28 public questions tagged with this topic.
Smaller DNA fragments migrate faster through the gel than larger fragments due to lower resistance.
Ref: NCERT Class 12 Biology Chapter 9: Biotechnology: Principles and Processes Processes - Isolation of DNA and Gel Electrophoresis
DNA is negatively charged and moves toward the positive electrode (anode) in gel electrophoresis, not the negative electrode.
Ref: NCERT Class 12 Biology Chapter 9: Biotechnology: Principles and Processes Processes - Isolation of DNA and Gel Electrophoresis
DNA is negatively charged and moves toward the positive electrode (anode) in gel electrophoresis.
Ref: NCERT Class 12 Biology Chapter 9: Biotechnology: Principles and Processes Processes - Isolation of DNA and Gel Electrophoresis
Electrophoresis separates charged macromolecules under influence of uniform electric field applied across a conductive buffer system. Molecules placed in buffer between electrodes experience electrostatic force proportional to net charge, causing directed migration toward oppositely charged electrode with velocity determined by charge-to-mass ratio. Migration rate is further modulated by molecular size, shape, and frictional interaction with support matrix providing sieving. This electrokinetic phenomenon underlies all variants including agarose, polyacrylamide, capillary, and isoelectric focusing. It does not rely on precipitation, magnetic fields, or simple diffusion, but specifically on electric field-based directed movement of polyelectrolytes.
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.
The running buffer in agarose electrophoresis, typically TAE or TBE, fulfills multiple physicochemical roles. It provides conductivity for current flow, maintains pH around 8.0 to 8.5 to ensure DNA remains fully deprotonated and negatively charged, and supplies counterions that establish electric field uniformity. Tris buffers protons, while EDTA chelates divalent cations, inhibiting nucleases. Maintaining constant pH prevents alterations in charge or band distortion. It does not increase sample volume, fix DNA, or stain proteins. Thus pH and ionic strength maintenance are its principal functions.
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.
Electroendosmosis is bulk fluid flow toward cathode opposite to DNA migration, caused by fixed charges on support matrix. Agarose contains residual sulfate and pyruvate groups introduced during purification from agar, imparting negative charge to gel fibers. Under electric field, counterions in diffuse layer move toward cathode, dragging water with them. This electroosmotic flow retards anionic DNA and causes band broadening. Highly purified low-EEO agarose minimizes this effect. Thus phenomenon originates not from pH or temperature alone, but from intrinsic charged substituents within agarose polymer.
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.
Loading dyes require a dense component to ensure samples settle at the bottom of wells immersed in running buffer. Glycerol at 5 to 10 percent (v/v) increases specific gravity and viscosity without adding charge, so it does not alter electric field migration. It is chemically inert, transparent, and inexpensive. Sucrose and Ficoll perform similar functions as alternatives. Glycerol does not stain DNA, nor affect conductivity or voltage significantly; its role is strictly physical, preventing sample dispersal and ensuring sharp, well-defined entry into the gel matrix during loading.
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.
Single-stranded nucleic acids form intramolecular secondary structures via base pairing that alter electrophoretic mobility independent of length. Denaturing agents prevent renaturation. Urea at 6 to 8 M concentration disrupts hydrogen bonding and base stacking, maintaining RNA and denatured DNA as extended random coils. It is compatible with polyacrylamide electrophoresis for sequencing gels. TEMED catalyzes polymerization, sucrose increases density, and bromocresol green is a tracking dye. Formaldehyde and formamide serve similar purposes, but urea is the standard denaturant for ssDNA and RNA gels.
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.
Ethidium bromide is a cationic phenanthridinium dye that intercalates between stacked base pairs of double-stranded DNA, with approximately one dye per 2.5 base pairs. Intercalation extends the helix and, upon UV excitation at 300 to 360 nm, emits orange fluorescence at 590 nm, enabling visualization of DNA bands within agarose or polyacrylamide gels. It does not participate in gel polymerization, does not denature DNA, nor catalyze reactions. While mutagenic and now often replaced by safer dyes, its mechanism remains classic DNA staining by intercalation.
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.
Tris-Borate-EDTA possesses greater buffering capacity than Tris-Acetate-EDTA due to borate's multivalent buffering action and lower conductivity. During electrophoresis of small DNA fragments, which require longer runs or higher voltages, TAE exhausts quickly, leading to pH rise, buffer depletion, and band broadening. TBE maintains stable pH and ionic environment, reducing heating and providing sharper resolution for fragments below 1 kb. Although TAE allows easier DNA recovery and is preferred for large fragments, TBE's superior capacity favors small fragment separation in high-resolution applications.
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.
Tracking dyes are anionic dyes co-migrating with nucleic acids to visually monitor electrophoretic progress without staining DNA. Xylene cyanol FF migrates at approximately 4 kb in 1 percent agarose, while bromophenol blue migrates near 300 bp, allowing estimation for different fragment ranges. Xylene cyanol is specifically preferred for larger fragments because its slower mobility prevents premature exit. Ethidium bromide is an intercalating stain, not a front marker. TEMED is a polymerization catalyst, and agarose is the support matrix. Hence xylene cyanol serves as a reliable migration indicator.
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.
Agarose forms a porous matrix with large, tunable pore sizes determined by concentration, typically 0.7 to 2 percent. This range allows efficient sieving of DNA fragments from approximately 50 bp to over 50,000 bp, covering routine plasmids, restriction digests, and genomic DNA preparations. Polyacrylamide offers much smaller pores, ideal for 5 to 500 bp resolution at single-basepair discrimination. Cellulose acetate and thin-layer matrices lack molecular sieving for nucleic acids. Agarose provides low electroendosmosis, low UV absorption, and non-denaturing separation, making it the standard choice for broad-range DNA electrophoresis.
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.
