Genetic code tables

The genetic code (Table 28 3) is the message earned by mRNA It is made up of triplets of adjacent nucleotide bases called codons Because mRNA has only four dif ferent bases and 20 ammo acids must be coded for codes using either one or two nucleotides per ammo acid are inadequate If nucleotides are read m sets of three how ever the four mRNA bases generate 64 possible words more than sufficent to code for 20 ammo acids... 

What is the amino acid sequence of the fusion protein Where is the junction between /3-galactosidase and the sequence encoded by the insert (Consult the genetic code table on the inside front cover to decipher the amino acid sequence.)... 

When writing protein sequences, you write the amino terminus on the left. If you have to use the genetic code tables to figure out a protein sequence from the DNA sequence, it is not necessary to write down the complementary RNA sequence first it s the same as that of the sense strand (the one on top) with the Ts replaced by Us. 

Most genetic code tables designate the codons for amino adds as mRNA sequences (Figure 1-4-1). Important features of the genetic code include ... 

Mutations are often classified according to the effect they have on the structure of the gene s protein product. This change in protein structure can be predicted using the genetic code table in conjunction with the base sequence of DNA or mRNA. A variety of such mutations is listed in Table 1-4-1. Point mutations and frameshifts are illustrated in more detail in Figure 1-4-2. 

The genetic code (Table 26.4) is the message carried by mRNA. It is made up of triplets of adjacent nucleotide bases called codons. Because mRNA has only four different... 

A protein is biosynthesized from its N-terminal end to its C-terminal end by a process that reads the bases along the mRNA strand in the 5 3 direction. The amino acid that is to be incorporated into a protein is specified by a three-base sequence called a codon. The bases are read consecutively and are never skipped. The three-base sequences and the amino acid that each sequence codes for are known as the genetic code (Table 26.2). A codon is written with the 5 -nucleotide on the left. For example, the codon UCA on mRNA codes for the amino acid serine, whereas CAG codes for glutamine. 

The information contained in the base sequence of the mRNA template is interpreted in sequences of three bases called codons each codon represents one amino acid. Therefore, the unit of information is the codon. Since there are four major bases in mRNA, 4 (i.e. 64) different codons are possible. The 64 triplets constitute the genetic code (Table 17.1). All codons have been assigned to amino acids or punctuation signals. Three triplets (UAA, UAG and UGA) are not complemented by anticodons on tRNAs and serve to signal that the polypeptide chain has been completed. Of the other 61 triplets which have complementary tRNAs, two (AUG and GUG) have additional roles in the initiation of protein synthesis. Since there are only 20 amino acids, most amino acids are specified by more than one codon, i.e. the code is degenerate. The genetic code applies to prokaryotes and eukaryotic nuclear and chloroplast mRNAs but not to... 

It is now known that each codon consists of a sequence of three nucleotides ie, it is a triplet code (see Table 38—1). The deciphering of the genetic code depended heavily on the chemical synthesis of nucleotide polymers, particularly triplets in repeated sequence. 

Table 38-1. The genetic code (codon assignments in mammalian messenger RNA)J...

The anticodon region consists of seven nucleotides, and it recognizes the three-letter codon in mRNA (Figure 38-2). The sequence read from the 3 to 5 direction in that anticodon loop consists of a variable base-modified purine-XYZ-pyrimidine-pyrimidine-5h Note that this direction of reading the anticodon is 3 " to 5 whereas the genetic code in Table 38—1 is read 5 to 3 since the codon and the anticodon loop of the mRNA and tRNA molecules, respectively, are antipar-allel in their complementarity just like all other inter-molecular interactions between nucleic acid strands. 

Table 8.1 The genetic code as a three-letter code, with the normal abbreviations for the 20 pro-teinogenic amino acids. The first base corresponds to the 5 end, the third to the 3 end of the nucleotide chain... 

Table 8.2 According to the co-evolution theory proposed by Wong, the biosynthetic routes to amino acids from their precursors could perhaps provide information on the evolution of the genetic code (Wong, 1975)...

It is worth looking at the various base sequences with the help of the genetic code. All the pairs of codon units which are linked by arrows in Table 8.2 differ by only one single base exchange. 

Fig. 8.1 Wong s evolutionary map shows possible relationships between code words. The codons in boxes correspond to today s code words (compare Table 8.1). The codons for Asp and Glu in dotted boxes refer to these amino acids in a very early evolutionary stage of the genetic code. Single-headed arrows refer to the biosynthetic relationships between precursor and product, while double-headed arrows refer to reciprocal formation possibilities. All pairs of codon units (irrelevant of whether they are linked by single- or double-headed arrows) differ only in one single base change (Wong, 1975)...

Twenty amino acids are constituents of the peptide chains found in nature. These are generally referred to as the common amino acids (Table 6.1). Less-common amino acids are listed in Table 6.2, including selenocysteine, often called the 21st natural amino acid . Although of little relevance here, we note that the 22nd amino acid , pyrrolysine, was discovered in 2002 in the genetic code of certain Archea and eubacteria [1],... 

To transcribe information from DNA to mRNA, one strand of the DNA is used as a template. This is called the anticoding, or template, strand and the sequence of mRNA is complementary to that of the template DNA strand (Fig. A2.8) (i.e., C->G, G->C, T->A, and A U note that T is replaced by U in mRNA). The other DNA strand, which has the same base sequence as the mRNA, is called the coding, or sense, strand. There are 64 (4 x 4 x 4) possible triplet codes of the four bases 61 are used for coding amino acids and three for termination signals. As there are 20 amino acids for the 61 codes, some triplets code for the same amino acid. A table of the genetic code is presented in Exhibit A2.2. 

Table 1-4-3. Important Points About the Genetic Code, Mutations, and Dranstation...

There are 64 ways to order four things three at a time when the order in which they are taken (permutations) matters n = 4 = 64. So there are 64 words in the language of DNA. This is more words than we need to specify the 20 amino acids of proteins. A few of these words are used as punctuation marks—start and stop signals. Beyond that, most of the amino acids are specified by more than one word. The genetic code is provided in table 12.1. Note that the code words in table 12.1 refer to those in messenger RNA, mRNA, the complement to the code words in DNA. 

NUCLEOTIDES, NUCLEIC ACIDS, AND GENES 1 57 Table 12.1 Genetic Code mRNA to Amino Acids... 

Table 12.1. The genetic code presented in this table is very nearly universal. There are isolated exceptions in the genome of mitochondria, which is described later in this chapter. Beyond that, the genetic code has been expanded to include codons for two unusual amino acids that occur in a modest number of proteins. These amino acids are selenomethionine, in which an atom of selenium replaces the sulfur atom of methionine, and pyrrolysine, a cychzed form of lysine. For details, see A. Ambrogelly, S. Palioura, and D. Soil, Nat Chem Biol 3 29-35 (2007). 

The following table indicates the standard codons that serve as the basis of the genetic code. Note (a) Stop codons have no amino acids assigned to them (b) Met indicates the AUG start codon. 

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