Stop codon

Definition

A stop codon is a nucleotide triplet within messenger RNA (mRNA) that signals the termination of protein synthesis during translation. It plays a crucial role in ensuring that proteins are produced with the correct length and sequence by indicating when the ribosome should stop adding amino acids to the growing polypeptide chain. There are three specific stop codons: UAA, UAG, and UGA, each of which does not code for any amino acid and instead promotes the release of the newly synthesized protein.

5 Must Know Facts For Your Next Test

1. Stop codons are essential for ensuring that proteins are synthesized correctly; without them, ribosomes would continue adding amino acids indefinitely.
2. The presence of a stop codon causes release factors to bind to the ribosome, prompting the release of the finished polypeptide from the tRNA.
3. The genetic code is degenerate, meaning that multiple codons can code for the same amino acid, but stop codons uniquely signal termination of translation.
4. In eukaryotic cells, the process of recognizing stop codons involves additional complexities such as post-translational modifications and ribosomal recycling.
5. Mutations that alter a stop codon can lead to truncated proteins, which may have serious consequences for cellular function and contribute to various diseases.

Review Questions

• How do stop codons function in the termination of protein synthesis, and what would happen if they were not present?
  • Stop codons function by signaling the ribosome to halt translation when it encounters one of these specific nucleotide sequences. When a stop codon appears, release factors bind to it, prompting the disassembly of the ribosome and releasing the completed polypeptide. Without stop codons, translation would continue indefinitely, resulting in excessively long and non-functional proteins that could disrupt cellular processes.
• Discuss the role of release factors in relation to stop codons during translation termination.
  • Release factors play a critical role in recognizing stop codons during translation termination. When the ribosome encounters a stop codon on the mRNA, release factors bind to the ribosome instead of tRNA. This binding triggers hydrolysis reactions that break the bond between the polypeptide and the tRNA, leading to the release of the newly synthesized protein and the disassembly of the ribosomal complex. This process ensures that protein synthesis is accurately concluded at the right time.
• Evaluate how mutations in stop codons can affect protein synthesis and contribute to disease development.
  • Mutations in stop codons can lead to significant changes in protein synthesis by altering or eliminating signals for termination. If a stop codon is mutated into a coding sequence (nonsense mutation), this could result in elongated proteins due to continued translation until another stop signal is reached. Such truncated proteins may lose their normal function or gain toxic properties, leading to cellular dysfunction. This can be linked to various diseases, including certain cancers and genetic disorders, where improper protein function has critical consequences.

Related terms

start codon:

The start codon is the first codon of an mRNA transcript that is translated by a ribosome, typically AUG, which codes for the amino acid methionine.

codon:

A codon is a sequence of three nucleotides on mRNA that corresponds to a specific amino acid or a stop signal during protein synthesis.

ribosome:

A ribosome is a molecular machine made of RNA and proteins that facilitates the translation of mRNA into a polypeptide chain by reading codons and assembling amino acids.