Cutting of DNA

Correct answer: A. Less excess agarose improves DNA binding and recovery

Keeping the gel slice as small as possible reduces the amount of agarose carried into purification. Less agarose helps the DNA bind efficiently to a silica column or other purification matrix and can improve DNA recovery. A larger slice does not increase the size of the DNA fragment; it mainly adds unnecessary agarose and possible impurities. Exam tip: cut only the minimum gel surrounding the required DNA band.

Correct answer: A. To prevent DNA contamination

Gel extraction aims to recover a selected DNA fragment with as much purity and integrity as possible for a later step such as ligation, cloning, or amplification. A scalpel that has contacted another sample, old gel, dust, or biological material can transfer unwanted DNA into the excised gel slice. It may also introduce nucleases or other contaminants that damage the target DNA or interfere with downstream enzymes. Using a clean, suitable cutting tool and avoiding unnecessary gel mass reduces this risk and improves the reliability of the recovered fragment. The scalpel does not sweeten the gel, activate the restriction enzyme, or prepare buffer. Therefore preventing DNA and nuclease contamination makes A correct.

Correct answer: A. To reduce DNA damage

UV light can damage DNA by causing lesions such as pyrimidine dimers and strand breaks. This may reduce the ability of the recovered restriction fragment to undergo ligation or cloning. Therefore, the DNA band should be viewed under UV for the shortest possible time. Exam tip: during gel-based DNA recovery, prolonged UV exposure is avoided because it damages DNA.

Correct answer: A. By comparing with ladder and expected map

After a restriction digest, DNA fragments appear as bands at positions related to their sizes. The desired fragment should have a size predicted by the restriction map or experimental design. A DNA ladder provides reference bands of known sizes, allowing the sample band to be estimated and compared with the expected fragment length.

The correct procedure is therefore to compare the band with the ladder and with the expected restriction map before cutting it from the gel. This reduces the chance of purifying an unwanted fragment, which could lead to an unsuccessful cloning or downstream experiment. Enzyme smell, tube colour, and buffer taste provide no reliable information about DNA fragment identity and are not appropriate laboratory methods. Thus option A is correct.

Correct answer: A. DNA concentration can increase

The governing concept is concentration, which is the amount of DNA divided by the volume of solution. If approximately the same recovered DNA mass is eluted in a smaller volume, the DNA concentration increases, making the sample more convenient for ligation, cloning, or another downstream reaction. Therefore, option A is correct. Options B, C, and D do not describe the effect of elution volume: DNA does not become absent, the elution volume does not directly inactivate an enzyme, and it does not make the gel harder. However, the volume should not be made excessively small, because incomplete wetting or poor elution can reduce total DNA recovery.

Correct answer: A. To keep insert-vector ratio correct

Fragment amount and molar ratio are important in ligation setup. Quantification improves cloning efficiency.

Correct answer: A. Ligation efficiency can decrease

Excess salts can affect ligase and DNA end interaction. Purification and wash step are necessary.

Correct answer: A. It can inhibit ligase activity

Residual ethanol can inhibit downstream enzyme reactions. Dry the DNA pellet or column membrane.

Correct answer: A. By keeping DNA under nuclease-free conditions and handling it gently

DNA ends produced after restriction digestion are important for later steps such as ligation. Nuclease-free conditions prevent degradation by enzymes such as DNases, while gentle handling reduces mechanical shearing. Vigorous vortexing can shear DNA, and excessive UV exposure can damage it. Exam tip: For DNA preservation, look for nuclease-free tools and reagents.

Correct answer: B. Restriction endonuclease

Restriction endonucleases recognize specific, usually palindromic DNA sequences and cut both DNA strands. DNA ligase does the opposite: it joins cut DNA fragments. Exam tip: associate DNA cutting at recognition sites with endonuclease.

Correct answer: A. DNA integrity may decrease, and small fragments may break

Repeated freeze-thaw cycles can physically damage DNA, reducing its integrity and sometimes breaking small DNA fragments. This can affect later cloning or ligation steps. Restriction fragments do not become longer on their own, and cut DNA ends do not join without ligase. Exam tip: Store DNA samples in small aliquots to avoid repeated freeze-thaw cycles.

Correct answer: A. Enzyme name fragment identity and date

The direct answer is A: write the enzyme name, fragment identity, and date. A useful label tells what sample is present, which enzyme or treatment it received, and when it was prepared. This information prevents tube mix-ups and helps trace problems during later ligation, electrophoresis, or troubleshooting. Option A is correct because these details identify both the sample and its history. Option B, gel taste, is irrelevant and unsafe; laboratory gels must never be tasted. Option C, enzyme smell, is not a reliable identification method and should not be used. Option D, water color, does not identify the DNA product or its treatment. Labels should also be legible and securely attached, with extra details added when required by the laboratory protocol. Memory cue: label identity, treatment, and date before storing.

