Amino acids codons

To make things a little more complicated (or interesting), it is worth remembering that some amino acids are specified by more than one codon (termed redundancy, there are 20 basic amino acids and 64 available codons). Quite often, where an amino acid has associated codon redundancy, an organism will more frequently use one or more codons over the others to specify that particular amino acid (codon bias). 

Transfer RNAs base-pair with mRNA codons at a three-base sequence on the tRNA called the anticodon. The first base of the codon in mRNA (read in the 5 —>3 direction) pairs with the third base of the anticodon (Fig. 27-8a). If the anticodon triplet of a tRNA recognized only one codon triplet through Watson-Criclc base pairing at all three positions, cells would have a different tRNA for each amino acid codon. This is not the case, however, because the anticodons in some tRNAs include the nucleotide inosinate (designated I), which contains the uncommon base hypoxanthine (see Fig. 8-5b). Inosinate can form hydrogen bonds with three different nucleotides (U, C, and A Fig. 27-8b), although... 

The sequence of bases (A, G, T, and C) in a strand of DNA specifies the order in which amino acids are assembled to form proteins. The genetic code is the collection of base sequences that correspond to each amino acid, codon. Since there are only four bases in DNA and twenty amino acids in protein, each codon must contain at least three bases.5 Two bases cannot serve as codons because there are only 42 possible pairs of four bases, but three bases can serve because there are 43 = 64 possible triplets. Since the number of possible triplets are more than enough, several codons designate the same ammo acid. In other words, the genetic code is highly redundant as shown in Table 7.1. For example, UCU, UCC, UCA, UCG, AGU, and AGC are all codes for serine. 

The use of synthetic trinucleotides as building blocks would obviate the problems of stop codons or other unwanted amino acids turning up randomly in hypervariable sequences, and would indeed allow the predetermined definition of amino acid codon frequency bias. This has been demonstrated by Glaser et al. [97] for phage-display library construction see also Ref. 98 for a review of current DNA synthesis technology. 

X 106 codons) (1 amino acid/codon)/(400 amino acids/protein) = 4,000 proteins... 

Question Is the dictionary of amino acid codons shown in Table 17.1 the same for all organisms ... 

This flow of information is dependent on the genetic code, which defines the relation between the sequence of bases in DNA (or its mRNA transcript) and the sequence of amino acids in a protein. The code is nearly the same in all organisms a sequence of three bases, called a codon, specifies an amino acid. Codons in mRNA are read sequentially by tRNA molecules, which serve as adaptors in protein synthesis. Protein synthesis takes place on ribosomes, which are complex assemblies of rRNAs and more than 50 kinds of proteins. 

The answer is b. (Murray, pp 452-467. Scriver, pp 3-45. Sack, pp 245-257. Wilson, pp 151-180.) The replacement of the codon UAG with UAA would be a silent mutation since both codons are stop signals. Thus, transcription would cease when either triplet was reached. There are three termination codons in mRNA UAG, UAA, and UGA. These are the only codons that do not specify an amino acid. A missense or a substitution mutation is the converting of a codon specifying one amino acid to another codon specifying a different amino acid. A nonsense mutation converts an amino acid codon to a termination codon. A suppression counteracts the effects of another mutation at another codon. The addition or deletion of nucleotides results in a frame-shift mutation. 

CODON AMINO ACID CODON ACID CODON ACID... 

Some 30-40 different tRNAs have been Identified in bacterial cells and as many as 50-100 In animal and plant cells. Thus the number of tRNAs In most cells is more than the number of amino acids used In protein synthesis (20) and also differs from the number of amino acid codons In the genetic code (61). Consequently, many amino acids have more than one tRNA to which they can attach (explaining how there can be more tRNAs than amino acids) In addition, many tRNAs can pair with more than one codon (explaining how there can be more codons than tRNAs). 

Table of the genetic code showing the 61 amino acid codons and three termination codons (STOP). The codon AUG specifies methionine (Met) and is often referred to as the initiator... 

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