Exons are specific segments of DNA within a gene that carry the direct instructions for constructing proteins. These “coding” regions contain the blueprint cellular machinery reads to assemble the molecular components that perform most of the cell’s functions. Exons provide the precise genetic information necessary for protein synthesis and contribute to the final functional product of a gene.
Exons and Gene Structure
A gene consists of a DNA sequence that serves as a template for genetic information. In complex organisms, genes are not continuous coding sequences; instead, they are organized with coding regions interrupted by non-coding segments. These non-coding, intervening sequences are called introns, while exons are the functional segments interspersed between them.
Both exons and introns are initially copied from the DNA template into a precursor RNA molecule during transcription. This initial RNA transcript contains a mixture of both coding and non-coding information. Genes can vary significantly in their exon and intron content; for instance, the human dystrophin gene contains 79 exons and 103 introns.
From Exons to Proteins
The journey from a gene’s DNA to a functional protein begins with transcription, where the DNA sequence is copied into a pre-messenger RNA (pre-mRNA) molecule. This pre-mRNA contains both exon and intron sequences. For the genetic information in the exons to be utilized, the introns must be precisely removed.
This removal process is called splicing. During splicing, non-coding introns are cut out from the pre-mRNA molecule, and coding exons are accurately joined together to form a continuous, mature messenger RNA (mRNA) sequence. The precision of splicing is paramount, as errors can lead to a faulty protein. This final mRNA molecule, composed solely of exon sequences, then travels to ribosomes where its genetic code is translated into a functional protein.
The Significance of Exons
Exons carry the specific instructions that determine the structure and function of proteins. Each set of three nucleotides within an exon sequence, known as a codon, specifies a particular amino acid, dictating the protein’s composition. The sequence of exons ultimately defines the protein’s characteristics and capabilities.
Exons also contribute to protein diversity through alternative splicing. This mechanism allows different combinations of exons from a single gene to be selectively included or excluded in the final mRNA molecule. As a result, a single gene can produce multiple distinct protein variants, each with potentially different structures and functions. This process significantly increases the diversity of proteins an organism can create from a limited number of genes, contributing to biological complexity and adaptability.
When Exons Malfunction
Errors involving exons or their processing can have considerable biological consequences. Mutations within an exon’s sequence can alter the genetic code, potentially leading to the production of non-functional, truncated, or improperly functioning proteins. Such changes can disrupt the protein’s ability to perform its designated role.
Mistakes during the splicing process, where introns are removed and exons are joined, also pose a risk. These errors can include exons being skipped entirely or extra segments being included in the mature mRNA. These splicing aberrations can result in protein abnormalities that disrupt cellular processes and are associated with various biological issues.