Correct answer: A. To track sample identity and enzyme history

The direct answer is A: labeling tracks sample identity and enzyme history. A restriction-digestion product may later be used for gel analysis, purification, ligation, or comparison with another sample. A clear label can record the sample name, enzyme used, date, and relevant reaction information. This prevents mix-ups and helps the researcher know how the fragment was produced when making later cloning decisions. Option A is correct because traceability is essential for reliable laboratory work. Option B is wrong because a label cannot cut DNA; restriction enzymes perform cutting. Option C is wrong because staining the gel requires a stain and an appropriate staining procedure, not a storage label. Option D is wrong because labels do not heat enzymes, and enzymes should normally be handled under suitable temperature conditions rather than heated unnecessarily. Label before storage and use a consistent naming system. Memory cue: label now, trust the sample later.

Correct answer: A. While handling multiple DNA samples and clones at the same time

A is correct. During restriction digestion, handling several DNA samples, clones, or tubes together increases the chance of sample mix-ups or incorrect labelling. A chain-of-sample record documents the sample identity, source, label, and sequence of handling steps. Such a record can still be useful for one tube, but it is most important when multiple samples are being processed. Exam tip: Choose sample tracking or chain-of-custody procedures when the risk of mix-up is high.

Correct answer: A. It is against scientific honesty

A negative result is useful for troubleshooting. Correct reporting is part of responsible science.

Correct answer: A. Suspected variable, such as the correct buffer, enzyme amount, or DNA purity

When repeating a failed restriction digestion, check and modify the suspected variable, such as the appropriate buffer for the enzyme, the amount of active enzyme, or DNA purity. Changing one factor at a time helps identify the actual cause of failure. Changing all conditions together makes troubleshooting difficult. Exam tip: a restriction enzyme requires the correct buffer and sufficiently pure DNA for efficient digestion.

Correct answer: A. Detailed notes and checklist

A checklist reduces enzyme buffer time and temperature errors. Notes make repeat work reliable.

Correct answer: A. Enzyme name and buffer compatibility

Restriction digestion requires the correct restriction enzyme and a compatible buffer recommended for that enzyme. The buffer’s pH, salts, and cofactors affect enzyme activity; an unsuitable buffer may cause poor or no DNA cutting. Water taste, bench colour, and student height do not affect the reaction. Exam tip: check the enzyme, buffer, DNA, reaction volume, and incubation temperature before setting up digestion.

Correct answer: A. Verify the reaction plan and enzyme tube labels first

To select the correct enzyme, compare the enzyme named in the reaction plan with the label on the tube before thawing it. Restriction enzyme names may look similar, but their recognition sites can differ; a wrong enzyme may cut DNA at an unintended site. Thawing all enzymes is also poor practice because repeated thawing can reduce enzyme activity. Exam tip: check the enzyme name, buffer, and required incubation temperature before starting the reaction.

Correct answer: A. Names are similar but enzymes are different

The direct answer is A: the names look similar, but EcoRI and EcoRV are different restriction enzymes. To understand this, restriction enzymes are proteins that recognise particular DNA base sequences and cut DNA at or near those sequences. In the name EcoRI, “Eco” refers to the organism from which the enzyme was obtained, while the remaining letters and Roman numeral identify a particular enzyme. EcoRV has a different recognition sequence and can produce a different kind of DNA end from EcoRI. Option A is correct because similar names do not mean identical enzymes. Option B is wrong: they are not always the same enzyme. Option C is wrong: restriction enzymes are not DNA ligases; ligase joins DNA fragments. Option D is wrong: they are not proteinases, which digest proteins. Always read the complete enzyme name and check its recognition site and cut pattern. Memory cue: similar name, separate enzyme, specific site.

Correct answer: A. Wrong digestion plan can form

Wrong enzyme selection can change compatible ends and fragment pattern. Write exact name in documentation.

Correct answer: A. To track actual reaction duration

The governing concept is controlled reaction timing and reproducibility. Restriction enzymes require a defined incubation period under specified temperature and buffer conditions. Recording the start time allows the operator to calculate when the planned endpoint has been reached and to compare the actual duration across experiments. This helps identify incomplete digestion caused by too little time or unexpected over-incubation that may increase unwanted activity in some conditions. Thus option A is correct. Option B concerns staining and occurs separately during gel analysis. Option C has no scientific role, and option D is not the purpose of recording time; enzyme inactivation, if required, is performed by a specified heat or purification step.

Correct answer: A. To verify digestion duration

End time tells actual incubation length. It is useful in partial digest or star activity analysis.

Correct answer: A. To know whether incubator temperature was correct

Temperature deviation can change enzyme activity. A log helps troubleshooting and quality assurance.

Correct answer: A. Providing stable incubation temperature

Stable temperature is needed for restriction enzyme activity. Device choice depends on lab setup.