M. Klemke et al. (2001) discovered an interesting coding phenomenon in which an exon within a neurologic hormone receptor gene in mammals appears to produce two different protein entities ( and ALEX). Following is the DNA sequence of the exon's  end derived from a rat.

  5'-gtcccaaccatgcccaccgatcttccgcctgcttctgaagATGCGGGCCCAG

The lowercase letters represent the initial coding portion for the  protein, and the uppercase letters indicate the portion where the ALEX entity is initiated. (For simplicity, and to correspond with the RNA coding dictionary, it is customary to represent the coding (non-template) strand of the DNA segment.)

Locate the initiator codon within the XLαs segment.

Key Concepts

Exons and Introns

Exons are the coding regions of a gene that are expressed and translated into proteins, while introns are non-coding regions that are spliced out during RNA processing. Understanding the distinction between exons and introns is crucial for analyzing gene structure and function, particularly in the context of alternative splicing, where different combinations of exons can lead to the production of multiple protein variants from a single gene.

Initiator Codon

The initiator codon, typically AUG, is the specific sequence in mRNA that signals the start of translation, marking where ribosomes begin synthesizing proteins. Identifying the initiator codon is essential for understanding how proteins are produced from genetic information, as it determines the reading frame and the subsequent amino acid sequence of the protein.

Alternative Splicing

Alternative splicing is a regulatory mechanism by which different combinations of exons are joined together to produce multiple mRNA variants from a single gene. This process allows for the generation of diverse protein products, such as the two entities mentioned in the question, and plays a critical role in increasing the functional complexity of the genome without the need for additional genes.