Correct option is A
The correct option is (a)
EXPLANATION
Proteins linked by non-covalent interactions can sometimes form stable complexes that resist dissociation during the first SDS-PAGE, especially if the conditions are mild or partial.
DTT can help break certain interactions or facilitate complete denaturation in the second dimension.
Hence, the observation fits a complex stabilized by non-covalent interactions that remains intact during the first SDS-PAGE but dissociates into components in the second SDS-PAGE with reducing conditions.
Incorrect options-
Option (b) : Covalent bonds here mainly refer to disulfide bonds between protein subunits. If proteins were linked by disulfide bonds, SDS alone wouldn’t break them, so the complex would remain as one band in both dimensions until DTT was added. But the separation seen here indicates more complex dynamics.
Option (c) : They would separate into two bands even in the first dimension.
Option (d) : A single protein with disulfide bonds wouldn’t separate into two distinct spots in the second dimension.
