Isolation of DNA

DNA is negatively charged due to phosphate backbone. Therefore it moves toward the positive electrode in an electric field.

Cross-contamination means that material from one sample accidentally enters another sample. It can produce a false result or make it impossible to know which sample produced an observed DNA band or measurement. Using a fresh sterile pipette tip for each transfer greatly reduces the movement of cells, DNA, enzymes, or reagents between tubes. Careful handling and keeping tubes closed are also helpful.

Correct labelling is equally important because it preserves sample identity and prevents tubes from being confused. Therefore, option A is correct. Using one tip for all samples increases contamination, leaving samples open exposes them to unwanted material, and removing labels destroys the information needed to identify them. Good technique combines fresh tips, clear labels, organized work, and clean surfaces.

Aerosol-resistant tips reduce aerosol contamination risk. They are useful in sensitive DNA work.

Dirty gloves can transfer contaminants or nucleases. Clean handling is needed between different samples.

Nuclease-free tubes and tips protect DNA from breakdown. Sterility reduces sample contamination.

A chain-of-custody record documents, in sequence, who handled or transferred a sample, when it was handled, and for what purpose. It therefore helps maintain sample identification, tracking, and accountability. Translation of DNA into protein is a cellular biological process, not a function of record keeping. Exam tip: Information about who handled a sample and when usually indicates chain of custody.

The governing concept is scientific traceability and reproducibility. A laboratory notebook should preserve the information needed to understand, repeat, compare, and troubleshoot an experiment. Recording the sample source and identity establishes traceability. Recording the extraction method, reagent lots or changes, dates, timings, and deviations explains how the work was performed. Yield and purity measurements allow the result to be evaluated and compared with another preparation. Therefore option B is correct. Taste and smell are not reliable scientific measurements, and tube color alone cannot identify a sample or reproduce a protocol. Complete records also help reveal procedural errors and support responsible interpretation.

The governing concept is experimental reproducibility and quality control. In duplicate extraction, equivalent portions of one sample are processed independently using the same defined protocol. If the preparations give similar yield, purity, and downstream results, confidence increases that the observation is reproducible rather than the result of an accidental pipetting error, contamination event, incomplete mixing, or unusual tube-specific problem. Therefore option C is correct. Duplicates are not intended to destroy DNA, form a cell wall, or compare taste. They do not remove every source of variation, but they provide useful evidence that the extraction procedure is performing consistently.

Biological samples may contain unknown hazards. PPE and biosafety rules are needed for safe handling.

The direct answer is A: used lysis waste should be disposed of safely to reduce chemical and biological risk. Lysis is the step in which cells are broken open. The remaining liquid can therefore contain biological material as well as detergents, salts, enzymes, or other laboratory chemicals. Safe disposal means following the institution’s biohazard and chemical-waste rules, using suitable containers and protective equipment. Option A is correct because it prevents exposure, contamination, accidental reactions, and environmental release. Option B is wrong: DNA amplification requires a specific laboratory reaction, such as PCR, not waste disposal. Option C is wrong: throwing waste away does not cool the sample. Option D is wrong: gel preparation uses agarose and buffer and is a separate step. The exact disposal route depends on the chemicals used, so one should follow the laboratory protocol rather than guess. Memory cue: after lysis, ask “what biological and chemical hazards remain?”

Ethanol is a flammable solvent, and its vapours can also ignite easily. Therefore, during DNA isolation and other laboratory work, ethanol must be kept away from open flames, sparks, and heat sources. A nuclease or restriction enzyme cuts DNA; ethanol does not. Exam tip: Remember that alcohols such as ethanol are generally classified as flammable chemicals.

Phenol is a toxic, corrosive chemical, and its vapours can be harmful to health. A fume hood removes vapours from the work area and reduces the risk of inhalation exposure; therefore, option C is correct. PPE protects the skin and eyes but does not replace a fume hood. A fume hood is not used to stain DNA or to speed up PCR. Exam tip: associate volatile or toxic chemicals with use of a fume hood and appropriate PPE.

Biological samples should be used according to rules and consent. Ethical practice is part of responsible biotechnology.

The governing concept is sample traceability, which connects a measurement to the correct biological source and experimental condition. If a tube is mislabeled, its label is lost, or the records do not identify its origin, a technically valid DNA measurement may be assigned to the wrong individual, tissue, treatment group, or time point. The resulting conclusion can therefore be scientifically or clinically misleading even when extraction and PCR worked properly. Option A is correct. Loss of identity does not purify DNA, automatically improve PCR, or make ethanol disappear. Unique identifiers, clear labels, and contemporaneous records help prevent this serious error.

Yield tells amount purity tells contamination and integrity tells DNA breakage status. All three are important.

TE buffer gives DNA stability but EDTA can affect some enzyme reactions. Choose buffer according to downstream use.

When DNA is divided into small aliquots, a separate tube can be used for each experiment. This avoids repeatedly freezing and thawing the main DNA stock, reducing the risk of DNA degradation and loss of quality. Converting DNA into protein is not a function of aliquoting. Exam tip: aliquoting is mainly used to preserve sample stability and reduce contamination and repeated freeze-thaw cycles.

In DNA isolation challenges change with sample and method. Remember quality control and safety together.

Plant bacterial fungal and plasmid samples can have different needs. Clean intact DNA is used further.

Sample type and DNA type affect method selection. Genomic and plasmid DNA approaches can differ.

Total DNA can include nuclear mitochondrial and sometimes chloroplast DNA. It depends on the sample.

Plant cells have chloroplasts that contain genetic material. Organelle DNA can appear in total DNA extraction.

The direct answer is A: mitochondria have their own DNA. Eukaryotic cells contain nuclear DNA, but mitochondria are organelles with their own small genomes. During total DNA extraction, the method usually breaks open the whole cell and releases DNA from multiple compartments. The reasoning is: the cell is lysed; the nucleus releases nuclear DNA; mitochondria are also broken open; mitochondrial DNA is released; because total DNA is collected rather than only purified nuclear DNA, mitochondrial DNA can be present in the final preparation. This fits the endosymbiotic origin of mitochondria, although that historical explanation is not required to choose the answer. Option A is correct. Option B is wrong because mitochondria do contain proteins, and that statement does not explain mitochondrial DNA. Option C is wrong because ethanol production is not the relevant reason. Option D is absurd: mitochondria are organelles, not agar. The amount of mitochondrial DNA can vary with tissue and extraction method. Memory cue: total DNA may include nuclear plus mitochondrial DNA.

Different samples can have different cell walls and impurities. Therefore buffer composition can change.

CTAB helps remove polysaccharides and some plant impurities. Therefore it is useful in such tissues.