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Termination codons

TERMINATION CODONS

Termination codons, also known as stop codons, are specific sequences of three nucleotides in mRNA that signal the termination of protein synthesis during translation. There are three stop codons in the standard genetic code, and they do not code for any amino acids. Instead, they serve as signals for the ribosome to release the newly synthesized polypeptide chain. The three stop codons are:

  1. UAA (UAG in DNA):
    • UAA is one of the three termination codons, and it is known as the "ochre" stop codon.
  2. UAG (UAA in DNA):
    • UAG is another termination codon, and it is often referred to as the "amber" stop codon.
  3. UGA (TGA in DNA):
    • UGA is the third termination codon, and it is also known as the "opal" stop codon.

Key Features of Termination Codons:

  1. Role in Protein Synthesis:
    • Termination codons signal the end of the translation process, instructing the ribosome to release the nascent polypeptide chain.
  2. No Corresponding Amino Acid:
    • Unlike other codons that code for specific amino acids, stop codons do not specify any amino acid. Instead, they act as signals to terminate translation.
  3. Interaction with Release Factors:
    • The recognition of stop codons by release factors triggers the hydrolysis of the bond between the final tRNA and the last amino acid in the polypeptide chain.
    • Release factors facilitate the release of the completed polypeptide from the ribosome.
  4. Presence of Multiple Stop Codons:
    • There are three different stop codons in the genetic code, providing redundancy. This redundancy contributes to the flexibility and robustness of the translation process.
  5. Codon Usage and Variation:
    • The frequency of occurrence of stop codons can vary among different organisms, and there may be variations in the frequency of stop codon usage for the same organism in different genes.
    • The variation in stop codon usage has implications for gene expression and may be influenced by factors such as tRNA availability and translation efficiency.

Examples of Termination Codons in an mRNA Sequence:

  • ... AUG (start codon) ... amino acid sequence ... UAA (stop codon) ...
  • ... AUG (start codon) ... amino acid sequence ... UAG (stop codon) ...
  • ... AUG (start codon) ... amino acid sequence ... UGA (stop codon) ...

In summary, termination codons play a crucial role in the accurate and controlled termination of protein synthesis during translation. Their recognition by release factors ensures the proper release of the synthesized polypeptide chain from the ribosome, completing the translation process.

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