Amino acid mRNA codons

The genes for all three deiodinases contain the TGA triplet in the coding region of the DNA, while the mRNA contains a UGA triplet at the corresponding position. Part of the coding sequence of type II deiodinase is shown in what follows. The entire protein consists of 266 amino acids. The codon resulting in selenocysteine is indicated by "SeC," where this amino acids resides in the context -glycine-serine-alanine-threonine-selenocysteine-proline-proline-phenylalanine-threonine- (Pallud et al., 1997) ... 

The synthesis of proteins is a more complex process (Fig. 124—11). Proteins consist of chains of amino acids in very specific seqnences. As in DNA synthesis, the double helix must unwind. However, in protein synthesis, only the portion of the DNA molecnle that codes for the desired protein is exposed. The enzyme RNA polymerase matches free complementary RNA nncleotides to the exposed DNA nucleotides, and the resultant chain of nncleotides is called mRNA. This process is called transcription. The mRNA travels to ribosomes in the cytoplasm, where protein synthesis occurs. Each three nucleotides of the mRNA chain compose a codon, whose seqnence is specific for a particular amino acid. The codon is recognized by tRNA, which then carries the amino acid to the ribosome, where it is added to the growing peptide chain. This process is known as translation. The... 

The sequence of bases on the mRNA specifies the polypeptide to be synthesized. Each sequence of three consecutive bases (base triplet) comprises a codon which codes or specifies for a particular tRNA which carries a particular amino acid. The codon is written using the one-letter symbols for the bases, e.g., ACC designates the codon with the base sequence adenine-cytosine-cytosine. 

Three contiguous nucleotides is the minimum that can serve as a codon. There are four kinds of nucleotides in mRNA. A codon consisting of only two nucleotides (either of which could be any of the four possible nucleotides) allows only 16 possible combinations (4 x 4 = 16). This would not be sufficient to specify all 20 of the amino acids. A codon consisting of three nucleotides, however, allows 64 possible combinations (4x4x4 = 64), more than enough to specify the 20 amino acids. 

Codon (Section 25.5C) A sequence of three bases on messenger RNA (mRNA) that contains the genetic information for one amino acid. The codon associates, by hydrogen bonding, with an anticodon of a transfer RNA (tRNA) that carries the particular amino acid for protein synthesis on the ribosome. 

Genetic code (Sections 25.5C and 25.5D) The correspondence of specific three-base sequences in mRNA (codons) that each code for a specific amino acid. Each codon pairs with the anticodon of a specific tRNA, which in turn carries the corresponding amino acid. 

The Genetic Code mRNA Codons and the Amino Acid Each Codon Directs... 

As described in the preceding sections protein synthesis involves transcription of the DNA to rtiRNA followed by translation of the mRNA as an amino acid sequence In addition to outlining the mechanics of transcription we have described the relationship among mRNA codons tRNA anticodons and ammo acids... 

When the cell requires instructions for protein production, part of the code on DNA, starting at an initiator and ending at a stop codon, is converted into a more mobile form by transferring the DNA code into a matching RNA code on a messenger ribonucleic acid (mRNA), a process known as transcription. The decoding, or translation, of mRNA then takes place by special transfer ribonucleic acids (tRNA), which recognize individual codons as amino acids. The sequence of amino acids is assembled into a protein (see Proteins section). In summary, the codes on DNA... 

Cellular protein biosynthesis involves the following steps. One strand of double-stranded DNA serves as a template strand for the synthesis of a complementary single-stranded messenger ribonucleic acid (mRNA) in a process called transcription. This mRNA in turn serves as a template to direct the synthesis of the protein in a process called translation. The codons of the mRNA are read sequentially by transfer RNA (tRNA) molecules, which bind specifically to the mRNA via triplets of nucleotides that are complementary to the particular codon, called an anticodon. Protein synthesis occurs on a ribosome, a complex consisting of more than 50 different proteins and several stmctural RNA molecules, which moves along the mRNA and mediates the binding of the tRNA molecules and the formation of the nascent peptide chain. The tRNA molecule carries an activated form of the specific amino acid to the ribosome where it is added to the end of the growing peptide chain. There is at least one tRNA for each amino acid. 

