Stop Codon

Definition

A stop codon is a sequence of three nucleotides in messenger RNA (mRNA) that signals the end of the coding sequence and the termination of protein synthesis during translation. These codons do not specify any amino acid and instead indicate that the ribosome should release the completed polypeptide chain and dissociate from the mRNA.

5 Must Know Facts For Your Next Test

1. There are three stop codons in the genetic code: UAA, UAG, and UGA.
2. Stop codons do not specify any amino acid and instead signal the ribosome to terminate protein synthesis.
3. The release of the completed polypeptide chain from the ribosome is facilitated by specialized proteins called release factors.
4. Mutations in stop codons can lead to the production of longer or truncated proteins, which can have significant impacts on cellular function.
5. The presence of a stop codon is essential for ensuring the proper length and structure of the final protein product.

Review Questions

• Explain the role of stop codons in the process of translation and protein synthesis.
  • Stop codons play a crucial role in translation by signaling the ribosome to terminate protein synthesis. When the ribosome reaches a stop codon in the mRNA sequence, it recognizes this as the end of the coding region and releases the completed polypeptide chain. This ensures that the final protein product has the correct length and structure, as the ribosome will not continue adding amino acids beyond the stop codon. The release of the polypeptide is facilitated by specialized proteins called release factors, which interact with the stop codon and trigger the dissociation of the ribosome from the mRNA.
• Describe the different types of stop codons and their significance in the genetic code.
  • There are three stop codons in the genetic code: UAA, UAG, and UGA. These three codons do not specify any amino acid; instead, they signal the ribosome to terminate protein synthesis. The presence of multiple stop codons in the genetic code provides redundancy and ensures that the ribosome can reliably identify the end of the coding sequence, even in the event of a mutation that changes one of the stop codons. This redundancy helps to maintain the fidelity of protein synthesis and prevent the production of incomplete or abnormal protein products, which could have detrimental effects on cellular function.
• Analyze the potential consequences of mutations in stop codons and their impact on protein structure and function.
  • Mutations in stop codons can have significant consequences for protein structure and function. If a mutation changes a stop codon to a sense codon (one that specifies an amino acid), the ribosome will continue translating the mRNA beyond the original stop site, resulting in the production of a longer, potentially aberrant polypeptide chain. Conversely, a mutation that creates a new stop codon upstream of the original one will lead to the synthesis of a truncated protein, which may lack essential functional domains or structural elements. Both scenarios can have profound impacts on the stability, localization, and overall function of the protein within the cell. Understanding the critical role of stop codons in ensuring the proper length and structure of proteins is essential for recognizing the potential consequences of genetic mutations and their implications for cellular processes and human health.

Related terms

Genetic Code: The set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins by living cells.

Ribosome: A complex molecular machine found within all living cells that is responsible for the synthesis of proteins.

Polypeptide Chain: A long, linear sequence of amino acids that is the basic structural component of a protein.