Isolation of DNA

Wash buffer helps remove salts proteins and other impurities. Purified DNA is better for downstream use.

If the same DNA dissolves in less volume concentration can increase. But very low volume can reduce recovery.

Warm buffer can improve DNA recovery from silica column or dried pellet. Keep temperature according to protocol.

PCR requires an adequate amount of good-quality DNA. Protein, phenol, salt, or other contaminants may inhibit DNA polymerase, so DNA concentration and purity should be checked. The colour of a DNA solution is not a reliable indicator of its suitability for PCR. Exam tip: Remember that DNA purity is often assessed using the A260/A280 ratio, while concentration can be measured with a spectrophotometer or fluorometer.

Phenol contamination can affect reactions like PCR and restriction digestion. Proper purification is necessary.

Salt left over after DNA isolation can reduce the efficiency of PCR, restriction-enzyme digestion, and other enzyme-based reactions. Excess salt interferes with enzyme activity and the required ionic conditions of the reaction. It does not permanently make DNA single-stranded or spontaneously alter its sequence. Exam tip: the wash step during DNA purification helps remove inhibitors such as salts.

Proper drying helps remove residual ethanol. Ethanol carryover can inhibit downstream reactions.

Nuclease-free water should not contain DNA-cutting enzymes. It is useful for DNA storage and dilution.

Certified water or proper treatment may be needed for nuclease-free quality. Autoclaving alone is not always enough.

Maintaining sample identity ensures that the isolated DNA is linked to the correct individual, organism, or source. Proper labels, sample codes, and records prevent sample mix-ups, wrong analysis, and incorrect conclusions. Burning DNA or converting it into protein is not a purpose of DNA isolation. Exam tip: Link sample labelling and record keeping with the reliability of laboratory results.

A chain-of-custody record documents the sample’s identity, who received or transferred it, when this happened, and its storage details. This helps maintain correct identification and reliability of the sample. Producing many DNA copies is the role of PCR, not of a record. Exam tip: If a question asks about the sequence of sample transfer and responsibility, choose chain of custody.

The direct answer is A: duplicate extraction is done to check result reliability. A duplicate means carrying out the extraction independently two times, preferably from the same relevant sample or from matched material. The reasoning is: perform extraction 1; perform extraction 2; measure DNA yield or quality in both; compare the results; similar results support reproducibility, while a large difference signals possible pipetting error, sample inconsistency, contamination, inhibition, or a procedural problem that needs investigation. Option A is correct because repeated independent results increase confidence that the finding is genuine rather than accidental. Option B is wrong because duplication is not intended to destroy DNA. Option C is irrelevant and has no scientific role in DNA extraction. Option D is wrong because extraction does not aim to make a cell wall; cell walls are biological structures made by cells. Duplicate work does not guarantee perfection, but it is a useful quality-control check. Memory cue: duplicate means “do it again and compare.”

A negative extraction control should not contain DNA. If DNA signal appears reagent contamination may exist.

A positive extraction control uses a known sample that is expected to yield DNA. Obtaining the expected DNA from this control shows that the extraction protocol, reagents, and procedure are functioning properly. A negative extraction control, not a positive control, is mainly used to check for contamination. Exam tip: a positive control should give an expected positive result, whereas a negative control should contain no target DNA.

Yield indicates the amount of DNA, while purity indicates the absence of contaminants such as proteins, RNA, or other impurities. However, even a large amount of pure DNA may be degraded into small fragments. Checking integrity confirms whether the DNA is intact, high molecular weight, and suitable for downstream work such as PCR, restriction digestion, or cloning. Option B is incorrect because high yield does not prove that DNA is unbroken. Exam tip: remember yield, purity, and integrity as amount, freedom from contaminants, and intactness of DNA, respectively.

A high DNA yield does not guarantee that the preparation is suitable for downstream work. Poor purity may indicate carryover of proteins, phenol, salts, carbohydrates, RNA, or other extraction reagents. Such contaminants can inhibit restriction digestion, PCR, ligation, sequencing, or accurate concentration measurement. The sample should first be assessed using appropriate quality checks and then subjected to an additional cleanup, re-purification, or other validated treatment selected for the contaminant. Option A is correct because it aims to retain the valuable DNA while removing inhibitory substances. Using it without checking risks unreliable results; automatic disposal wastes potentially recoverable material, and adding protein would worsen contamination.

Cleanup helps remove residual salts proteins phenol or ethanol. This improves downstream reactions.

The direct answer is A: clean DNA that is compatible with the restriction enzyme reaction is needed. Restriction enzymes recognize specific DNA sequences and cut them under defined buffer, salt, temperature, and ionic conditions. The reasoning is: DNA is isolated from cells; remaining proteins, alcohol, salts, or other contaminants may interfere with enzyme activity; purified DNA is placed in the enzyme’s compatible buffer; the correct temperature and time allow sequence-specific cutting. Option A is correct because clean DNA and suitable reaction conditions are essential for reliable restriction digestion. Option B is wrong because protein-contaminated DNA may inhibit or complicate digestion rather than improve it. Option C is wrong because excess ethanol can interfere with enzymes and must be removed or reduced before the reaction. Option D is wrong because salt crystals alone are not DNA and cannot provide the required substrate. Restriction digestion also needs the correct enzyme, buffer, DNA amount, and incubation conditions, so “clean DNA” is necessary but not the only requirement. Memory cue: “restriction enzymes need the right DNA and the right reaction environment.”

During DNA isolation, contaminants such as plant polyphenols, excess salts, and residues of phenol or ethanol may remain with the DNA. They can interfere with DNA polymerase activity and reduce or stop PCR amplification. Properly purified DNA itself is not an inhibitor, so option A is correct. Exam tip: When PCR fails, consider contaminant carryover as well as the amount of DNA used.

Dilution can adjust both template amount and inhibitor concentration. It is useful in PCR setup.

Repeated pipetting forces long DNA molecules through the narrow pipette tip, creating mechanical stress that can shear them into smaller fragments. DNA samples are therefore mixed gently to preserve DNA integrity. Pipetting does not increase DNA concentration or remove RNA contamination. Exam tip: Handle high-molecular-weight genomic DNA gently; wide-bore tips can help reduce shearing.

Low-bind tubes are designed to reduce adsorption of DNA to the tube walls. Therefore, they can improve DNA recovery when the sample contains a very small amount or low concentration of DNA. Restriction enzymes, not low-bind tubes, cut DNA. Exam tip: For low-yield samples, remember tools that minimise sample loss.

Final DNA should have enough amount and quality for downstream experiments. Purity integrity and storage are important.

DNA isolation is a stepwise process from sample to purified DNA. Quality control is necessary before final use.

DNA isolation is a foundation step of recombinant DNA technology. Clean intact DNA is useful in PCR cloning and analysis.