A gene is a fundamental unit of heredity, a segment of DNA that carries instructions for building and maintaining an organism. Traditionally, each gene was thought to dictate the creation of a single type of protein. However, biological reality reveals an efficiency in how genetic information is utilized. A single gene often has the capacity to produce multiple distinct polypeptides, which are amino acid chains that fold into functional proteins. This process allows for an expansion of the protein repertoire from a limited number of genes.
From Gene to Protein: The Basic Process
The journey from a gene to a functional protein begins with transcription, where the DNA sequence of a gene is copied into messenger RNA (mRNA). This mRNA molecule carries the genetic blueprint out of the cell’s nucleus into the cytoplasm. In the cytoplasm, mRNA encounters ribosomes, cellular machines responsible for synthesizing proteins.
This second step is called translation, during which the ribosome “reads” the mRNA sequence in sets of three nucleotides, known as codons. Each codon specifies a particular amino acid. As the ribosome moves along the mRNA, it links these amino acids together, forming a polypeptide chain. This polypeptide then folds into a three-dimensional structure to become a functional protein.
Alternative Splicing: Creating Diversity from a Single Gene
Within a gene’s DNA sequence, exons contain coding information for the protein, while introns are intervening sequences that do not directly code for it. After a gene is transcribed into a precursor mRNA, introns are removed, and exons are joined together in a process known as RNA splicing. This splicing typically occurs before the mRNA leaves the nucleus.
Alternative splicing is a mechanism where different combinations of exons from the same gene are included or excluded from the final mature mRNA. Depending on which exons are selected, the resulting mRNA will have a unique sequence.
Each distinct mRNA variant, produced through alternative splicing, serves as a template for translation. Different combinations of exons lead to polypeptides with varying lengths, amino acid sequences, and distinct functions. This process expands the diversity of proteins an organism can produce from its fixed number of genes, contributing to biological complexity.
Varying the Transcript: Promoters and Polyadenylation
Beyond alternative splicing, a single gene can generate multiple polypeptides by varying the start and end points of its mRNA transcript. Genes often have multiple promoters, DNA sequences that signal where transcription should begin. The cell can activate different promoters within the same gene, leading to transcription initiation at various points along the gene’s length.
When transcription starts from a different promoter, the resulting mRNA will have a distinct 5′ (start) end. This alteration can change the initial part of the polypeptide, potentially leading to a protein with a different N-terminal sequence or altering which start codon is recognized. Such variations can result in polypeptides that differ in cellular localization, stability, or enzymatic activity.
Similarly, genes can contain multiple polyadenylation sites, signals for where the mRNA transcript should be cleaved and a poly-A tail added. Choosing different polyadenylation sites alters the 3′ (end) of the mRNA. While primarily influencing mRNA stability and transport, if a polyadenylation site is located within a coding region, its selection can lead to a truncated polypeptide. This mechanism, alongside alternative promoter usage, adds another layer to the protein products derived from a single gene.
Beyond the Transcript: RNA Editing and Translational Regulation
Even after an mRNA has been fully processed, further modifications can occur that alter the resulting polypeptide. RNA editing is a process where enzymes modify individual nucleotides within an mRNA sequence, changing the code the ribosome reads. This alteration means the mRNA sequence differs from the DNA, leading to the incorporation of a different amino acid during translation and creating a distinct polypeptide.
In addition to changes in the mRNA itself, translation can also be regulated to produce multiple polypeptides from a single mRNA. One mechanism involves alternative start codons, where the ribosome might initiate protein synthesis at different methionine codons within the same mRNA. This can result in polypeptides of varying lengths, each with a unique N-terminal sequence.
Another mechanism is ribosomal frameshifting, where the ribosome “slips” or shifts its reading frame during translation. Instead of reading codons in consecutive sets of three, it might move forward or backward by one or two nucleotides. This shift causes subsequent codons to be read differently, leading to an altered sequence of amino acids from that point onward, producing a distinct polypeptide from the original mRNA template.