Stop codon

Definition

A stop codon is a specific sequence of three nucleotides in mRNA that signals the termination of protein synthesis during translation. These codons are essential for ensuring that proteins are made with the correct length and structure, as they instruct the ribosome to stop adding amino acids to the growing polypeptide chain. The presence of stop codons allows for precise regulation and completion of protein synthesis, linking directly to the broader concepts of the genetic code and the flow of information within a cell.

5 Must Know Facts For Your Next Test

1. There are three main stop codons in the genetic code: UAA, UAG, and UGA, which do not correspond to any amino acids.
2. When a ribosome encounters a stop codon during translation, it releases the newly synthesized polypeptide chain from the tRNA.
3. Stop codons play a critical role in preventing the translation machinery from reading past the intended end of a gene, which could result in malfunctioning proteins.
4. The recognition of stop codons involves release factors that bind to the ribosome and facilitate the disassembly of the translation complex.
5. Mutations that create or disrupt stop codons can lead to serious consequences, such as extended or truncated proteins that can cause diseases.

Review Questions

• How do stop codons contribute to the accuracy of protein synthesis?
  • Stop codons ensure that protein synthesis is accurate by signaling when the ribosome should terminate translation. This prevents any additional amino acids from being added beyond what is specified by the mRNA. If stop codons did not exist or were misread, it could result in proteins that are too long or improperly folded, leading to potential loss of function or harmful effects.
• Discuss the role of release factors in relation to stop codons during translation.
  • Release factors are proteins that recognize stop codons on the mRNA during translation. When a ribosome encounters a stop codon, release factors bind to it and prompt the release of the completed polypeptide chain from the ribosome. This process also involves disassembling the ribosomal complex, allowing for new rounds of translation to occur. Therefore, release factors are essential for maintaining proper protein synthesis and cellular function.
• Evaluate how mutations affecting stop codons can impact gene expression and overall cellular health.
  • Mutations that alter or create stop codons can significantly impact gene expression and cellular health. For instance, a premature stop codon can lead to truncated proteins that may be nonfunctional or harmful. Conversely, mutations that eliminate stop codons can result in longer proteins due to continued translation past the intended termination point. Both scenarios can disrupt normal cellular processes and lead to various diseases, highlighting the critical role of stop codons in genetic regulation and protein homeostasis.

Related terms

start codon:

The start codon is the first codon of a messenger RNA (mRNA) transcript that is translated by a ribosome, typically AUG, which codes for methionine.

ribosome:

A ribosome is a complex molecular machine found within all living cells that facilitates the translation of mRNA into protein.

amino acid: An amino acid is a building block of proteins, coded by sequences of three nucleotides (codons) in mRNA during translation.