In eukaryotic organisms, such as animals and plants, the DNA contains segments known as exons and introns. Exons are the coding regions that provide instructions for protein synthesis, while introns are non-coding regions that do not contribute to the final protein product. During the transcription process, both exons and introns are transcribed into a precursor messenger RNA (pre-mRNA), also referred to as heterogeneous nuclear RNA (hnRNA).
The next critical step is the processing of pre-mRNA, which involves the action of spliceosomes. These molecular machines are responsible for removing introns from the pre-mRNA and splicing the exons together. A helpful mnemonic to remember this is that exons are "expressed," meaning they are the sequences that will ultimately be translated into proteins.
To visualize this process, consider a gene that contains both exons and introns. Initially, during transcription, the entire sequence is copied into pre-mRNA, which still includes both types of segments. As processing occurs, spliceosomes cut out the introns and join the exons, resulting in a mature mRNA molecule that consists solely of exons.
Once mature mRNA is formed, it exits the nucleus and travels to the ribosomes in the cytoplasm, where it serves as a template for protein synthesis. This processing step is essential, as it ensures that only the necessary coding sequences are retained for the accurate production of proteins, highlighting the importance of exons in the overall gene expression process.
