Isolation of DNA

Correct answer: A. They can make the solution viscous and affect DNA purity and enzyme reactions

During plant DNA isolation, polysaccharides may co-precipitate with DNA. This can make the sample viscous, reduce DNA purity, and interfere with enzyme-based reactions such as PCR or restriction digestion. Option C describes the action of restriction enzymes, not polysaccharides. Exam tip: The CTAB method is especially useful for removing polysaccharide contaminants from plant DNA preparations.

Correct answer: A. To help remove polysaccharides

CTAB helps remove polysaccharides and some impurities in plant samples. It is common in plant DNA isolation.

Correct answer: A. Helping phase separation and protein removal

Chloroform is an organic solvent used during extraction-based DNA isolation. It does not create new DNA, copy DNA, make RNA, or dissolve agar. Its important practical role is to help separate the mixture into different liquid phases. Proteins and other unwanted cell materials can move into the organic phase or collect at the boundary, while DNA is retained in the suitable aqueous phase under the proper conditions.

Thus, option A is correct because chloroform supports phase separation and helps remove proteins and other impurities. The exact separation also depends on the complete extraction mixture and careful centrifugation. Option B describes amplification, which requires enzymes and nucleotides, while option C is transcription and option D is unrelated to chloroform’s extraction function. The supplied answer and explanation are scientifically consistent.

Correct answer: A. Because it is a hazardous chemical

The governing concept is laboratory chemical safety. Phenol can damage skin and eyes, is corrosive at relevant concentrations, and may be absorbed through the skin; its vapour and contact therefore require careful handling. In DNA extraction, phenol may help separate proteins from nucleic acids, but its usefulness does not make it harmless. Option A is correct. Option B is wrong because phenol is not a vitamin, and option C incorrectly treats a hazardous reagent as a nutrient. Option D is directly false because phenol is not water and is not harmless. Appropriate practice includes a fume hood where required, suitable gloves and eye protection, labelled containers, spill procedures, and disposal according to institutional safety rules.

Correct answer: A. To remove salts and impurities from bound DNA

Wash buffer removes impurities while DNA remains on the column. DNA is then recovered in elution.

Correct answer: A. In releasing DNA from silica

The governing concept is reversible nucleic-acid binding to a silica membrane. Under high-salt and suitable alcohol conditions, DNA binds to silica in a spin column. During elution, the buffer usually has low salt and an appropriate pH, often near neutral or slightly alkaline. These conditions weaken the interaction between DNA and silica and allow the DNA to dissolve into the collected liquid. Therefore option A is correct. Low salt does not burn DNA, create a cell wall, or serve as a general protein-binding step, so options B, C, and D are incorrect. Elution efficiency also depends on buffer volume, contact time, pH, membrane dryness, and whether the sample is allowed to stand before centrifugation.

Correct answer: A. Incomplete cell lysis

DNA can be recovered only after it is released from the cells during lysis. If lysis is incomplete, a significant amount of DNA remains trapped inside the cells, resulting in a low DNA yield. Complete lysis and efficient DNA precipitation generally improve yield rather than reduce it. Exam tip: For low-yield questions in DNA isolation, check errors in lysis, DNA degradation, and precipitation steps.

Correct answer: A. Precipitation or elution step

DNA loss can occur during precipitation washing or elution. Check these steps during troubleshooting.

Correct answer: A. DNA is clean but its amount is low

DNA purity and DNA yield describe two different properties of an extracted sample. Purity indicates how much the preparation is free from substances such as proteins, salts, phenol, or other nucleic acids, whereas yield indicates the total quantity of DNA recovered. Therefore, a sample can have a satisfactory absorbance ratio and still contain only a small amount of DNA because the starting material was limited, cell lysis was incomplete, or DNA was lost during transfer, washing, or elution. Option A is correct. Low yield alone does not prove contamination, and it certainly does not mean that DNA became protein or that no DNA is present.

Correct answer: A. By preparing an appropriate dilution

PCR requires a suitable amount of template DNA, not simply the highest possible concentration. An excessively concentrated template may introduce too much genomic DNA, salts, or other substances into the reaction and can reduce amplification quality or promote nonspecific products. The concentration should first be estimated, then a measured aliquot should be diluted with an appropriate nuclease-free buffer or water to obtain a working concentration. Option A is correct because dilution preserves the DNA while making its input controllable and reproducible. Adding DNA blindly can inhibit or unbalance the reaction; drying and discarding wastes the sample, while ethanol is used for precipitation or storage steps, not as the normal PCR template medium.

Correct answer: A. DNA integrity may be affected and the risk of degradation can increase

Repeated freeze-thaw cycles can cause long DNA molecules to shear or break, reducing DNA integrity and increasing the risk of degradation. Such cycles do not automatically increase DNA purity. Exam tip: Store DNA in small aliquots to avoid thawing the same sample repeatedly.

