Stop codon

Stop codon refers to a nucleotide triplet within messenger RNA (mRNA) that signals a termination of translation and results in the release of the newly synthesized protein from the ribosome. The process of translation is crucial for protein synthesis, where mRNA is decoded by a ribosome to produce a specific amino acid chain, or polypeptide, that will then fold into an active protein.

Overview

In the genetic code, which is universal to nearly all organisms, there are three stop codons: UAG (amber), UAA (ochre), and UGA (opal). These codons are recognized by release factors rather than tRNAs, which helps to terminate protein synthesis. When a stop codon is encountered by a ribosome during translation, it signals the end of the polypeptide chain, thus completing the process of protein synthesis. This mechanism ensures that proteins are made to their correct lengths and structures.

Function

The primary function of stop codons is to signal the termination of protein synthesis. This is achieved through the recruitment of protein factors known as release factors, which recognize the stop codons and promote the cleavage of the polypeptide chain from the tRNA molecule. This release allows the polypeptide to fold into its functional three-dimensional structure and perform its biological functions.

Genetic Code and Evolution

The existence of stop codons is a fundamental aspect of the genetic code, which is remarkably conserved across all domains of life. This conservation suggests that the genetic code, including stop codons, has ancient origins. The universality of the genetic code implies a common evolutionary ancestor and highlights the efficiency and robustness of this coding system in guiding protein synthesis.

Clinical Significance

Mutations that affect stop codons can have significant biological consequences. A point mutation that changes a stop codon into a sense codon (one that codes for an amino acid) can result in a frameshift mutation or the production of a longer, potentially dysfunctional protein. Conversely, a mutation that converts a sense codon into a stop codon can lead to a truncated protein, potentially causing disease. Such mutations are implicated in various genetic disorders and are a focus of medical research.

See Also

• Genetic code
• Translation (biology)
• Mutation
• Protein synthesis