What experimental information verifies that certain codons in mRNA specify chain termination during translation?

Verified step by step guidance

1. Understand that the question asks about experimental evidence showing that specific codons in mRNA act as stop signals during translation, causing the protein chain to terminate.
2. Recall that the key experimental approach involved using synthetic mRNAs with known sequences and observing the resulting polypeptides produced in cell-free translation systems.
3. One classic experiment used synthetic mRNAs composed of repeating codons (e.g., poly-U, poly-A) to determine which amino acids were incorporated, and then introduced mutations or specific codons suspected to be stop codons to see if translation halted.
4. Analyze how researchers identified that certain codons (UAA, UAG, UGA) do not correspond to any amino acid but instead cause the ribosome to release the polypeptide chain, by observing truncated proteins or lack of amino acid incorporation beyond these codons.
5. Summarize that the experimental verification comes from comparing protein products synthesized from mRNAs with and without these codons, demonstrating that the presence of these codons leads to termination of translation.

Key Concepts

The Genetic Code and Codons

The genetic code consists of triplet codons in mRNA, each specifying an amino acid or a stop signal during translation. Stop codons (UAA, UAG, UGA) do not code for amino acids but signal the termination of protein synthesis, ensuring the polypeptide chain is released at the correct point.

Experimental Evidence for Stop Codons

Experiments using synthetic mRNAs and in vitro translation systems demonstrated that specific codons cause translation to halt. For example, researchers observed that when stop codons were present, no corresponding amino acid was incorporated, and the polypeptide chain was released, confirming their role in termination.

Role of Release Factors in Translation Termination

Release factors are proteins that recognize stop codons during translation and promote the release of the newly synthesized polypeptide from the ribosome. Their discovery and characterization provided biochemical proof that certain codons signal chain termination rather than coding for amino acids.