Methods in Biology

DNA & RNA isolation

What is the emission wavelength of EtBr when bound to DNA?

Ethidium bromide is a fluorescent intercalator whose photophysics change dramatically upon DNA binding. Free EtBr in aqueous solution emits weakly, but when intercalated between base pairs, hydrophobic environment shields it and fluorescence quantum yield increases about 20-fold. Upon excitation with UV around 302 nm or 365 nm, the DNA-EtBr complex emits orange-red fluorescence with emission maximum near 590 nm. This property enables visualization of DNA bands in agarose gels under UV transilluminator. Shorter wavelengths like 260 nm or 400 nm are absorption peaks, not emission maxima.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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DNA & RNA isolation

The interphase in organic extraction contains:

During phenol-chloroform extraction, lysate separates into three distinct zones after centrifugation. Upper aqueous phase contains hydrophilic nucleic acids, lower organic phase contains lipids and hydrophobic molecules dissolved in phenol-chloroform. Denatured proteins, containing both hydrophobic and hydrophilic residues, are insoluble in either phase and accumulate as white flocculent precipitate at interphase between aqueous and organic layers. Centrifugation traps them there firmly. DNA and RNA remain aqueous, buffers remain aqueous, but interphase is diagnostic of successful protein denaturation and effective purification during extraction.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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DNA & RNA isolation

Which of the following is used to quantify DNA using fluorescence?

Fluorescent quantification of DNA employs DNA-binding fluorochromes for sensitivity beyond UV absorbance. SYBR Green I is a cyanine dye that exhibits extremely low fluorescence when free but over 1000-fold enhancement upon intercalation into minor groove of double-stranded DNA. This allows specific quantitation of dsDNA even with contaminants present. Coomassie Blue stains proteins, DAPI binds AT-rich dsDNA but is used mainly for microscopy, and TRIS is a buffer. SYBR Green is standard for real-time PCR quantification and fluorometric assays with high specificity and linearity.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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DNA & RNA isolation

What determines the location of DNA in organic extraction layers?

Organic extraction partitioning of DNA depends on physicochemical properties like solubility and polarity rather than size alone. Nucleic acids are highly hydrophilic polyanions due to sugar-phosphate backbone, favoring aqueous phase. Proteins, especially after phenol denaturation, expose hydrophobic residues and partition into organic phase or interphase. Phenol-chloroform mixture creates immiscible layers where hydrophilic molecules dissolve in water and hydrophobic in organic. Temperature, pH of phenol influences RNA vs DNA partitioning, and size influences precipitation, but phase preference is fundamentally determined by relative solubility differences.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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DNA & RNA isolation

In alcohol precipitation, DNA becomes less soluble because it is:

In alcoholic precipitation, DNA hydration is critical for solubility. Water molecules form hydration shell around negatively charged phosphates, keeping DNA hydrophilic and soluble. Addition of ethanol lowers dielectric constant of medium, reducing water activity and allowing monovalent cations to neutralize phosphates. This strips hydration shell, exposes hydrophobic nitrogenous bases, and makes DNA more hydrophobic and less soluble in polar solvent. Aggregation and precipitation follow. DNA does not become more hydrophilic, denatured at room temperature precipitation, or ionized further; hydrophobicity increase drives insolubility and aggregation in alcohol.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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DNA & RNA isolation

Which is *not* used in RNA isolation?

RNA isolation requires potent RNase inhibitors and denaturants such as GITC which chaotropically inactivates RNases, and phenol-chloroform which partitions RNA into aqueous phase while denaturing proteins. Chloroform aids phase separation and removes lipids. SDS-PAGE, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, is an analytical method for separating proteins by size based on SDS-denatured migration, not an extraction method for RNA. It is used post-extraction for protein analysis and would degrade RNA preparations. Thus SDS-PAGE has no role in RNA isolation workflow and is unrelated.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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DNA & RNA isolation

Which homogenization tool forces cells through a narrow space?

Dounce homogenizer is a manual tissue grinder consisting of a glass tube and tightly fitting pestle with narrow clearance. Cells suspended in buffer are forced through this narrow annular space, generating mechanical shear and pressure differentials that rupture plasma membranes while preserving organelles if clearance is controlled. This contrasts with bead beater which uses bead collision, sonicator which uses acoustic cavitation, and freeze-thaw which uses ice crystal damage. Dounce is gentle, ideal for preparing nuclei and subcellular fractions without vigorous denaturation of sensitive complexes.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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DNA & RNA isolation

Which technique separates DNA based on conformation and density?

CsCl density gradient centrifugation uniquely separates DNA species based on conformation and buoyant density in an isopycnic gradient. Supercoiled, nicked, and linear DNA differ in capacity to intercalate EtBr, resulting in distinct buoyant densities and band positions. Silica columns and magnetic beads purify by adsorption based on chemical interaction with matrix, not conformation. GITC lysis denatures proteins for extraction but does not resolve topology. Therefore only CsCl centrifugation discriminates DNA forms by supercoiling, linking conformational state to measurable density differences for high-resolution purification of plasmids.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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DNA & RNA isolation

What does an A260/A280 ratio

Spectrophotometric purity ratio A260/A280 assesses protein contamination in nucleic acid preparations. Nucleic acids absorb maximally at 260 nm, proteins due to aromatic residues absorb at 280 nm. Pure DNA shows ratio about 1.8, pure RNA about 2.0. If ratio is below 1.8 for DNA, it indicates excess absorbance at 280 nm contributed by residual proteins, phenol, or other contaminants absorbing near 280 nm. Ratios below threshold suggest inadequate deproteinization by phenol or protease. Pure RNA would show higher ratio, while pure DNA falls below that threshold.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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DNA & RNA isolation

Which of the following shows the correct elution condition in mRNA purification?

Affinity purification of mRNA using oligo dT relies on reversible hybridization. Binding of poly-A tail to oligo dT is stabilized by high ionic strength, which shields repulsion between negatively charged phosphates and favors base pairing. Elution requires conditions that destabilize hydrogen bonds between A and T. Low salt concentration reduces shielding, increases phosphate repulsion, and weakens duplex stability, causing mRNA to detach and elute. High salt promotes binding, low pH damages RNA, and high temperature is unnecessary and risky for RNA; low ionic strength is optimal for gentle elution.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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DNA & RNA isolation

The function of TRIS in TE buffer is to:

TE buffer, composed of Tris and EDTA, is standard for DNA storage. Tris (tris-hydroxymethyl-aminomethane) is a buffering agent with pKa 8.1, effective around pH 7.5-8.5. Its primary function is to maintain stable alkaline pH, typically 8.0, which minimizes acid-catalyzed depurination and keeps DNA deprotonated and soluble. Alkaline pH also inhibits acid-dependent nucleases and preserves phosphodiester backbone integrity. EDTA chelates divalent ions, while Tris does not precipitate DNA or chelate. Thus Tris provides pH homeostasis essential for long-term DNA integrity and enzymatic compatibility.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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DNA & RNA isolation

Oligo dT column binds to:

Oligo dT chromatography exploits the polyadenylation of eukaryotic mRNA. Mature mRNAs possess 3' poly-A tail of 150-250 adenine residues added post-transcriptionally, crucial for stability and translation. Oligo dT column contains short oligomers of deoxythymidine immobilized on resin. Under high salt conditions, adenines in poly-A tail form stable hydrogen bonds with thymines via A-T base pairing, retaining eukaryotic mRNA specifically. rRNA, tRNA, and non-polyadenylated RNAs lack poly-A tails and flow through. Elution at low salt disrupts hydrogen bonding and releases purified mRNA intact.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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