20 public questions tagged with this topic.
SDS-PAGE (Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis) is used to separate proteins based on their molecular weight.
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.
Native PAGE omits SDS and reducing agents, so proteins retain native conformation, oligomeric association, and intrinsic charge determined by amino acid composition and pH relative to isoelectric point. Without uniform charge masking, mobility depends on both charge density and hydrodynamic size and shape, as described by Ferguson plots. This differs from SDS-PAGE where SDS confers uniform negative charge per unit mass, separating primarily by size. Native PAGE therefore resolves based on combined size and charge, preserving enzymatic activity and protein-protein interactions.
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.
Isoelectric focusing separates macromolecules based on differences in isoelectric point rather than solely molecular weight or total charge at fixed pH. A stable pH gradient is created using carrier ampholytes or immobilines immobilized in gel matrix. Proteins migrate under electric field until reaching zone where local pH equals pI, resulting in zero net charge and cessation of movement. This focusing concentrates proteins into extremely narrow bands at characteristic pH values. Distinction from SDS-PAGE which separates by size, or ion-exchange which separates by constant charge, underlies its ability to resolve isoforms differing by single charged modification.
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.
Proteins encoded by single gene often exist as multiple isoforms created by alternative splicing, phosphorylation, acetylation, deamidation or limited proteolysis, producing subtle variations in charge and mass indistinguishable in one-dimensional systems. SDS-PAGE masks charge differences and poorly resolves small mass changes, while isoelectric focusing alone cannot separate mass variants. Two-dimensional electrophoresis combining isoelectric focusing for pI separation and SDS-PAGE for molecular mass provides orthogonal resolving power, displaying isoforms as distinct spots across pH and size axes. This reveals charge variants and post-translational modifications co-migrating in single dimension, essential for proteomic isoform analysis.
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.
Under denaturing SDS-PAGE conditions, proteins saturated with SDS possess nearly identical charge-to-mass ratios because anionic detergent binding overwhelms native ionization of acidic and basic residues. Consequently, intrinsic charge, isoelectric point and solubility differences become negligible for electrophoretic velocity. Separation matrix then functions as molecular sieve where larger polypeptide-SDS complexes experience greater frictional retardation and migrate slower than smaller ones, making molecular size and chain length the dominant determinant of relative mobility. Parameters like charge and pI govern separation in native PAGE or isoelectric focusing, not under denaturing SDS conditions with uniform coating.
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.
Accurate molecular weight estimation by SDS-PAGE requires both anionic detergent and reducing thiol for complete linearization. Sodium dodecyl sulfate disrupts non-covalent interactions and imparts uniform negative charge but cannot cleave covalent disulfide linkages between cysteine residues, which constrain polypeptide loops and maintain compact domains. Addition of beta-mercaptoethanol or dithiothreitol reduces disulfides to free sulfhydryls, fully extending chain. Without reducing agent, proteins remain denatured yet partially constrained with intact disulfides, resulting in anomalously fast or slow migration, incomplete unfolding and inaccurate mass estimation and possible multimer persistence.
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.
One-dimensional electrophoresis resolves proteins by single parameter, typically molecular mass in SDS-PAGE, leading to significant spot overlap in whole cell lysates containing thousands of polypeptides with similar sizes. Two-dimensional electrophoresis combines isoelectric focusing followed by SDS-PAGE, separating first by isoelectric point and subsequently by molecular weight, providing orthogonal resolution. This dramatically increases peak capacity and resolving power, revealing post-translational isoforms, modified variants and low-abundance species that co-migrate in 1D systems. Isoelectric focusing, native PAGE, or SDS-PAGE alone provide only one-dimensional information, insufficient for proteome complexity.
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.
Two-dimensional electrophoresis couples orthogonal separation principles to resolve highly complex proteomes containing thousands of polypeptides. First dimension is isoelectric focusing in immobilized pH gradient strips or carrier ampholyte tubes where proteins migrate under electric field until net charge becomes zero at their isoelectric point, effectively focusing at pI independent of size. This separates isoforms differing by charged modifications. Second dimension is SDS-PAGE where focused proteins are further separated by molecular mass. Charge alone or solubility does not define focusing position; isoelectric point focusing is the discriminating parameter for first dimension.
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.
SDS-PAGE employs a discontinuous buffer system with distinct pH zones to achieve stacking and separation. Stacking gel is cast at pH 6.8 in Tris-HCl to keep glycine largely as zwitterion with low electrophoretic mobility, enabling isotachophoretic concentration of proteins between chloride leading ions and glycine trailing ions. Resolving gel is cast at pH 8.8, where glycine acquires full negative charge, moves rapidly ahead, and allows proteins to separate by molecular sieving through smaller pores. This alkaline pH maintains consistent Tris buffering and stable migration. Values like 6.8, 7.5 or 9.2 do not match standard resolving gel composition.
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.
Stacking gel with low acrylamide percentage and pH 6.8 Tris-HCl concentrates dilute protein samples into extremely sharp zones before resolution through isotachophoresis, also called Kohlrausch discontinuity. Chloride from Tris-HCl acts as highly mobile leading ion, while glycine at this pH is predominantly zwitterionic with low mobility as trailing ion. Proteins possess intermediate mobility and become sandwiched between them, stacking into narrow bands of micrometer thickness. Upon entering resolving gel at pH 8.8, glycine becomes fully anionic, overtakes proteins, and sieving separation begins. This ensures superior band sharpness.
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.
Sodium dodecyl sulfate is an anionic detergent that binds cooperatively along the polypeptide backbone at approximately one molecule per two amino acid residues, corresponding to about 1.4 grams SDS per gram protein. This extensive binding imparts a large uniform negative charge that overwhelms intrinsic acidic and basic residues, disrupts non-covalent interactions, and unfolds proteins into rod-like micelles. Electrophoretic mobility then becomes proportional to chain length rather than native charge. Glycerol adds sample density, bromophenol blue tracks the front, and TEMED catalyzes polymerization without contributing charge.
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.
