Cutting of DNA

Correct answer: A. When the sample contains contaminants such as salts, phenol, or ethanol

Contaminants such as salts, phenol, and ethanol can reduce or inhibit restriction enzyme activity and may interfere with the required buffer conditions. Therefore, cleaning a contaminated DNA sample makes restriction digestion more efficient and reliable. In option B, the DNA is already pure, so additional cleanup is usually unnecessary. Exam tip: remove contaminants before enzyme-based molecular biology procedures.

Correct answer: A. Concentration can change in small volume

The reaction needs correct amounts of enzyme buffer and DNA. Pipetting error can change digestion result.

Correct answer: A. The relative effect of pipetting error can decrease

In a very small reaction volume, a 1–2 µL pipetting error can cause a relatively large change in the total volume and in the concentrations of DNA, enzyme, and buffer. In a larger reaction volume, the same error forms a smaller fraction of the total, making results more reproducible. Increasing volume does not automatically degrade DNA or inactivate the restriction enzyme. Exam tip: For small-volume digests, accurate pipetting and the correct buffer-to-enzyme ratio are especially important.

Correct answer: A. To reduce evaporation and maintain the intended reaction conditions

Restriction digestion is performed in a small reaction volume with defined concentrations of DNA, enzyme, salts, magnesium, and buffer. During incubation, especially at an elevated temperature, evaporation can reduce the volume and concentrate the remaining components. This changes the intended salt, substrate, and enzyme concentrations and may produce less reproducible digestion or even inhibit the enzyme. A properly closed tube helps retain water and keeps the reaction close to its designed composition. Therefore option A is correct. The cap does not stain DNA, destroy the restriction enzyme, or freeze agarose. Good practice also includes briefly spinning down the mixture and following the recommended incubation temperature and time.

Correct answer: A. Bringing drops to tube bottom

The direct answer is A: bringing drops to the tube bottom. During pipetting, small droplets may remain on the tube wall or under the cap. A brief spin-down uses centrifugal force to move those droplets to the bottom, where they mix with the other reaction components. This helps ensure that the intended complete volume participates in digestion. Option A is correct. Option B is wrong because the enzyme performs DNA cutting; spinning does not cut DNA. Option C is wrong because spin-down cannot make an enzyme. Option D is wrong because gel staining happens after electrophoresis and is not caused by centrifugation. Spin-down may also collect liquid and reduce loss, but it does not replace mixing or correct a bad reaction.

Correct answer: A. Components collect at bottom for proper reaction

Direct answer: Option A, a brief centrifuge spin collects droplets at the tube bottom and helps make the reaction uniform. Small drops can remain on the tube wall or lid after pipetting. A short spin brings them together, so enzyme, DNA, buffer, and water are in the same reaction volume and can mix during incubation. Option A is correct, although spinning alone does not replace careful pipetting or mixing. Option B is wrong: a brief spin does not intentionally inactivate the enzyme; excessive heat or unsuitable conditions could do that, not the normal quick spin. Option C is wrong because DNA does not become protein by centrifugation. Option D is wrong because a brief spin does not evaporate buffer. Use a suitable centrifuge and follow laboratory instructions. Memory cue: quick spin gathers, it does not change the chemistry.

Correct answer: A. To reduce the risk of shearing long DNA molecules and affecting enzyme activity

A restriction digest should usually be mixed gently. Harsh shaking can mechanically shear long DNA molecules, producing damaged DNA rather than the expected restriction fragments; it is also unsuitable handling for optimal enzyme activity. Increasing the number of DNA copies is the role of PCR, not restriction digestion. Exam tip: restriction digestion cuts DNA, whereas PCR amplifies DNA.

Correct answer: A. To mix the components uniformly without causing damage

Gently tapping the final reaction mixes DNA, restriction enzyme, buffer, and water uniformly. This ensures proper contact between the enzyme and DNA while avoiding vigorous agitation, which may shear long DNA molecules. Heating would inactivate the enzyme, so it is not the purpose of tapping. Exam tip: In DNA reactions, “gently mix” usually means mixing uniformly without vigorous agitation.

Correct answer: A. To maintain enzyme stability and prevent premature activity before incubation

Restriction enzymes are temperature-sensitive. Preparing the reaction mixture on ice keeps the enzyme stable and limits its activity until the mixture is placed at the required incubation temperature, preventing premature DNA cutting. DNA migration in an agarose gel depends on electrophoresis conditions, not on setting up the digest on ice. Exam tip: Add the restriction enzyme last and mix the tube gently.

Correct answer: A. The risk of loss of enzyme activity increases

Repeated thawing can destabilise the protein structure of a restriction enzyme, increasing the risk that its catalytic activity will decrease. Therefore, enzymes are commonly stored in small aliquots and thawed only when needed. Thawing does not alter the restriction sites on DNA. Exam tip: in enzyme-storage questions, repeated freeze-thaw is mainly associated with loss of enzyme activity.

Correct answer: A. Many reactions can fail

A contaminated enzyme stock can spoil multiple experiments. Sterile tips and quick handling are important.

