Stop Codons (stɑp ˈkoʊdɑnz) are essential to the genetic code. They signal the end of a protein-coding sequence and help to ensure that the protein is correctly produced. The spelling of Stop Codons is pronounced with the IPA phonetic transcription, which is a universal method of representing the sounds of speech. The first syllable begins with the consonant cluster st and is followed by the vowel sound ɑ. The second syllable features the open-mid back unrounded vowel ɔ, and the final syllable ends with the voiced consonant sound z.
Stop codons, also known as termination codons or nonsense codons, are specific sequences of nucleotides within a messenger RNA (mRNA) molecule that signal the termination of protein synthesis during translation, leading to the release of the newly synthesized polypeptide chain. Stop codons are one of the three types of codons found in the genetic code, the other two being start codons and sense codons.
In the universal genetic code, there are three specific stop codons: UAG (amber), UAA (ochre), and UGA (opal). These codons do not code for any amino acid but instead act as signals for the ribosomes to halt translation. When a ribosome encounters a stop codon during translation, it recognizes specific release factors that bind to the ribosome and cause the release of the polypeptide chain. This marks the end of protein synthesis.
Stop codons are crucial in maintaining the correct reading frame and integrity of the genetic code. Mutations that alter a sense codon into a stop codon can lead to premature protein termination and may result in non-functional or truncated proteins. Additionally, the presence of stop codons within the mRNA molecule prevents the ribosome from continuing translation beyond the intended open reading frame, ensuring proper protein synthesis and functionality.
Overall, stop codons play a vital role in the precise and controlled process of protein synthesis, allowing for the accurate termination and release of the nascent polypeptide chain.
The term "Stop codons" is derived from the combination of two etymological elements.
1. "Stop": The word "stop" comes from the Old English word "stoppian", meaning "to block, to plug up". It has been used since the 14th century to indicate halting or restricting movement or action.
2. "Codons": A codon is a sequence of three nucleotides on the mRNA (messenger RNA) that corresponds to a specific amino acid or serves as a signal for the termination of protein synthesis. The term "codon" originated by combining "co-" from "coding" and "-don" from "nucleotide".
So, "Stop codons" refers to the specific sequences of nucleotides that signal the termination of protein synthesis during the translation process, essentially causing the ribosome to stop or halt its activity.