Arginine codons

Each tRNA has an anticodon sequence that allows it to pair with the codon for its cognate amino acid in the mRNA. Because base pairing is involved, the orientation of this interaction wEl be complementary and antiparallel as shown in Figure T4-6. The arg-tRNA 8 has an anticodon sequence, UCG, allowing it to pair with the arginine codon CGA. 

In an effort to reduce the competition encountered with naturally occurring tRNAs, even when rare codons are used, Hardesty and co-workers have devised a strategy based on 4-nucleotide codon-anticodon pairs [48]. An extra thymidine was inserted either 5 or 3 to the rare arginine codon AGG to yield TAGG... 

Gia-Fen, T. C. and Inouye, M. (1990). Suppression of the negative effect of minor arginine codons on gene expression preferential usage of minor codons within the first 25 codons of the Escherichia coli genes. Nucleic Acids Res. 18,1465-1473. 

Table 6.1 shows the relationship between the codon sequence in mRNA and its corresponding amino acid in the new protein. Because there are 64 (43) different anticodon combinations and only 20 encoded amino acids, some different anticodon sequences encode for the same amino acid. Generally, all the anticodons matching a given amino acid will have the same first two nucleotides. Exceptions are arginine, serine, and isoleucine. For example, the codon for proline will always start with CC, but the arginine codon may start with either AG or CG. The 3 end of the tRNA anticodon pairs with the 5 end of the mRNA codon. In other words, the codon and anticodon align and bind in an antiparallel fashion. 

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 hyperthermophiles employ the universal genetic code but some biases are detectable, including preferential use of only two of the six arginine codons and rare usage of cysteine codons. 

There are no AG A or AGG (arginine) codons in mtDNA genes. Also, no tRNAs are made in mitochondria for these codons. 

McNulty DE, ClafFcc BA, Huddleston MJ et al. Mistranslational errors associated with the rare arginine codon CGG in Escherichia coli. Protein E]q)r Purif 2003 27(2) 365-374. 

A mutation in the human Factor V gene plays a major role in causing thrombosis (Factor V Leiden mutation R506Q). The mutation (a G A transversion) that changes the amino acid arginine (codon CGA) to glutamine (cod i CAA) is located at codon 506. The site of this mutation is selected and amplified by PCR in furnishing a DNA of 224 base pairs of which a piece of the antisense sequaice is shown below ... 

The information contained in the DNA (i.e., the order of the nucleotides) is first transcribed into RNA. The messenger RNA thus formed interacts with the amino-acid-charged tRNA molecules at specific cell organelles, the ribosomes. The loading of the tRNA with the necessary amino acids is carried out with the help of aminoacyl-tRNA synthetases (see Sect. 5.3.2). Each separate amino acid has its own tRNA species, i.e., there must be at least 20 different tRNA molecules in the cells. The tRNAs contain a nucleotide triplet (the anticodon), which interacts with the codon of the mRNA in a Watson-Crick manner. It is clear from the genetic code that the different amino acids have different numbers of codons thus, serine, leucine and arginine each have 6 codewords, while methionine and tryptophan are defined by only one single nucleotide triplet. 

Many amino acids are specified by more than one codon (redundancy). Frequently, a tRNA can translate more than one of these codons, sparing the ceE from making multiple tRNAs to carry the same amino acid. For instance, in Figure 1-4-6 the arg-tRNA shown can translate both the CGA and the CGG codons that specify arginine. This phenomenon is known as Wobble" and can be summarized as follows ... 

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