Translation codons

Figure 1 Structure of the mouse DOR gene. The bottom line represents mouse genomic DNA encompassing the DOR gene from 390 bp upstream of the ATG translation codon in exon 1 to 1.24 kb downstream of the TGA translation stop codon in exon 3. The 1.8-kb DOR-1 cDNA molecule is drawn above the DOR gene, relative to a fully spliced, undegraded DOR message (top line). (From Ref. 16.)...

As was evident in the previous section, the structural characteristics of the mRNA transcripts play a significant role in translation. An overwhelming majority of the structurally dependent mechanisms of translation control dealt with inhibition or improvement of ribosomal access to the initiation sites or altering the rate at which ribosomes scan across the transcripts. This section aims to complement the previous discussion by considering a more fundamental aspect of translation -codon usage. 

Table 2. Categories and Names of the Most Common Amino Acids, Their Abbreviations, and Respective Translatable Codons in mRNA Abbreviations...

Name Three letter One letter Translatable Codons... 

Adenia racemosa, ArHNL (NH )2S0 fractionation and chromatography, tryptic digest, sequencing, back-translation, codon usage of yeast 12.4 (S) E. coli, P. pastoris [69]... 

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... 

FIGURE 28 12 Translation of mRNA to an ammo acid sequence of a protein starts at an mRNA codon for methionine Nucleophilic acyl substitution transfers the N formylmethionme residue from Its tRNA to the ammo group of the next ammo acid (shown here as alanine) The process converts an ester to an amide... 

Section 28 11 Three RNAs are involved m gene expression In the transcription phase a strand of messenger RNA (mRNA) is synthesized from a DNA tern plate The four bases A G C and U taken three at a time generate 64 possible combinations called codons These 64 codons comprise the genetic code and code for the 20 ammo acids found m proteins plus start and stop signals The mRNA sequence is translated into a prescribed protein sequence at the ribosomes There small polynucleotides called... 

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... 

A potentially general method of identifying a probe is, first, to purify a protein of interest by chromatography (qv) or electrophoresis. Then a partial amino acid sequence of the protein is deterrnined chemically (see Amino acids). The amino acid sequence is used to predict likely short DNA sequences which direct the synthesis of the protein sequence. Because the genetic code uses redundant codons to direct the synthesis of some amino acids, the predicted probe is unlikely to be unique. The least redundant sequence of 25—30 nucleotides is synthesized chemically as a mixture. The mixed probe is used to screen the Hbrary and the identified clones further screened, either with another probe reverse-translated from the known amino acid sequence or by directly sequencing the clones. Whereas not all recombinant clones encode the protein of interest, reiterative screening allows identification of the correct DNA recombinant. 

Translation of the Foreign Gene. The translation of a mRNA into a protein is governed by the presence of appropriate initiation sequences that specify binding of the mRNA to the ribosome. In addition, not all the codons of the genetic code are used equally frequently by all organisms. 

Efficient translation depends on matching the preferred pattern of host codon usage in the heterologous gene. 

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. 

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. 

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]. 

There may be no detectable effect because of the degeneracy of the code. This would be more likely if the changed base in the mRNA molecule were to be at the third nucleotide of a codon such mutations are often referred to as silent mutations. Because of wobble, the translation of a codon is least sensitive to a change at the third position. 

---