Correct options are (B) and (C).
Options (A) and (D) are incorrect in the context of SDS-PAGE.

Correct answer and explanation

• Correct statements: (B) β-mercaptoethanol is used to reduce disulphide bonds; (C) The protein migrates towards the anode.

• Incorrect statements: (A) Ethidium bromide is used to track the progress; (D) Lower molecular weight proteins migrate slower.

Explanation of each option

Option (A): Ethidium bromide as tracking dye

• Ethidium bromide is a fluorescent intercalating dye used mainly to visualize nucleic acids (DNA/RNA) in agarose gels, not to track proteins in SDS-PAGE.

• In SDS-PAGE, a small anionic dye such as bromophenol blue is used in the sample/loading buffer to monitor the electrophoresis front and track progress of the run.

• Therefore, statement (A) is false.

Option (B): β-mercaptoethanol and disulphide bonds

• SDS-PAGE is usually performed under denaturing and often reducing conditions to ensure proteins are fully unfolded and subunits are separated.

• β-mercaptoethanol (β-ME) or DTT is added as a reducing agent in the sample buffer to cleave disulfide (S–S) bonds between cysteine residues, breaking tertiary/quaternary structure and separating subunits.

• This makes all polypeptide chains linear so that separation reflects polypeptide length (molecular weight), not higher-order structure.

• Therefore, statement (B) is true.

Option (C): Direction of protein migration

• SDS, an anionic detergent, coats polypeptides with a large excess of negative charge approximately proportional to their length.

• This overwhelms the native charge of the protein and gives all proteins a similar charge-to-mass ratio and a net negative charge.

• In an electric field, negatively charged SDS–protein complexes migrate towards the positive electrode (anode) through the polyacrylamide gel.

• Therefore, statement (C) is true.

Option (D): Mobility of low vs high molecular weight proteins

• The polyacrylamide gel acts as a molecular sieve:

  • Smaller (lower molecular weight) proteins experience less friction and move more easily through the pores, so they migrate faster and farther.

  • Larger (higher molecular weight) proteins are retarded more strongly and migrate slower and a shorter distance.

• Hence, lower molecular weight proteins do not migrate slower; they migrate faster than larger proteins.

• Therefore, statement (D) is false.

SDS-PAGE concept overview

• SDS-PAGE (Sodium Dodecyl Sulphate–Polyacrylamide Gel Electrophoresis) is a denaturing electrophoretic technique for separating proteins almost exclusively on the basis of molecular weight.

• Key components:

  • SDS: denatures proteins and confers uniform negative charge.

  • Reducing agent (β-mercaptoethanol/DTT): breaks disulfide bonds.

  • Polyacrylamide gel: provides a sieving matrix; pore size depends on % acrylamide.

• Result: migration distance is inversely related to log(molecular weight); smaller proteins migrate farther toward the anode.

SEO-optimized introduction

SDS-PAGE principle and protein migration are central concepts for analyzing proteins based on molecular weight in biochemistry and molecular biology. This technique uses SDS, a strong anionic detergent, and polyacrylamide gel to standardize protein charge and shape so that separation depends mainly on the size of denatured polypeptides. By combining SDS with β-mercaptoethanol and an electric field, SDS-PAGE provides high-resolution separation of protein subunits and is essential for Western blotting, protein purity assessment, and molecular weight estimation.