Final answer:
The statement in the question is false because the 5' cap is a 7-methylguanosine, not thymine, and it is added to nascent pre-mRNA, not a completed peptide. This 5' cap plays crucial roles in mRNA protection and translation initiation. The correct option is B.
Step-by-step explanation:
The statement that a 5′-cap describes the addition of a base, usually thymine, to the 5′ end of a completed peptide is false.
Instead, the 5' capping of mRNA involves the enzymatic addition of a modified guanine nucleotide to the 5' end of the nascent pre-mRNA. This modification is a 7-methylguanosine cap that is added via a unique 5'-to-5' triphosphate bridge.
During the process of transcription, as the pre-mRNA is synthesized, this cap is added to protect the mRNA from degradation. This 5' cap also facilitates the egress of mRNA from the nucleus and plays an important role in initiation of translation by enabling the mRNA to be recognized by the ribosome.
Thymine is not involved in this process as it is not a component of the 5' cap, and the 5' cap is not related to peptides, but to mRNA. Notably, in RNA the base thymine (T) is replaced by uracil (U). Option B. is the correct one.