Amino acid genetic code

Mehl RA, Anderson JC, Santoro SW, Wang L, Martin AB, King DS, Horn DM, Schultz PG Generation of a bacterium with a 21 amino acid genetic code. J. Am. Chem. Soc. 2003 125 935-939. 

A completely autonomous bacterium with a 21 amino acid genetic code was engineered. The bacterium can generate p-aminophenylalanine from basic carbon sources and incorporate this amino acid into proteins in response to the amber nonsense codon [132]. 

See also Table 5.1, Amino Acids, Genetic Code, Y-Carboxyglutamic Acid, Glutamine, Glutamate as a Precursor of Other Amino Acids (from Chapter 21), Transamination in Amino Acid Metabolism (from Chapter 20), Citric Acid Cycle Intermediates in Amino Acid Metabolism (from Chapter 21), Essential Amino Acids... 

See also Table 5.1, Amino Acids, Genetic Code, Methionine, Metabolism of Sulfur-Containing Amino Acids, Essential Amino Acids... 

As the genetic code provides 4 = 64 codons for the 20 amino acids, there are several synonymous codons for most amino acids— the code is degenerate. Three triplets do not code for amino acids, but instead signal the end of translation (stop codons). Another special codon, the start codon, marks the start of translation. The code shown here is almost universally applicable only the mitochondria (see p. 210) and a few microorganisms deviate from it slightly. 

The genetic code involves sequences of three bases called codons, each of which translates to a specific amino acid. The code is degenerate (that is, there is more than one codon per amino acid), and some codons are "stop" signals that terminate synthesis. 

Transfer RNA (tRNA) molecules mediate translation of the nucleic acid genetic code into the amino acid building blocks of proteins, thus ensuring the survivability of cells. The dynamic properties of tRNA molecules are crucial to their functions in both activity and specificity. This chapter summarizes two methods that have been recently developed or improved upon previous protocols to introduce fluorophores to site-specific positions in tRNA. One method enables incorporation of fluorophores carrying a primary amine (such as proflavin or rhodamine) to dihydrouridine (D) residues in the tRNA tertiary core, and a second method enables incorporation of pyrroloC and 2-aminopurine to positions 75 and 76, respectively, of the CCA sequence at the 3 end. These site-specific fluorophore labeling methods utilize tRNA transcripts as the... 

As we can see from the figure, three codons are used as protein synthesis termination signals, while the other 61 specify the amino acids and the initiation signal. Between 61 codons and 20 amino acids there cannot be a one-to-one correspondence, and in fact some amino acids are specified by six codons, some by four, others by two, and only two amino acids are coded by a single codon. In technical terms, this is expressed by saying that the genetic code is degenerate. 

Because there are 20 amino acids and only four nucleotides, there must be a combination of at least three nucleotides to define each amino acid. A code based on two nucleotides would provide only 42 or 16 combinations, which is insufficient. Proof that the codon for each amino acid consists of three nucleotides was provided by genetic studies of the effects on the polypeptide product of nucleotide addition to or deletion from a gene. 

The precise sequence of bases along the DNA carries the genetic information. An individual amino acid is coded by a triplet of bases. The genetic code for all the amino acids is shown in table 3.3. 

An anticodon is a sequence of three nucleotides in a transfer RNA (tRNA) that is complementary to a codon of messenger RNA (mRNA). The relationship between codons and the amino acids they code for is called the genetic code. The process of converting mRNA sequence information to the amino acid sequence of a protein is called translation. 

By definition, a sense peptide is one whose sequence is coded for by the nucleotide sequence (read 5 3 ) of sense mRNA, whose sequence contains the same coding information as the sense strand of DNA. Gonversely, a complementary peptide is coded for by the nucleotide sequence (read 5 3 ) of complementary mRNA, with the same sequence information as the complementary strand of DNA. Frequently, sense and complementary peptides are capable of specific interactions, in a process that may involve an amino-acid interaction code embedded within the genetic code and its complement (Figure 7.40). One application of sense-complementary peptide interactions may be in the design of complementary... 

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