Final answer:
Transfer RNA (tRNA) is responsible for recognizing codons and delivering the appropriate amino acid during protein synthesis.
Step-by-step explanation:
Transfer RNA (tRNA) plays a crucial role in protein synthesis by transferring the appropriate amino acid to the growing polypeptide chain. tRNA molecules have between 73 and 93 nucleotides in their structures, with eight or more nucleotides being unusual modified bases. tRNAs have four arms that contribute to their cloverleaf shape, including the aminoacyl arm, anticodon arm, dihydrouridine arm, and ribothymidine-pseudouridine arm.
Based on the information provided, it is implied that semi-invariant nucleotides (nt) would refer to the nucleotides that do not change within the tRNA molecule, while invariant nt would refer to nucleotides that vary and can recognize different codons. Therefore, the answer to your question is Transfer RNA (tRNA) as it is responsible for recognizing codons and delivering the appropriate amino acid during protein synthesis.