Final answer:
Exon 18b-carrying mRNA encodes for a part of a polypeptide chain, based on the exons that remain after RNA splicing during the gene expression process.
Step-by-step explanation:
Exon 18b-carrying mRNA encodes a specific segment within a protein, based on the genetic information that is transcribed from DNA into mRNA during gene expression. Once the mRNA is transcribed, it undergoes processing that includes splicing, where non-coding regions called introns are removed. The remaining coding sequences, known as exons, are spliced together to form the final mRNA molecule that is used for protein synthesis at the ribosome. Thus, exon 18b refers to a protein-coding sequence of a eukaryotic gene that, once spliced into the mature mRNA, will dictate the inclusion of a specific amino acid sequence in the synthesized polypeptide based on the genetic code.