What is the definition of an exon in the context of mRNA?

An exon is defined as a segment of a gene that is retained in the final, mature messenger RNA (mRNA) transcript after the process of splicing. During the transcription of a gene, both exons and introns (non-coding regions) are included in the initial RNA transcript. However, introns are removed during processing, while exons are linked together to form the continuous coding sequence that will be translated into a protein. Therefore, the role of exons is critical as they contribute to the functional parts of mRNA that ultimately dictate the amino acid sequence of proteins. This makes them essential for gene expression and the production of proteins.

In contrast, non-coding DNA sequences, sequences that contribute to ribosomal RNA (rRNA) synthesis, and promoter regions serve different functions in molecular biology and do not represent the coding segments that are part of the processed mRNA.