Correct answer: A. To avoid repeated freeze-thaw cycles of the DNA sample

DNA is divided into small portions (aliquots) and stored in separate tubes. When needed, only one aliquot is thawed, preventing repeated freeze-thaw cycles of the main DNA stock and helping preserve DNA quality. Cutting DNA into fragments is done by restriction enzymes, not by aliquoting. Exam tip: remember that aliquoting is mainly used to protect sample stability and quality during storage.

Correct answer: A. To check contamination

A no-template or blank control can indicate contamination. Controls are useful in quality assurance.

Correct answer: A. To subtract buffer background

The blank is the same solvent or buffer in which DNA is dissolved. This makes absorbance reading more accurate.

Correct answer: A. To record the sample source, method, yield, and purity

A laboratory notebook creates a traceable record of what was done and what was observed during DNA isolation. Important entries can include sample identity and source, date, reagent or kit used, volumes, incubation conditions, centrifugation settings, deviations from the protocol, DNA concentration, purity ratio, and storage conditions. These details allow another person to repeat the procedure and help identify the cause of an unexpectedly low yield or poor-quality preparation. Option A is correct because it records scientifically relevant information and supports reproducibility, troubleshooting, and sample tracking. The other options describe irrelevant observations and would not document the experimental procedure or DNA quality adequately.

Correct answer: A. By fresh tips and proper labelling

Fresh tips and careful handling prevent samples from mixing. This is necessary for reliable DNA analysis.

Correct answer: A. To reduce contamination risk

Aerosol-resistant tips help reduce sample-to-sample contamination. They are useful in sensitive DNA work.

Correct answer: A. To avoid contamination while handling different samples

Gloves protect both the worker and the sample, but they can themselves become contaminated during handling. When moving from one sample to another, touching contaminated surfaces, or dealing with spilled material, gloves may carry DNA, nucleases, salts, chemicals, or microorganisms into a new tube. Changing gloves, together with clean pipetting and separate work areas, reduces cross-contamination and protects the integrity of the isolated DNA. Option A is correct because different samples must be kept separate for reliable identification and downstream analysis. Gloves do not convert DNA into protein, heat samples, or transform water into RNA; those distractors do not describe the purpose of glove changes.

Correct answer: A. To handle biological samples safely

Biological samples may contain pathogens or hazards. Biosafety is linked with safe handling and disposal.

Correct answer: A. To reduce biological and chemical risks

The direct answer is A: used lysis material should be disposed of safely to reduce biological and chemical risks. Lysis breaks cells open, so the waste may contain cell material, microorganisms, DNA, proteins, and chemicals such as detergents, salts, or enzymes. Therefore, it must go into the correct biohazard or chemical-waste container according to laboratory rules; it should not be poured carelessly into a sink or touched without protection. Option A is correct because safe disposal protects students, staff, the environment, and later experiments. Option B is wrong: amplification is done by methods such as PCR, not by throwing away waste. Option C is wrong: disposal does not cool a sample. Option D is wrong: agar gel is prepared separately for DNA separation. Memory cue: lysis waste may contain both biological material and chemicals, so think “protect people and environment.”

Correct answer: A. Through an approved chemical-waste route

Phenol is a hazardous chemical that can cause serious injury through skin or eye contact and can also harm the environment if released improperly. Waste containing phenol should therefore be collected in a compatible, clearly labelled chemical-waste container and handled through the institution’s approved hazardous-waste procedure. The exact process may depend on local regulations, concentration, and whether the waste is mixed with other chemicals, so laboratory safety instructions must be followed rather than improvising disposal. Option A is correct. Pouring phenol into a sink can expose people and contaminate water, while placing it in a garden or leaving it on a bench is unsafe and unacceptable.

Correct answer: A. It can ignite and burn easily

Ethanol is a flammable organic solvent. Its vapours can also ignite, so it must be kept away from open flames, sparks, and hot surfaces during DNA isolation. Ethanol is used to precipitate DNA; it does not synthesize DNA. Exam tip: “Flammable” means a substance can ignite easily.

Correct answer: A. Personal protective equipment

PPE stands for Personal Protective Equipment. During DNA isolation, PPE such as gloves, a lab coat, and safety goggles helps ensure safe handling of samples and chemicals. A protein purification enzyme may be used in an experiment, but it is not what PPE means. Exam tip: Associate PPE with laboratory safety gear.

Correct answer: A. Remember purity yield integrity contamination and safety together

In DNA isolation both quantity and quality are important. Controls records and safe disposal are also important.

Correct answer: A. Sample lysis, cleanup, precipitation, quality check, and storage

A practical DNA-isolation workflow begins with collecting and identifying the sample, followed by lysis to disrupt cells and release nucleic acids. The lysate is then cleared and cleaned so proteins, lipids, cell debris, and inhibitory chemicals are removed. DNA may subsequently be precipitated or bound to and eluted from a purification matrix, depending on the protocol. The preparation is checked for concentration, purity, and sometimes integrity before being stored under suitable conditions for later use. Option A gives this logical sequence. The other choices list unrelated biological processes or omit the essential recovery and quality-control steps required to obtain usable DNA.