Isolation of DNA

Silica-column purification depends on the chemical environment of the binding mixture, not simply on temperature or appearance. In the presence of a suitable concentration of chaotropic salt, DNA becomes relatively dehydrated and can interact with the silica surface, while many proteins and other contaminants remain in the liquid or are removed during washing. After washing, a low-salt aqueous buffer or water rehydrates the DNA and releases it during elution. Therefore option B is correct. Low-temperature light and agar solidification do not create the necessary binding chemistry. Protein digestion may improve cleanliness, but by itself it does not guarantee DNA binding to silica.

The wash step removes salts and contaminants while DNA remains on the column. This makes eluted DNA clean.

Direct answer: Option A, DNA concentration can increase. Elution means washing or releasing DNA into a liquid. If the same amount of DNA is dissolved in a smaller volume, the DNA is more concentrated: concentration = amount of DNA ÷ volume. For example, 10 micrograms in 100 microlitres is less concentrated than 10 micrograms in 50 microlitres. However, an extremely small volume may not wash all DNA from the column, so total recovery can decrease. Option A is correct because concentration may rise. B is wrong because low volume does not always degrade DNA. C is wrong because protein contamination does not automatically increase. D is wrong because DNA does not become absent. Memory cue: smaller volume usually means higher concentration, but possibly lower recovery.

In gel electrophoresis, DNA fragments separate mainly according to size and appear as bands. Good-quality, intact genomic DNA usually gives a clear high-molecular-weight band, whereas degraded DNA may appear as a smear. Thus, the gel is useful for assessing DNA integrity. An exact DNA concentration is not obtained reliably from a gel; a spectrophotometer or fluorometer is more suitable for that purpose. Exam tip: a clear band suggests relatively intact DNA, while a smear suggests degradation.

Smear can indicate broken DNA fragments. Gentle handling and nuclease protection are necessary.

A spectrophotometer estimates DNA amount and purity using absorbance reading. It is useful in quality control.

For pure DNA A260 by A280 is considered about 1.8. It is useful for checking protein contamination.

A260 by A230 ratio can indicate impurities like salts phenol and carbohydrates. It is especially useful in plant samples.

Correct DNA amount and purity are needed for PCR template. Contaminants can inhibit amplification.

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

Salt remaining after DNA extraction can reduce the efficiency of enzyme-based reactions such as PCR, restriction digestion, and ligation. Excess salt interferes with suitable ionic conditions and proper enzyme activity. Therefore, wash steps are important for salt removal. Exam tip: In DNA-purity questions, remember that salt carryover mainly inhibits downstream enzymatic reactions.

Nuclease-free water should be free from DNA-cutting enzymes. This makes DNA storage and dilution safer.

When a DNA sample is divided into small aliquots, only one portion needs to be thawed when required. This prevents repeated freeze-thaw cycles of the main stock and helps preserve DNA quality and integrity. Making aliquots does not increase DNA concentration. Exam tip: Avoiding repeated freeze-thaw cycles is important when storing DNA, RNA and enzymes.

A label tracks sample identity and date. This is necessary for reproducibility and correct analysis.

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

The direct answer is D: to check reagent contamination. A DNA extraction negative control contains all relevant reagents and follows the extraction steps but contains no biological sample. The reasoning is: prepare the control without sample; process it exactly like the real samples; test the final control for DNA or the expected signal; if a signal appears, DNA may have entered through a contaminated reagent, tube, tool, or handling step. Therefore, a positive negative control weakens confidence in sample results and calls for investigation and repetition. Option A is wrong because a negative control is not added to increase DNA yield. Option B is wrong because it contains no cells and is not used for cell lysis. Option C is wrong because it is not a protein-production reaction. Option D is correct because the control reveals contamination introduced during extraction or associated handling. It cannot identify every possible error, but it is an important contamination check. Memory cue: no sample plus signal equals suspect contamination.

A DNA extraction positive control uses a known sample that is expected to yield DNA. If DNA is obtained from it as expected, the extraction steps and reagents are considered to be functioning properly. Contamination is usually checked using a negative control, not a positive control. Exam tip: a positive control should give the expected positive result, whereas a negative control should not give a result.

The direct answer is C: PPE helps maintain both sample safety and worker safety. Personal protective equipment includes items such as gloves, a lab coat, and eye protection, chosen according to the biological and chemical risks of the procedure. The reasoning is: laboratory samples may be contaminated or contain hazardous chemicals; gloves and protective clothing reduce contact with skin and clothing; eye protection helps prevent splashes from reaching the eyes; careful PPE use also reduces the chance that a worker will contaminate the sample. Option C is correct because PPE protects the person and helps maintain cleaner handling of the sample. Option A is wrong because PPE does not increase DNA concentration; concentration depends on extraction and elution conditions. Option B is wrong because PPE does not manufacture ethanol; ethanol is a chemical reagent made by separate industrial or biological processes. Option D is wrong because agar solidification depends on its composition and cooling, not on protective clothing. PPE is only one part of safety: proper technique, labeling, disposal, hand hygiene, and risk assessment are also needed. Memory cue: “PPE protects people and prevents contamination; it does not change the sample’s chemistry.”

Phenol is a hazardous chemical. It should be disposed through chemical waste route according to safety rules.

In DNA isolation, cells are first broken open by lysis to release DNA. Proteins, RNA and other contaminants are then removed, DNA is precipitated, and its purity and quality are checked. Therefore, option A is the correct revision point because it covers the main workflow. Remembering photosynthesis alone is unrelated to DNA isolation. Exam tip: recall the sequence—lysis, purification, precipitation, quality check.

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

Genomic DNA consists of long chromosomal DNA molecules. Vigorous vortexing, repeated pipetting, or rough mixing can shear these molecules into smaller fragments, so they should be kept as intact as possible. Breaking small RNA fragments is not the main concern during genomic DNA isolation. Exam tip: gentle mixing and wide-bore pipette tips help reduce DNA shearing.

In a plasmid-DNA isolation procedure, the target is the plasmid molecule rather than the cell’s main chromosome. A plasmid is usually a small, circular, double-stranded DNA molecule that exists separately from the bacterial chromosome. It can replicate inside a suitable host cell and may carry useful genes, such as a selectable marker or an inserted recombinant sequence.

Thus, option B is correct: the aim of a plasmid preparation is to collect this small circular extra-chromosomal DNA. Chromosomal DNA may be unwanted carryover, while RNA and proteins are different biological materials and are removed during purification. The word “target” means the nucleic-acid product the procedure is designed to recover, not every substance present in the cell.

Alkaline lysis is common in bacterial plasmid DNA preparation. It separates plasmid and chromosomal DNA.

Alkaline lysis separates plasmid DNA from cellular material by using detergent and alkaline conditions, followed by neutralization. Plasmid DNA is small and circular, so it can often re-form correctly, whereas long chromosomal DNA is fragile and easily sheared. Vigorous vortexing or rough pipetting breaks chromosomal DNA into smaller pieces that remain in the solution and may contaminate the plasmid preparation. Gentle inversion mixes the reagents while limiting this shearing. Therefore option D is correct; the other choices do not explain the purpose of gentle handling.