16 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
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 l
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 proteins exhibit variable net charge depending on relationship between buffer pH and isoelectric point, affecting electrophoretic direction. Sodium dodecyl sulfate is strong anionic detergent possessing sulfate headgroup with persistent negative charge that binds along unfolded polypeptide chain at high density, approximately one molecule per two residues. Excess negative charges overwhelm intrinsic acidic and basic side chains, conferring uniform anionic character to all protein-SDS complexes proportional to length. Resulting micelles migrate toward positively charged anode during elec
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 fa
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 proteins maintain folding through hydrogen bonds, ionic and hydrophobic interactions that define secondary and tertiary structure crucial for biological activity. Binding of anionic SDS along with heating with reducing agents disrupts all non-covalent forces, abolishing alpha-helices, beta-sheets and compact domains. Polypeptides convert to extended rod-like structures surrounded by negatively charged detergent micelles with random coil characteristics. Biological function and folded architecture are lost, allowing all proteins to adopt similar shape and charge density. There is no incr
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 denatures proteins and coats unfolded chains with uniform negative charge essential for size-based separation. Quantitative binding studies demonstrate approximately 1.4 grams SDS bound per gram of protein, corresponding to one SDS molecule per two amino acid residues. This constant charge-to-mass ratio cancels intrinsic charge contributions, ensuring migration proportional to chain length. Ratios such as one-to-one, two-to-one, or 1.4-to-one per amino acid misrepresent established stoichiometry. Understanding this binding ratio explains why SDS-PAGE separates primarily
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.
Coomassie Brilliant Blue G-250 and R-250 are triphenylmethane dyes that bind non-covalently to proteins through combined electrostatic attraction to protonated amine groups of lysine, arginine and histidine and hydrophobic interactions with aromatic residues phenylalanine, tyrosine and tryptophan. Binding shifts absorption maximum from 465 to 595 nanometers, producing intense blue color proportional to protein amount, forming basis for Bradford assay. It does not efficiently intercalate DNA base pairs, bind RNA or stain polysaccharides, unlike ethidium bromide, SYBR Green or periodic acid-Schi
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.
Sensitivity differences among protein dyes determine detection limits in gels. Coomassie Brilliant Blue R-250 detects roughly 50 to 100 nanograms per band by binding electrostatically to basic and aromatic residues, suitable for routine quantification. Silver staining involves impregnation of gel with silver ions that bind to carboxyl, sulfhydryl and amino side chains, followed by reduction to metallic silver by formaldehyde, achieving 50 to 100-fold greater sensitivity down to 1 to 5 nanograms. Bromophenol blue merely tracks migration front, while ethidium bromide and SYBR Green intercalate n
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.
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 beg
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.
Polyacrylamide gel formation requires initiation of vinyl polymerization. Ammonium persulfate provides free radicals by decomposing into sulfate radicals that attack the double bonds of acrylamide and bisacrylamide, starting chain growth and crosslinking into a sieving matrix. Tetramethylethylenediamine accelerates this process by catalyzing radical formation and stabilizing intermediates, but it does not generate radicals independently, thus acting as catalyst rather than initiator. Glycine serves as trailing ion in discontinuous buffer system, and Tris-HCl buffers pH. APS is therefore recogn
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.
Beta-mercaptoethanol is a thiol-based reducing agent present in Laemmli sample buffer to cleave both interchain and intrachain disulfide bridges by reducing cystine to cysteine sulfhydryl groups. Disulfide bonds maintain tertiary and quaternary structure, preventing complete unfolding even after SDS binding and heating. Their reduction ensures proteins become fully linearized extended polypeptides uniformly coated with SDS, allowing accurate size estimation and preventing aggregation. It does not initiate acrylamide polymerization, impart charge to proteins, or enhance Coomassie staining, whic
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.
