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Three sequences, UAG, UGA, and UAA, known as stop codons, [note 1] do not code for an amino acid but instead signal the release of the nascent polypeptide from the ribosome. [7] In the standard code, the sequence AUG—read as methionine—can serve as a start codon and, along with sequences such as an initiation factor, initiates translation.
A tRNA is commonly named by its intended amino acid (e.g. tRNA-Asn), by its anticodon sequence (e.g. tRNA(GUU)), or by both (e.g. tRNA-Asn(GUU) or tRNA Asn GUU). [19] These two features describe the main function of the tRNA, but do not actually cover the whole diversity of tRNA variation; as a result, numerical suffixes are added to differentiate.
For example, if the amino acid that attach to the end is phenylalanine, the reaction will be catalyzed by phenylalanine-tRNA synthase to produce tRNA phe. [4] The other end—the bottom often called the "DNA arm"—consists of a three base sequence that pairs with a complementary base sequence in a mRNA. [5]
The deduced amino acid sequence can be saved in various formats and searched against the sequence database using the basic local alignment search tool (BLAST) server. The ORF Finder should be helpful in preparing complete and accurate sequence submissions. It is also packaged with the Sequin sequence submission software (sequence analyser).
An open reading frame (ORF) is a reading frame that has the potential to be transcribed into RNA and translated into protein. It requires a continuous sequence of DNA which may include a start codon, through a subsequent region which has a length that is a multiple of 3 nucleotides, to a stop codon in the same reading frame.
For example, early tRNA-like ribozymes may have had different affinities for amino acids, with codons emerging from another part of the ribozyme that exhibited random variability. Once enough peptides were coded for, any major random change in the genetic code would have been lethal; hence it became "frozen".
The anticodon is an RNA triplet complementary to the mRNA triplet that codes for their cargo amino acid. Aminoacyl tRNA synthetases catalyze the bonding between specific tRNAs and the amino acids that their anticodon sequences call for. The product of this reaction is an aminoacyl-tRNA.
At the end of the initiation step, the mRNA is positioned so that the next codon can be translated during the elongation stage of protein synthesis. The initiator tRNA occupies the P site in the ribosome, and the A site is ready to receive an aminoacyl-tRNA. During chain elongation, each additional amino acid is added to the nascent polypeptide ...