Codons specified

Fig. 8.2 Model for the synthesis of amino acids from alpha-keto acid precursors covalently attached to dinucleotides. The dinucleotide that is capable of catalyzing synthesis of particular amino acids is proposed to contain the first two bases of the codon specifying that amino acid (Copley et al., 2005)...

As our ability to synthesize large DNA molecules keeps improving, it may be possible to generate a synthetic E. coli variant in which rare or degenerate codon-tRNA pairs are eliminated from the wild-type genome by replacing them with sense codons specifying the same amino acid. The liberated codons can instead be used to... 

The code is unambiguous. Each codon specifies no more than one amino acid. 

Silent new codon specifies same amino add i None... 

Missense new codon specifies different amino acid Possible decrease in function variable effects... 

In the genetic code of human nuclear DNA, one of the codons specifying the amino acid tyrosine is UAC. Another codon specifying this same amino acid is ... 

The codon specifying the next amino acid to be added is displayed in the open A site. 

A striking feature of the genetic code is that an amino acid may be specified by more than one codon, so the code is described as degenerate. This does not suggest that the code is flawed although an amino acid may have two or more codons, each codon specifies only one amino acid. The degeneracy of the code is not uniform. Whereas methionine and tryptophan have single codons, for example, three amino acids (Leu, Ser, Arg) have six codons, five amino acids have four, isoleucine has three, and nine amino acids have two (Table 27-3). 

When several different codons specify one amino acid, the difference between them usually lies at the third base position (at the 3 end). For example, alanine is coded by the triplets GCU, GCC, GCA, and GCG. The codons for most amino acids can be symbolized by XYq or XYg. The first two letters of each codon are the primary determinants of specificity, a feature that has some interesting consequences. 

When a mutation introduces a termination codon in the interior of a gene, translation is prematurely halted and the incomplete polypeptide is usually inactive. These are called nonsense mutations. The gene can be restored to normal function if a second mutation either (1) converts the misplaced termination codon to a codon specifying an amino acid or (2) suppresses the effects of the termination codon. Such restorative mutations are called nonsense suppressors they generally involve mutations in tRNA genes to produce altered (suppressor) tRNAs that can recognize the termination codon and insert an amino acid at that position. Most known suppressor tRNAs have single base substitutions in their anticodons. 

All the amino acids that eventually appear in the finished protein must be present at the time of protein synthesis. [Note If one amino acid is missing (for example, if the diet does not contain an essential amino acid), that amino acid is in limited supply in the cell, and translation, therefore, stops at the codon specifying that amino acid. This demonstrates the importance of having all the essential amino acids in sufficient quantities in the diet to ensure continued protein synthesis (see p. 259 for a discussion of the essential amino acids).]... 

Anticodon Each tRNA molecule also contains a three-base nucleotide sequence—the anticodon—that recognizes a specific codon on the mRNA (see Figure 31.6). This codon specifies Ihe insertion into the growing peptide chain of the amino acid carried... 

There are 20 naturally occurring amino acids that are assembled into proteins. If codons consisted of only two base pairs, each of which could be one of four nitrogenous bases, directions could be given for only 4x4= 16 amino acids. Using three bases per codon gives a total of 4 x 4 x 4 = 64 possibilities, which is more than sufficient. This provides for some redundancies for example, six different codons specify arginine. Codons also signal initiation and termination of a protein chain. 

The genetic code is not universal but is the same in most organisms. Exceptions are found in mitochondrial genomes where some codons specify different amino acids to that normally encoded by nuclear genes. In mitochondria, the UGA codon does not specify termination of translation but instead encodes for tryptophan. Similarly, in certain protozoa UAA and UAG encode glutamic acid instead of acting as termination codons. 

CODON Three contiguous nucleotides in mRNA that specify the amino acid to be inserted at a specific position in a polypeptide during translation of the 64 possible codons that are formed by the four types of nucleotides in SNA (nucleotides of adenine, guanine, cytosine, and uracil), 61 specify an amino acid and three (nonsense codons) specify no amino acid, but rather serve as termination signals in protein synthesis. 

Incorporation of amino acids into a polypeptide chain occurs through the involvement of a class of small RNA molecules known as transfer RNA or tRNA. For each codon in mRNA, there is a corresponding tRNA to which the amino acid specified by the codon is covalently attached. Since multiple codons specify the same amino acid (degenerate code), a given amino acid may be linked to a set of tRNAs. 

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 code is nearly universal. The same codon specifies the same amino acid in almost all species studied however, some differences have been found in the codons used in mitochondria. 

Fig. 12.4 Outline of the process of protein synthesis (translation of messenger RNA) in bacterial cells. The four stages of synthesis are shown initiation, elongation, translocation and termination with the sites of action of antibiotics. AUG is the start codon on messenger RNA (mRNA) specifying the first amino acid in bacterial proteins, N-formylmethionine. UAG, UAA and UGA are termination codons specifying no amino acid. 30S and 50S are the subunits of the ribosome. Other protein factors involved in protein synthesis are initiation factors (IF-1,2,3), elongation factors (EF-Tu and EF-G) and release factors (RF-1,2,3).

Several nondeletion a+-thalassemias have been described. Four mutations in the normal stop codon for the a-globin gene have been identified (Table 28-3). In these cells, the mutated codon specifies an amino acid, and translation continues beyond the normal point, extending the a chains by 31 additional amino acids. Synthesis is terminated by a stop codon downstream from the normal stop codon. Hb Constant Spring is relatively common in Southeast Asian populations. In heterozygotes, about 1% of the a-globin is Hb Constant Spring (25% is expected... 

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. 

According to the central dogma, when one strand of DNA is transcribed into RNA and translated to make proteins, three consecutive nucleotides form a codon. Each codon specifies an amino acid or amino acid chain termination. For example, the nucleotide sequence, or codon, GGA specifies the amino acid glycine. The genetic code has substantial redundancy, in that two or more codons code for the same amino acid. For example, both GGA and GGC code for glycine. Amino acids are the basic constituents of proteins, which mediate all cellular functions. Only 20 different amino acids, in various arrangements, form the basic units of all the proteins in the human body. 

The first two base pairings in a codon-anticodon interaction confer most of the specificity required during translation. Recall that most redundant codons specifying a certain amino acid possess identical nucleotides in the first two positions. These interactions are standard (i.e., Watson-Crick) base pairings. 

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