The genetic code (Table 28.3) is the message caiiied by mRNA. It is made up of triplets of adjacent nucleotide bases called codons. Because mRNA has only four different bases and 20 amino acids must be coded for, codes using either one or two nucleotides per amino acid are inadequate. If nucleotides are read in sets of three, however, the four mRNA bases generate 64 possible words, more than sufficent to code for 20 amino acids. 

Anticodon (Section 27.28) Sequence of three bases in a molecule of tRNA that is complementary to the codon of mRNA for a particular amino acid. 

Codon (Section 28.11) Set of three successive nucleotides in mRNA that is unique for a particular amino acid. The 64 codons possible from combinations of A, T, G, and C code for the 20 amino acids from which proteins are constructed. 

Three-base codons on die mRNA corresponding to specific amino acids direct the sequence of building a protein. These codons are recognized by tRNAs (transfer RNAs) carrying die appropriate amino acids. Ribosomes are the machinery for protein syn diesis. 

The specific ribonucleotide sequence in mRNA forms a message that determines the order in which amino acid residues are to be joined. Each "word," or codon, along the mRNA chain consists of a sequence of three ribonucleotides that is specific for a given amino add. For example, the series UUC on mRNA is a codon directing incorporation of the amino acid phenylalanine into the growing protein. Of the 43 = 64 possible triplets of the four bases in RNA, 61 code for specific amino acids and 3 code for chain termination, fable 28.1 shows the meaning of each codon. 

As each successive codon on mRNA is read, different tRNAs bring the correct amino acids into position for enzyme-mediated transfer to the growing... 

Figure 28.7 A representation of protein biosynthesis. The codon base sequences on mRNA are read by tRNAs containing complementary anticodon base sequences. Transfer RNAs assemble the proper amino acids into position for incorporation into the growing peptide.

H Translation is the process by which mRNA directs protein synthesis. Each mRNA is divided into codons, ribonucleotide triplets that are recognized by small amino acid-carrying molecules of transfer RNA (tRNA), which deliver the appropriate amino acids needed for protein synthesis. 

Anticodon (Section 28.5) A sequence of three bases on tRNA that reads the codons on mRNA and brings the correct amino acids into position for protein synthesis. 

Codon (Section 28.5) A three-base sequence on a messenger RNA chain that encodes the genetic information necessary to cause a specific amino acid to be incorporated into a protein. Codons on mRNA are read by complementary anticodons on tRNA. 

Most of the aforementioned efforts are based on the nonsense suppression methodology as the method for peptide altering [100]. In this approach, a nonsense codon is introduced into the enzyme-coding mRNA in the site that has to be altered. Simultaneously, the tRNA-noncoded amino acid hybrids are prepared with nonsense anticodons. Finally, the translation of modified mRNA is performed in vivo [101] or in vitro [102]. 

THE NUCLEOTIDE SEQUENCE OF AN mRNA MOLECULE CONSISTS OF A SERIES OF CODONS THAT SPECIFY THE AMINO ACID SEQUENCE OF THE ENCODED PROTEIN... 

Twenty different amino acids are required for the synthesis of the cellular complement of proteins thus, there must be at least 20 distinct codons that make up the genetic code. Since there are only four different nucleotides in mRNA, each codon must consist of more than a single purine or pyrimidine nucleotide. Codons consisting of two nucleotides each could provide for only 16 (4 ) specific codons, whereas codons of three nucleotides could provide 64 4 ) specific codons. 

Elongation is a cycUc process on the ribosome in which one amino acid at a time is added to the nascent peptide chain. The peptide sequence is determined by the order of the codons in the mRNA. Elongation involves several steps catalyzed by proteins called elongation factors (EFs). These steps are (1) binding of aminoacyl-tRNA to the A site, (2) peptide bond formation, and (3) translocation. 

The now deacylated tRNA is attached by its anticodon to the P site at one end and by the open GGA tail to an exit (E) site on the large ribosomal subunit (Figure 38-8). At this point, elongation factor 2 (EE2) binds to and displaces the peptidyl tRNA from the A site to the P site. In turn, the deacylated tRNA is on the E site, from which it leaves the ribosome. The EF2-GTP complex is hydrolyzed to EF2-GDP, effectively moving the mRNA forward by one codon and leaving the A site open for occupancy by another ternary complex of amino acid tRNA-EFlA-GTP and another cycle of elongation. 

---