Codon stop sequences

Fig. 2. Attenuation of the trp operon. (a) When tryptophan is plentiful, sequences 3 and 4 base-pair to form a 3 4 structure that stops transcription (b) when tryptophan is in short supply, the ribosome stalls at the trp codons in sequence 1, leaving sequence 2 available to interact with sequence 3. Thus a 3 4 transcription terminator structure cannot form and transcription continues.

Selenium is incorporated into Se-requiring enzymes by the modification of serine. This serine is not modified when it is in the free state or when it occurs in a polypeptide chain. The serine residue in question is modified when it occurs boimd to transfer RNA, that is, eis the aminoacyl-tRNA derivative. Seryl-tRNA is converted to selenocysteinyl-tRNAby the action of selenocysteine synthase (Stur-chler et al, 1993). The codon for selenocysteine is UGA (TGA in DNA UGA in mRNA). The fact that this particular triplet of bases codes for an amino acid is very imusual, as UGA normally is a stop codon. Stop codons occur in mRNA and signal the termination of synthesis of the protein however, in the case of the UGA codons that code for selenocysteine residues, regions of the mRNA that lie beyond the coding sequence somehow convert the UGA from a codon that halts translation to one that codes for selenocysteine (Figure 10.55). The structure of selenocysteine is shown in Figure 10.56. 

Premature stop codon polymorphisms, in which there is premature termination of the polypeptide chain by a stop codon (specific sequence of three nucleotides that do not code for an amino acid but rather specify polypeptide chain termination)... 

C4 metabolism a photosynthetic pathway that produces a four-carbon molecule and avoids photorespiration in eukaryotic photo-synthesizing organisms coding strand the DNA strand that has the same base sequence as the RNA transcript (with thymine instead of uracil) codon a sequence of three nucleotides in mRNA that directs the incorporation of an amino acid during protein synthesis or acts as a start or stop signal coenzyme a small organic molecule required in the catalytic mechanisms of certain enzymes... 

Termination codon, stop codon, punctuation codon a sequence of three nucleotides in mRNA, which signals the end of polypeptide synthesis and release of the polypeptide in the process of Protein biosynthesis (see). 5 -tlAA (see Ochre codon), 5 -UAG (see Amber codon) and UGA are T.c. 

Analysis of tlie global statistics of protein sequences has recently allowed light to be shed on anotlier puzzle, tliat of tlie origin of extant sequences [170]. One proposition is tliat proteins evolved from random amino acid chains, which predict tliat tlieir length distribution is a combination of the exponentially distributed random variable giving tlie intervals between start and stop codons, and tlie probability tliat a given sequence can fold up to fonii a compact... 

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... 

As shown in Figure 45.1, the bases appear in complementary pairs, A with T and G with C in this particular example, the sequence for one strand of DNA is A-T-C-G-T- while the other strand is -T-A-G-C-A-. The sequences of the bases attached to the sugar-phosphate backbone direct the production of proteins from amino acids. Along each strand, groups of three bases, called codons, correspond to individual amino acids. For example, in Figure 45.1, the triplet CGT, acting as a codon, would correspond to the amino acid serine. One codon, TAG, indicates where synthesis should begin in the DNA strand, and other codons, such as ATT, indicate where synthesis should stop. 

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... 

The codon UAA stops protein synthesis. Why does the sequence UAA in the following stretch of mRNA not cause an) problems ... 

The complete sequence of the pgaX gene has been determined. Seven introns are present in the gene which were all confirmed by cDNA sequencing. The introns are between 50 and 58 nt long, have fungal consensus splice sequences and all but one have in frame stop codons. 

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