Isolation of DNA

After alkaline lysis the cleared supernatant can contain plasmid DNA. It is further purified according to protocol.

The direct answer is D: DNA usually remains in the aqueous phase. In phenol-chloroform extraction, the mixture separates into an upper water-based aqueous phase, a lower organic phase containing phenol and chloroform, and sometimes an interface containing denatured proteins and debris. DNA is negatively charged and, under the usual buffered conditions, is soluble in the aqueous layer. Option D is correct. Option A is wrong because DNA is not normally restricted to the organic phase; that phase mainly receives lipids and many hydrophobic substances. Option B is wrong because the interface mainly contains precipitated or denatured protein, although DNA can be lost there if the sample is handled badly. Option C is wrong because DNA is not normally a solid protein pellet in this step. After carefully collecting the aqueous layer, DNA may later be precipitated with alcohol. Memory cue: water-loving nucleic acids stay in the water layer.

In organic extraction proteins can move into organic phase or interface. DNA can remain in the aqueous layer.

During DNA isolation, the interface between the aqueous and organic layers often contains proteins, cell debris, and other impurities. If this layer is accidentally collected, the DNA sample becomes contaminated with proteins and cellular debris, lowering its purity. Option A is the opposite effect. Exam tip: Carefully remove the DNA-containing aqueous layer without disturbing the interface.

Polyphenols can become impurities in plant DNA samples. Therefore plant protocols try to remove them.

Polysaccharides from plant tissues may co-extract or co-purify with DNA. They make the DNA extract viscous and can inhibit enzymatic reactions such as PCR and restriction digestion. Option D is incorrect because polysaccharides do not synthesize DNA or increase its copy number. Exam tip: In plant DNA isolation, associate polysaccharide contamination with viscosity and enzyme inhibition.

CTAB helps remove polysaccharides and some secondary compounds from plant tissues. It is common in plant DNA isolation.

It can help reduce polyphenol oxidation as a reducing agent. It can be useful for plant DNA purity.

Mechanical forces produced by harsh vortexing and rough pipetting can break long genomic DNA molecules into smaller fragments; this is called DNA shearing. A correct buffer and proper cooling help maintain DNA stability and are not usual causes of shearing. Exam tip: To protect high-molecular-weight DNA during isolation, mix gently and use wide-bore pipette tips.

The governing concept is protection of DNA integrity during physical handling. High-molecular-weight DNA consists of very long strands, so it is especially vulnerable to mechanical shearing. Vortexing, forceful pipetting, repeated rapid mixing, and foaming can break those strands into smaller fragments. Gentle inversion or slow mixing distributes reagents while applying much less mechanical stress, allowing the DNA to remain sufficiently intact for electrophoresis, long-fragment analysis, or other applications. Therefore option D is correct. Rough vortexing and fast pipetting can shear DNA, while boiling can denature and damage it rather than preserve its high molecular weight.

The governing concept is recovery of DNA from the starting biological material. Efficient cell lysis is essential because intact cells or nuclei retain DNA, preventing it from entering the extraction solution. If the lysis buffer is unsuitable, the incubation is too short, the sample is not mixed adequately, or the material is difficult to disrupt, only part of the available DNA is released. The final measured yield is then low. Option A is correct. Good precipitation and correct elution generally improve recovery, while a perfect sample is not a cause of low yield. Losses can also occur later, but incomplete lysis is a common early cause.

The governing concept is the distinction between quantity and quality in a DNA preparation. Yield describes how much DNA has been recovered, commonly estimated from concentration multiplied by sample volume. Purity describes the relative absence of contaminants such as protein, salts, solvent, or residual reagents, often assessed through absorbance ratios or another quality test. These measurements are related but not identical. A preparation can therefore have a favorable purity result while containing only a small amount of DNA. Option B is correct. Good purity does not mean high yield, and low yield does not prove that no DNA was isolated or that the material is protein.

A high-yield sample can still be unusable due to impurities. Cleanup helps downstream reactions.

Cleanup helps remove residual impurities such as salts phenol protein or ethanol. This can improve PCR and digestion.

During DNA extraction, plant compounds such as polyphenols and polysaccharides, as well as residual salts, phenol, or ethanol from purification, can interfere with PCR enzyme activity. Therefore, option A is correct. Completely pure DNA is not itself a PCR inhibitor; the problem is contaminating substances. Exam tip: For PCR inhibitors, focus on contaminants that may remain after extraction and purification.

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

The direct answer is option C: enzyme-compatible clean DNA. Restriction digestion means cutting DNA at particular recognition sites with restriction enzymes. These enzymes work only when the DNA sample is sufficiently pure and placed in a suitable reaction buffer. Step by step: first, DNA is isolated; second, proteins, phenol, alcohol, excess salt and other chemicals are removed; third, the clean DNA is mixed with the correct enzyme and buffer; fourth, the enzyme cuts at its specific site. Option A is wrong because protein contamination can block or reduce enzyme activity, so “always protein contaminated” describes a poor sample, not a required one. Option B is wrong because leftover ethanol can inhibit enzymes and interfere with the reaction. Option C is correct because clean DNA with conditions compatible with the enzyme allows reliable cutting. Option D is wrong because salt crystals alone are not a DNA sample and cannot be digested. A useful exam cue is: restriction enzymes need pure DNA, the correct buffer, proper temperature and enough time; contaminants are obstacles, not requirements.

For cloning DNA should be free from contaminants and sufficiently intact. Poor quality can cause cloning failure.

The amount of DNA template in PCR should be within an appropriate range. When DNA concentration is too high, the sample is diluted so that a suitable amount of template is added; excess DNA or associated impurities can interfere with PCR. Ethanol is used for DNA precipitation, not as the usual step to reduce an overly concentrated PCR template. Exam tip: If the template is too concentrated for PCR, dilution is generally the first step.

Too little template can give weak signal in downstream reactions. Concentration check is important.

NanoDrop estimates DNA concentration and purity ratios by absorbance. It gives quick quality check.

The direct answer is D: fluorometric quantification is better when the specific amount of double-stranded DNA is needed. Absorbance methods measure light absorption, commonly at 260 nm, but proteins, RNA, phenol, salts, and other contaminants can affect the reading. Fluorometric methods use a dye that binds preferentially to the target nucleic acid, so they are often more specific and sensitive, especially when DNA concentration is low or the sample is impure. Option D is correct because the method is chosen to measure a particular DNA type more selectively. Option A is wrong because burning DNA is not a measurement purpose. Option B is wrong because agar preparation is unrelated to quantification. Option C is wrong because protein digestion is a separate treatment, not the reason for fluorometry. Fluorescence still requires correct standards, dye, instrument settings, and sample handling. Memory cue: absorbance sees general light absorption; fluorescence can identify DNA more specifically.

Band intensity can be compared with a known ladder or standard for approximate DNA amount. It is a rough estimate.

A DNA ladder is a mixture of DNA fragments with known base-pair lengths. Comparing the positions of its bands with sample bands on the gel helps estimate the sizes of sample DNA fragments. Cell lysis is an earlier step in DNA isolation; a ladder is only a molecular size marker. Exam tip: remember that a DNA ladder is used as a molecular size marker in gel electrophoresis.

In gel electrophoresis, smaller DNA fragments migrate farther down the gel than larger fragments. Therefore, very small fragments near the bottom can indicate DNA degradation or fragmentation, often caused by improper handling or nucleases such as DNase. Intact high-molecular-weight DNA usually remains closer to the top of the gel. Exam tip: the farther a DNA band moves down the gel, the smaller the fragment is generally expected to be.