11 public questions tagged with this topic.
Elution is the process of extracting DNA fragments from an agarose gel for further use in recombinant DNA experiments.
Ref: NCERT Class 12 Biology Chapter 9: Biotechnology: Principles and Processes Processes - Isolation of DNA and Gel Electrophoresis
Lysozyme breaks down the peptidoglycan cell wall of bacteria to release DNA for isolation.
Ref: NCERT Class 12 Biology Chapter 9: Biotechnology: Principles and Processes Processes - Isolation of DNA and Gel Electrophoresis
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.
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.
CTAB (cetyltrimethylammonium bromide) is a cationic detergent preferentially used for plant DNA extraction. Plant cells contain large amounts of polysaccharides, polyphenols, and secondary metabolites that co-precipitate with DNA and inhibit downstream PCR. CTAB forms insoluble complexes with nucleic acids at low salt, but at high salt concentrations above 0.7 M NaCl, it forms complexes with acidic polysaccharides and proteins while DNA remains soluble, allowing their removal. Subsequent dilution precipitates CTAB-DNA complex. It is less effective for animal tissues or RNA purification.
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.
Alcohol precipitation of DNA exploits solubility differences modulated by dielectric constant. DNA is soluble in water due to hydration of its negatively charged phosphate backbone. Addition of monovalent salt like sodium acetate neutralizes charges, while ethanol or isopropanol, having much lower dielectric constant than water, reduces the shielding of electrostatic interactions, allowing Na+ to bind phosphates. This decreases hydrophilicity and causes DNA to fall out of solution and precipitate. Charge repulsion is overcome, not increased, and pH shift or denaturation is not the primary mechanism for precipitation.
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.
Proteinase K is a broad-spectrum serine protease widely used during DNA extraction to digest proteins, including nucleases, histones, and cellular structural proteins. It exhibits high activity in presence of SDS and EDTA and at elevated temperatures around 50-65°C, conditions that simultaneously lyse cells and denature substrates. By hydrolyzing peptide bonds, it deproteinizes DNA, inactivates DNases and RNases, and improves yield and purity. RNase specifically degrades RNA, DNase degrades DNA, and Triton X-100 is a non-ionic detergent for membrane permeabilization, not for proteolysis.
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.
Standard phenol-chloroform DNA extraction involves phenol for protein denaturation, chloroform for phase separation and lipid removal, and isoamyl alcohol, added in a 25:24:1 ratio, as an antifoaming agent that stabilizes interphase and prevents bubbling. This mixture efficiently deproteinizes the lysate. SDS (sodium dodecyl sulfate), an anionic detergent used for membrane lysis and protein solubilization, is part of the initial lysis buffer but is not a component of the organic extraction mixture itself. It is removed prior to or during phenol treatment and precipitation steps.
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.
Phenol-chloroform extraction separates nucleic acids based on differential solubility. Phenol denatures proteins and chloroform improves phase separation and removes lipids. When mixed with aqueous lysate and centrifuged, two phases form: lower organic phase containing phenol-chloroform and hydrophobic molecules, and upper aqueous phase containing hydrophilic nucleic acids. DNA, being highly polar due to its phosphate backbone, remains soluble in the aqueous phase, while denatured proteins partition into organic phase or precipitate at interphase. Thus pure DNA is recovered from the aqueous layer for subsequent precipitation.
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.
EDTA (ethylenediaminetetraacetic acid) is a chelating agent essential in DNA isolation buffers. It binds divalent cations like Mg2+ and Ca2+ with high affinity, forming stable coordination complexes. Mg2+ is a crucial cofactor for DNases that degrade DNA and for nucleases that destabilize membranes. By sequestering Mg2+, EDTA irreversibly inhibits endogenous nuclease activity, protects genomic DNA from enzymatic degradation, and helps destabilize cell walls by removing membrane-stabilizing cations. It does not denature proteins directly, bind nucleic acids specifically, or degrade RNA; its function is strictly chelation-mediated protection and enzymatic inhibition.
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.
Ultrasonication is a mechanical lysis method that employs high-frequency sound waves, typically above 20 kHz, to disrupt cells. Acoustic cavitation generates localized microbubbles in the suspension that implode violently, producing intense shear forces, shock waves, and transient high pressure that rupture rigid cell walls and lipid membranes. This efficiently releases intracellular nucleic acids into buffer. In contrast, freeze-thaw relies on intracellular ice crystal formation, bead milling uses physical collision with glass beads, and liquid homogenization uses pressure-driven shear through a narrow gap, none involving ultrasonic cavitation.
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.
