Question

Why does translation terminate when the ribosome reaches a stop codon? What happens?

Short answer

Translation terminates when the ribosome reaches a stop codon because these codons signal the end of protein synthesis and do not encode for any amino acids. Release factors bind to the stop codon, which causes the ribosome to release the polypeptide chain and disassemble.

Step-by-step solution

Understanding Stop Codons

Understand that stop codons are specific nucleotide triplets within messenger RNA (mRNA) that signal the end of protein synthesis. The three stop codons are UAA, UAG, and UGA. They do not code for any amino acid.

Recognizing the Role of Release Factors

Recognize that when the ribosome encounters a stop codon during translation, it binds to release factors instead of aminoacyl tRNA. These release factors recognize the stop codons and prompt the ribosome to halt the translation process.

Mechanism of Termination

Learn that once release factors bind to the ribosome-mRNA complex, they induce the hydrolysis of the bond between the polypeptide chain and the tRNA in the P site. This releases the newly synthesized polypeptide, and the ribosome subunits, mRNA, and release factors disassemble, completing the translation process.

Key concepts

Stop Codons and Translation Termination

At the heart of protein synthesis, 'stop codons' serve as the genetic period at the end of the sentence. They are special sequences of nucleotides within the messenger RNA (mRNA) that do not correspond to an amino acid but instead mark the conclusion of the polypeptide chain's construction. Within the genetic code, there are three stop codons: UAA, UAG, and UGA. Each is a unique triplet that, upon entering the A site of the ribosome, does not recruit a tRNA bearing an amino acid. Instead, it signals that the protein's building blocks are complete and it's time to release the chain.

It's like reaching the final note in a musical composition; the stop codon tells everything involved in the process that the melody is finished. The absence of an amino acid being added prompts the next phase - termination - where another set of proteins come into play to wrap things up neatly and ensure the new protein is released to go and perform its functions within the cell.

Release Factors: The Bridge to Termination

When stop codons enter the orchestration of protein synthesis, 'release factors' take the stage. These are proteins that recognize the stop codons and bind to the ribosome-mRNA complex. Their binding is what sets them apart from the usual aminoacyl-tRNA which adds amino acids to the growing chain. Release factors essentially mistake the stop codon for a tRNA binding call, but instead of prolonging the chain, these factors incite a break in the process.

They act as a biological mechanism that prompts the ribosome to add water molecules instead of amino acids, effectively cleaving the bond between the polypeptide and the tRNA that holds it in place. This separation is akin to cutting the ribbon at the end of a race, signifying the protein is now free and the complex synthesis process has come to a close. The meticulous unraveling of the ribosome's components follows, as it readies itself for the potential construction of another protein molecule.

Protein Synthesis: The Cellular Assembly Line

Protein synthesis is a fundamental biological process that crafts the vast array of proteins necessary for life. It operates much like a factory assembly line; initiated at the 'start codon', the ribosome navigates along the mRNA strand, synthesizing a protein by sequentially adding amino acids together through the format of codons –each a set of three nucleotides. This entire process is known in the biological field as 'translation'.

During translation, tRNAs ferry each corresponding amino acid to the ribosome as it sequences the mRNA's coding region. However, this process is not indefinite and must be brought to a precise halt, guided by the aforementioned stop codons and aided by the release factors. In the big picture, understanding the delicate ballet of initiation, elongation, and termination in protein synthesis illuminates the remarkable efficiency of cellular machinery in producing life's building blocks.