A codon is a three-nucleotide sequence in messenger RNA (mRNA) that typically specifies an amino acid, the building blocks of proteins. A stop codon, also called a termination or nonsense codon, does not code for an amino acid. Instead, it signals the end of protein synthesis, ensuring the protein chain is completed at the correct length.
Understanding Genetic Instructions
The genetic code dictates how cells translate DNA or RNA information into proteins. This process begins with transcription, where genetic information from DNA is copied into an mRNA molecule. The mRNA then carries this message to the cytoplasm. Here, the mRNA sequence is read in three-nucleotide groups, each forming a codon. Most codons specify an amino acid, which are linked to form a growing protein chain.
How Stop Codons Work
Protein synthesis, or translation, occurs on cellular machinery called ribosomes, which move along the mRNA molecule, reading each codon. As the ribosome reads a codon, a corresponding transfer RNA (tRNA) molecule, carrying a specific amino acid, binds to the mRNA codon. The amino acid is then added to the growing polypeptide chain.
This elongation process continues until the ribosome encounters one of the three specific stop codons: UAA, UAG, or UGA. Unlike other codons, there is no tRNA molecule with a complementary anticodon that can bind to a stop codon. This absence of a matching tRNA signals the ribosome that the protein sequence is complete.
Instead of a tRNA, specialized proteins called release factors bind to the ribosome when a stop codon is encountered. These release factors trigger the release of the newly synthesized polypeptide chain from the ribosome. Following the release of the protein, the ribosome disassembles into its subunits, detaching from the mRNA, which can then be reused or degraded.
The Importance of Precise Termination
The accurate function of stop codons is important for producing proteins of the correct size and amino acid sequence. Precise termination ensures that proteins fold into their proper three-dimensional structures, which is necessary for their specific biological functions. Each protein has a defined length, and its activity depends on this exact structure.
If protein synthesis does not terminate correctly, several issues can arise. Premature termination, where a stop codon appears too early, results in a shortened, non-functional protein. Conversely, if a stop codon is ignored, a phenomenon known as “read-through” occurs, leading to an abnormally elongated protein.
Both truncated and extended proteins can disrupt cellular processes. These faulty proteins may be unstable, misfolded, or unable to perform their intended roles, potentially leading to cellular dysfunction. The precise signaling provided by stop codons is important for maintaining cellular health and proper biological operations.
What Happens When Stop Codons Malfunction
Errors involving stop codons can have consequences for an organism. One common type of error is a nonsense mutation, which occurs when a point mutation changes a regular amino acid-coding codon into a stop codon. This leads to premature termination of protein synthesis, resulting in a truncated protein that is often non-functional.
Such shortened proteins are frequently recognized by cellular quality control mechanisms and degraded, or they may accumulate and interfere with normal cell activities. Nonsense mutations are implicated in a number of human genetic conditions due to the production of these faulty proteins.
Less commonly, errors can cause a ribosome to “read through” a stop codon, incorporating an amino acid where termination should have occurred. This read-through results in an abnormally long protein with an extended C-terminus. These extended proteins may also be non-functional or have altered properties, potentially impacting cellular processes.