Our bodies are built from intricate instructions. This biological instruction manual is written in our DNA, a remarkable molecule that holds the genetic code. Deciphering this code requires understanding specific terms, and “exonic” is one such term that helps understand how our genetic information is organized and utilized. It points to a significant part of our genetic makeup, guiding the construction of the proteins that perform many tasks within us.
The Role of Exons in a Gene
Within genes, specific segments hold direct instructions for building proteins. These segments are known as exons. Think of a gene as a comprehensive recipe, and the exons as the precise, ordered list of ingredients. They are the actual coding information the cell uses.
Interspersed between these exonic segments are other sections called introns. Introns do not carry the direct instructions for protein synthesis; they are more like a chef’s notes not part of the final ingredient list. While introns are part of the initial gene sequence, they are removed before the protein-making process begins, ensuring only the exonic instructions are followed. This structural organization of genes, with exons separated by introns, is a common feature across many organisms.
From Gene to Protein
The journey from a gene to a functional protein begins with transcription, which occurs within the cell’s nucleus. During transcription, the entire gene, including both its exons and introns, is copied into a temporary RNA molecule known as pre-messenger RNA (pre-mRNA).
Following transcription, RNA splicing takes place. During splicing, specialized cellular machinery precisely identifies and removes the non-coding intron sequences from the pre-mRNA. The remaining exonic segments are then joined in a specific order, forming a continuous, mature messenger RNA (mRNA) molecule. This mature mRNA, now containing only the protein-coding information derived from the exons, then travels out of the nucleus into the cytoplasm, where its instructions are translated into a protein.
Exonic Mutations and Genetic Conditions
An “exonic mutation” refers to a permanent change within the DNA sequence of an exon. Because exons contain the direct instructions for building proteins, these mutations are often impactful. Any alteration in an exon can directly change the genetic code that dictates the sequence of amino acids in a protein, potentially leading to a protein that does not function correctly, or sometimes, no protein at all.
Different types of exonic mutations have varying consequences.
Missense Mutation
A “missense” mutation is like a single typo in a recipe, where one ingredient is replaced, potentially leading to an altered or non-functional protein.
Nonsense Mutation
A “nonsense” mutation is like an unexpected “stop” command inserted too early in the recipe, prematurely halting the protein’s assembly and resulting in a truncated, non-functional protein.
Frameshift Mutation
A “frameshift” mutation, caused by the insertion or deletion of one or two DNA bases, is like deleting a letter in the middle of a sentence, scrambling subsequent words and leading to an altered or non-functional protein.
These changes in protein structure or production are often the underlying cause of various genetic conditions, from mild traits to severe diseases.
Understanding the Exome
The “exome” is defined as the complete collection of all exons within an organism’s genome. While the exome constitutes a relatively small fraction, typically 1% to 2%, of the entire human genome, it holds significance. This is because the vast majority of known disease-causing mutations, approximately 85% of them, are located within these protein-coding exonic regions.
Understanding the exome has advanced the diagnosis of genetic disorders through exome sequencing. This genetic test focuses on analyzing the exonic regions of an individual’s DNA. By concentrating on this small yet informative portion of the genome, exome sequencing offers an efficient and cost-effective approach to identify genetic variations responsible for many inherited diseases, especially compared to sequencing the entire genome. This targeted analysis helps clinicians pinpoint genetic changes contributing to a patient’s condition.