Final answer:
During the elongation phase of protein synthesis, the anticodon of aminoacyl-tRNA is complementary to the mRNA codon at the ribosome's A site, which dictates the addition of the correct amino acid to the polypeptide chain.
Step-by-step explanation:
In step 4 of elongation during the process of protein synthesis, the aminoacyl-tRNA carries a specific amino acid and has an anticodon that is complementary to the mRNA's codon present at the ribosome's A site. The anticodon-codon interaction ensures that the correct amino acid is added to the growing polypeptide chain. This specific base-pairing follows the rules of complementary base-pairing where adenine (A) pairs with uracil (U), and cytosine (C) pairs with guanine (G). For example, if the mRNA codon is GCA, which specifies the amino acid alanine, the complementary anticodon on the tRNA that carries alanine would be UGC. This precision allows the ribosome to synthesize a polypeptide sequence accurate to the genetic code carried by the mRNA.