Final answer:
Translation termination occurs when a ribosome encounters a stop codon, signalling the end of protein synthesis. mRNAs lacking stop codons can lead to harmful proteins, but are targeted for degradation by the Exosome complex. This ensures that only correctly coded proteins are produced, maintaining cellular integrity.
Step-by-step explanation:
In the process of translation termination, a ribosome ends its synthesis of a protein when it encounters a stop codon (UAA, UAG, or UGA) in the mRNA. These stop codons do not code for an amino acid and instead signal the release of the newly synthesized polypeptide chain. If an mRNA lacks a stop codon, it can lead to nonfunctional or damaging proteins. To prevent this, a surveillance system exists where the ribosome stalls at the 3'-end of the transcript. This stalling triggers the recruitment of the Exosome, a multi-protein complex with 3’ to 5’ exonuclease activity that degrades the aberrant mRNA from the 3’ end.
Normal mRNA transcripts are protected from degradation by RNA-stabilizing proteins, a 5' cap, and a poly-A tail. These modifications not only protect mRNA from premature degradation but also facilitate translation initiation and export from the nucleus. In cases where translation ends appropriately at a stop codon, the release factors trigger the disassembly of the ribosomal subunits, freeing the mRNA for degradation and nucleotide recycling.
Terminating translation ensures that only properly coded proteins are synthesized, and the Exosome plays a crucial role in maintaining the integrity of the protein-coding process by removing faulty mRNAs that lack stop codons, thus safeguarding the cell from potential harm.