Correct answer: A. silica membrane / Silica membrane

The direct answer is A: under suitable salt and binding conditions, DNA can bind to the silica membrane of the column. Gel extraction first separates DNA in agarose, after which the desired band is dissolved in a suitable buffer. Chaotropic salts help remove water around DNA and promote its binding to silica. Step by step: cut out the desired gel piece; dissolve the agarose; load the solution onto the silica column; DNA binds to the membrane while many impurities pass through; wash away contaminants; elute the DNA with a low-salt buffer or water. A is correct. B is wrong because the column’s binding surface is silica, not protein fiber. C is wrong because agar taste has no chemical role. D is wrong because water vapor cannot serve as the DNA-binding matrix. Binding and elution depend on correct pH, salt, and volume, so follow the kit protocol. Memory cue: “bind, wash, elute.”

Correct answer: A. Purification efficiency can decrease

The direct answer is A: purification efficiency can decrease. In gel extraction, the desired DNA band is cut from agarose and then dissolved before the DNA is purified. A larger slice contains more agarose than necessary. Therefore, more dissolving buffer, more time, and sometimes more impurities or incomplete melting may be involved. The purification column may also become overloaded or work less efficiently, so DNA recovery can fall. Option A is correct because extra gel makes purification more difficult. Option B is wrong: taking more agarose does not make DNA double; DNA multiplication needs a suitable amplification reaction. Option C is wrong: a larger gel piece does not make a restriction enzyme cut better, and cutting and extraction are different steps. Option D is wrong: buffer does not simply disappear; the real problem is that more buffer may be needed and purification may become inefficient. Cut as little agarose as safely possible around the required band. Memory cue: less gel usually means easier dissolution and better recovery.

Correct answer: A. Wrong fragment can come along

If a nearby band is accidentally cut unwanted DNA can come. High resolution separation is helpful.

Correct answer: A. Making a definite claim without controls

Without controls result interpretation is weak. Scientific conclusion should be based on evidence.

Correct answer: A. Preserve the original gel image and its corresponding lane map

The correct rule is to preserve the original gel image along with its lane map. This allows every band to be verified against its sample, control, or DNA ladder and keeps the results transparent. Adding expected bands or changing lane order is misleading data presentation. Exam tip: In scientific data handling, retaining original records, proper labels, and truthful reporting are ethical practices.

Correct answer: A. For tracking used stains chemicals and waste

Disposal record of stains and chemicals helps safety compliance. Lab safety is also part of experiment.

Correct answer: A. Remember quality control expected observed bands partial digest star activity and cleanup

Verify restriction digestion result using map controls and gel pattern. In troubleshooting check DNA quality and enzyme conditions.

Correct answer: A. Correct enzyme condition controls gel interpretation and records are needed for reliable digestion

Only seeing a band is not enough in DNA cutting. Expected pattern controls and cleanup are also important.

Correct answer: A. Compatible DNA ends with suitable 5′ phosphate and 3′ hydroxyl groups

DNA ligase efficiently joins DNA ends that are compatible, such as compatible sticky ends or compatible blunt ends. Suitable 5′ phosphate and 3′ hydroxyl groups are also needed for phosphodiester bond formation. Equal fragment length or high GC content alone does not ensure ligation. Exam tip: base-pairing between sticky ends makes ligation more specific.

Correct answer: A. To separate desired fragment from unwanted DNA

Direct answer: Option A, purify the desired insert to separate it from unwanted DNA and reaction components. After restriction digestion, the tube may contain the wanted fragment, unwanted vector or fragments, enzyme, salts, buffer, and sometimes DNA of incorrect size. Purification, often by gel extraction or a suitable cleanup method, selects the intended fragment and removes contaminants. This helps ligase join the correct molecules and improves ligation specificity, though yield and DNA damage must also be considered. Option A is correct. Option B is wrong because purification does not make the insert a protein. Option C is wrong because the purpose is to preserve and recover the desired insert, not destroy it. Option D is wrong because enzyme colouring has no role in cloning. Memory cue: purify the right band before joining it.

Correct answer: A. To increase chance of recombinant ligation

Correct insert-vector ratio can increase recombinant clones. A very wrong ratio can reduce cloning efficiency.

Correct answer: A. Vector ends joining each other without insert

Self-ligation can give empty vector colonies. Dephosphorylation or directional cloning reduces it.

Correct answer: A. To reduce self-ligation

Removing phosphate can reduce self-joining of vector ends. This increases chance of insert-containing clones.

Correct answer: A. phosphate group / Phosphate group

DNA ligase joins DNA fragments by forming a phosphodiester bond between a 3′-hydroxyl group and a 5′-phosphate group. When a vector is dephosphorylated, its 5′ phosphate groups have been removed. This prevents the vector ends from easily joining to each other and reduces self-ligation during cloning.

For the insert to be joined to the dephosphorylated vector, the insert must provide suitable 5′ phosphate groups. Ligase can then make the strand connection, although the resulting molecule may initially contain a nick that is repaired inside a host cell. Thus, the required feature is a phosphate group, so option A is correct; protein or ethanol cannot replace the DNA phosphate needed for ligation.