube closure and incubation conditions are important.

Correct answer: A. Enzyme activity may decrease, causing slower DNA cleavage

At a very low incubation temperature, the molecular movement and catalytic activity of a restriction enzyme decrease. Therefore, DNA cleavage at its recognition site may become slow or incomplete. Option D is the opposite effect because low temperature generally does not increase enzyme activity. Exam tip: remember the optimum temperature and buffer conditions recommended for a restriction digestion enzyme.

Correct answer: A. Enzyme can denature or become inactive

The direct answer is option A: excessive temperature can denature a restriction enzyme or make it inactive. Restriction enzymes are proteins, and their activity depends on their three-dimensional shape. At the recommended temperature, the active site usually has the correct shape to recognize a DNA sequence and cut it. Too much heat can disturb protein bonds and unfold the enzyme. Once the active site is damaged, the enzyme may no longer bind or cut DNA efficiently. Option A is correct. Option B is wrong because heating a restriction reaction does not automatically amplify DNA; amplification needs a suitable method such as PCR, primers, nucleotides, and cycling conditions. Option C is wrong because buffer remains a solution of salts and other components; it does not turn into DNA. Option D is wrong because a high temperature does not always dissolve an agarose gel, and gel melting depends on the material and conditions, not on enzyme activity. Follow the enzyme supplier’s recommended temperature and time. Memory cue: heat can unfold proteins, so excess heat can stop enzyme action.

Correct answer: A. Providing stable temperature

The direct answer is A: the incubation device provides a stable, suitable temperature. Restriction enzymes work best within a particular temperature range. A water bath, heat block, or thermocycler can keep the reaction near that temperature instead of allowing repeated warming and cooling. Step by step: the enzyme is mixed with DNA and buffer; the device maintains the required temperature; stable temperature helps the enzyme recognize and cut its sites consistently; the digestion is therefore more reproducible. A is correct. B is wrong because staining is done later or separately to visualize DNA in a gel. C is wrong because an incubation device does not manufacture enzyme; the enzyme must already be supplied. D is wrong because gel cutting is a later recovery procedure and is unrelated to incubation. Exact temperature and time must follow the enzyme manufacturer’s instructions, since not every enzyme has the same optimum. Memory cue: “incubation means controlled warmth for enzyme action.”

Correct answer: A. To interpret band identity correctly

The direct answer is option A: lane order must be recorded so each band can be assigned to the correct sample. In a gel, DNA samples are loaded into separate lanes. A lane may contain a ladder, untreated control, digested sample, or another replicate. During electrophoresis and photography, the physical appearance of the gel does not by itself tell us which lane is which. A written lane map preserves that identity. Option A is correct because without it, a true band may be attributed to the wrong sample and the conclusion may be false. Option B is wrong: lane order does not make the gel brighter; staining and imaging conditions affect brightness. Option C is wrong because electrophoresis does not convert DNA into protein. Option D is wrong because lane documentation does not freeze an enzyme. Record the lane number, sample name, treatment, and marker position before loading, and keep the same orientation in the image. Memory cue: no lane map means no trustworthy band identity.

Correct answer: A. Which sample is loaded in each lane

A lane map records the identity of every gel lane, such as which DNA sample, control, or DNA ladder was loaded in it. It therefore allows the observed bands to be assigned to the correct sample during interpretation. Fragment size is not shown by a lane map; it is estimated by comparison with a DNA ladder. Exam tip: when reading a gel diagram, first match the lane number with the lane map.

Correct answer: A. Reporting observed results correctly and honestly

The direct answer is A: ethical reporting means recording and presenting the observed results accurately and honestly. In a restriction-digestion experiment, the gel image, band positions, controls, unexpected results, and relevant procedure details should be reported without inventing or hiding evidence. Step by step: observe the gel; record what is actually visible; compare it with the expected pattern; mention uncertainty or experimental error; then make only a conclusion supported by the data. A is correct. B is wrong because inventing bands is fabrication. C is wrong because hiding the use of a wrong enzyme conceals an important experimental fact and can mislead interpretation. D is wrong because removing labels can cause sample mix-ups and destroys traceability. Ethical reporting does not mean that every result must look successful; an unsuccessful result can still be valuable when described honestly. Memory cue: “report what you saw, not what you hoped to see.”