3/30/2024 0 Comments Current color coded periodic tableNew amino acids added in evolution to the initial group of early amino acids may in some cases have taken over codons previously assigned to their precursors. Gly, Ala, Asp and Val) which can be also synthesized via pathways with only a few steps (for a review see ref. The etiology of this pattern of redundancy is not entirely clear, but is thought to be related to co-evolution of the genetic code and amino acids, with the appearance of the modern group of 20 (+2) amino acids (additional 2 amino acids include selenocysteine and pyrrolysine that are decoded via the UGA an UAG stop codons, respectively) evolving from a relatively small number of early/prebiotic amino acids (such as e.g. The greatest degree of redundancy exists for Leu, Ser and Arg, which are each encoded by 6 codons (4 in one group and 2 in another group, again with groups defined as being identical at positions 1 and 2) ( Fig. 1A). Ile is encoded by a group of 3 codons and Val, Pro, Thr, Ala, and Gly are each specified by a group of 4 codons these also differ only at position 3 within a group. Phe, Tyr, His, Gln, Asn, Lys, Asp, Glu, and Cys are each encoded by 2 distinct codons, which in each case are identical at positions 1 and 2 but different in position 3 (for example, Phe is encoded by UUU and UUC) ( Fig. 1A). Only two amino acids (Met and Trp) are encoded by just a single codon in most of the organisms (although exceptions to this rule do exist 4,5). In most organisms, there are 20 common amino acids used in protein synthesis thus, the genetic code is redundant with most amino acids being encoded by more than one codon. 4,5 With 4 bases (A, G, U, and C), there are 64 possible triplet codons 61 sense (encoding amino acids) and 3 nonsense (UAA, UAG, and UGA, so-called stop codons that direct termination of translation). The genetic code is nearly universal, meaning that in almost all living organisms, the identity of the amino acid encoded by a given triplet codon is the same. 1-3 Note that this is a simplified overview of the process, which involves many additional/intermediate steps, which will not be considered here. 1-3 In the next step, if acceptable codon–anticodon base pairing has been established between the mRNA and the incoming aa-tRNA, decoding is accompanied by a peptide bond formation between the incoming amino acid and the previous amino acid. Wobbling occurs because the conformation of the tRNA anticodon loop permits flexibility at the first base of the anticodon. The specificity is such that perfect Watson–Crick base pairs are usually observed between the first 2 nucleotides in the codon and those in the anticodon, but altered base pairing is possible at the third, so-called “wobble,” position. The next aa-tRNA binds to the ribosomal A-site by forming base-pairs with the next codon in the mRNA. 1-3 Placement of the initiator tRNA in the ribosomal P-site (directed by the initiation/AUG codon) sets the reading frame for all subsequent incoming aminoacyl-tRNAs (aa-tRNAs) required for decoding of the message. Decoding, or translation, of mRNAs is performed by ribosomes, with addition of each new amino acid to the growing chain involving a cycle of complex reactions consisting of several major steps. The genetic code describes the correspondence between the sequence of a given nucleotide triplet in an mRNA molecule, called a codon, and the amino acid that it directs to be added to the growing polypeptide chain during protein synthesis.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |