Serine genetic coding

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. 

In the next step, translation, the sequence of nucleotides in the newly synthesized mRNA strand is used to determine the sequence of amino acids in the protein to be synthesized. This is done by way of a genetic code, which was fully deciphered by 1966 and is shown in Figure 13.34. According to the genetic code, it takes three mRNA nucleotides—each three-nucleotide unit is called a codon—to code for a single amino acid. The mRNA nucleotide sequence AGU, for example, codes for the amino acid serine, and AAG codes for lysine. (Note from Figure 13.34 that more than one codon can call for the same amino acid.) A few codons, such as AUG and UGA, are the signals for protein synthesis to either start or stop. 

Another important technique was based on the observation that synthetic trinucleotides induced the binding to ribosomes of tRNA molecules that were "charged" with their specific amino acids 38/39 For example, the trinucleotides UpUpU and ApApA stimulated the binding to ribosomes of 14C-labeled phenylalanyl-tRNA and lysyl-tRNA, respectively. The corresponding dinucleotides had no effect, an observation that not only verified the two codons but also provided direct evidence for the triplet nature of the genetic code. Another powerful approach was the use of artificial RNA polymers, synthesized by combined chemical and enzymatic approaches.40 For example, the polynucleotide CUCUCUCUCU led to the synthesis by ribosomes of a regular alternating polypeptide of leucine and serine. 

The genetic code is shown in Table S.A2. All the possible 64 combinations are shown and there are codes for either start and stop signals or for amino-acids. There are up to six codes for some amino-acids, e.g. arginine, leucine and serine, while tryptophan and methionine have unique codes. The major features of the genetic code are ... 

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. 

Fig. 20.2. Simplified scheme describing the central dogma in molecular biology. DNA is replicated and passed from one generation to the next. For protein biosynthesis, DNA sequence is first transcribed into complementary messenger RNA (mRNA) sequence which, by means of the adapter molecule transfer RNA (tRNA), is translated into protein sequence. The translation follows the genetic code where a nucleotide triplet (e.g., AGC) codes for an amino acid (e.g., serine) [522]...

Hatfield DL, Gladyshev VN. How selenium has altered our understanding of the genetic code. Mol. CeU. Biol. 2002 22 3565-3576. Wu XQ, Gross HJ. The length and the secondary structure of the D-stem of human selenocysteine tRNA are the major identity determinants for serine phosphorylation. EMBO J. 1994 13 241-248. Hubert N, Sturchler C, Westhof E, Carbon P, Krol A. The 9/4 secondary structure of eukaryotic selenocysteine tRNA more pieces of evidence. Rna 1998 4 1029-1033. 

The 11 nonessential amino acids are listed as follows alanine, asparagine, aspartate, cysteine, glutamine, glutamate, glycine, hydroxyproline, proline, serine, and tyrosine. They are synthesized within the body, for instance, from essential amino acids. Interestingly, however, hydroxyproline is not one of the 20 common amino acids — the latter being the 20 as established by means of the genetic code. 

The third approach used repeating ribonucleotide polymers containing known repeating sequences (Fig. 26.4C). When these were used as templates for in vitro protein synthesis, it was found that each ribonucleotide polymer could specify as many as three different repeating polypeptide products. Of the 64 possible codons, 61 were found to specify amino acids and 3 were later defined as termination codons. Figure 26.5 shows the genetic code for protein synthesis in E. coli, which is for the most part applicable to mRNA translation in mammalian cells. Note that methionine and tryptophan are only specified by single codons, whereas almost all the other amino acids have from two to four codons (except leucine and serine which are encoded by six codons). Methionine is the first amino acid in essentially all proteins however, methionine residues are also found within the polypeptide sequence. The amino-terminal methionine is called the initiator methionine. 

The complete genetic code is shown in Figure 24.16. We can make several observations about the genetic code. First, methionine and tryptophan are the only amino acids that have a single codon. All others have at least two codons, and serine and leucine have six codons each. The genetic code is also somewhat mutation-resistant. For those amino acids that have multiple codons the first two bases are... 

See also Table 5.1, Genetic Code, Metabolism of Serine, Glycine, and Threonine, Figure 2L24, Figure 21.25, Essential Amino Acids... 

Data from Palmer, J. D. (1997) Nature (London) 387,454-455.1 F One for each amino acid of the genetic code but two each for serine and leucine. 

One of the main tasks of the DNA is to initiate the synthesis of proteins as and when they are needed. Proteins are synthesised in the ribosomes of the cell cytoplasm. DNA, however, is found in the cell nucleus. So how is the information contained in the DNA passed out of the cell nucleus and into the cytoplasm First, the DNA helix unfolds, and, in a process called transcription, a complementary strand of RNA is synthesised along a crucial part of one of the single DNA strands. This is the messenger RNA (mRNA) which leaves the cell nucleus and is transported into the manufacturing centres for proteins, the ribosomes. In the ribosome, transfer RNA (tRNA) delivers the amino acids required for polypeptide synthesis. The sequence of each group of three bases on the mRNA determines which amino acid is next in the peptide sequence. For example, the sequence AGC in the mRNA specifies the incorporation of the amino acid serine. This process is referred to as translation (Fig. 1.27). The genetic code, i.e. which sequence of bases in the DNA strand refers to which amino acid is given in Table 1.5. 

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