UAA termination codon

A potential stem-loop transcription termination signal is found in the region [position 22-41] downstream from the 5 -UAA termination codon. 

The mRNA is read continuously from a start codon (AUG) to a termination codon (UAA, UAG, UGA). 

Deactivation of an RF is feasible, however, in prokaryotes. In E. coli, three RFs are involved in the termination process, and there is a degree of codon specificity for two of them. RFl recognizes the UAG and UAA stop codons, while RF2 recognizes the UGA and UAA stop codons [16, 42]. RFS, which is not codon specific, stimulates the activity of the other two. Thus, if RF 1 is deactivated, UAA and UGA remain functional termination codons, but UAG should no longer signal for termination of protein synthesis. To test this hypothesis, the Chamberlin lab investigated a mutant strain of E. coli that produces a faulty RFl [29]. 

The final step in protein biosynthesis is chain termination. Natural mRNA molecules contain termination codons UAA, UGA, or UAG There are no tRNAs that have anticodons which are complementary to these codons. When the growing peptide chain encounters one of these termination codons, the peptidyl-tRNAis transferred to water instead of another aminoacyl-tRNA. The peptidyl-tRNA is hydrolyzed to free the completed protein and the tRNA. Chain termination completes protein synthesis. 

The ribosome can carry two aminoacyl-tRNAs simultaneously. In the chain elongation stage, the growing polypeptide is carried on one of these tRNAs. The chain is transferred to the second tRNA, which adds its amino acid to the growing peptide, and displaces the first tRNA. The ribosome then moves one codon along the mRNA to allow the next to be read. Termination of protein synthesis involves the release of the completed polypeptide, expulsion of the last tRNA, and dissociation of the ribosome from the mRNA. This is signaled by specific termination codons (UAA, UAG, or UGA) in the mRNA and requires the participation of various release factors. 

Termination ("stop" or "nonsense ) codons Three of the codons, UAG, UGA, and UAA, do not code for amino acids, bit rather are termination codons. When one of these codons appears in an mRNA sequence, it signals that synthesis of the peptide chain coded for by that mRNA is completed. 

Nonsense mutation The codon containing the changed base may become a termination codon. For example, if the serine codon UCA is given a different second base—A—to become UAA, the new codon causes termination of translation at that point. The creation of a termination codon at an inappropriate place is called a "nonsense" mutation. 

In intestinal epithelial cells the same apoB gene that is used to synthesize apoB-100 in the liver is used to make the shorter apoB-48 (48%) protein. This is accomplished in an unusual way that involves "editing" of the mRNA that is formed. Codon 2153 in the mRNA for the protein is CAA, encoding glutamine. However, the cytosine of the triplet is acted on by a deaminase, an editing enzyme, to form UAA, a chain termination codon.14 15 A third form of apoB is found... 

In addition to those codons assigned to specific amino acids, three are designated as chain termination codons UAA, UAG, and UGA. These are frequently referred to as "nonsense" codons. The termination codons UAA and UAG are also known as ochre and amber, respectively, although these names have no scientific significance.41 The codons AUG (methionine)... 

In E. coli, termination codons that arrive at the A site on the ribosome are recognized by one of three protein release factors, RF-1 recognizes UAA and UAG, and RF-2 recognizes UAA and UGA. The third release factor, RF-3, does not itself recognize termination codons but stimulates the activity of the other two factors. 

The genetic code is the rules that specify how the nucleotide sequence of an mRNA is translated into the amino acid sequence of a polypeptide. The nucleotide sequence is read as triplets called codons. The codons UAG, UGA and UAA do not specify amino acids and are called termination codons or Stop codons. AUG codes for methionine and also acts as an initiation (Start) codon. 

The genetic code is not universal but is the same in most organisms. Exceptions are found in mitochondrial genomes where some codons specify different amino acids to that normally encoded by nuclear genes. In mitochondria, the UGA codon does not specify termination of translation but instead encodes for tryptophan. Similarly, in certain protozoa UAA and UAG encode glutamic acid instead of acting as termination codons. 

Eventually, one of three termination codons (also called Stop codons) becomes positioned in the A site (Fig. 7). These are UAG, UAA and UGA. Unlike other codons, prokaryotic cells do not contain aminoacyl-tRNAs complementary to Stop codons. Instead, one of two release factors (RF1 and RF2) binds instead. RF1 recognizes UAA and UAG whereas RF2 recognizes UGA. A third release factor, RF3, is also needed to assist RF1 or RF2. Thus either RF1 + RF3 or RF2 + RF3 bind depending on the exact termination codon in the A site. RF1 (or RF2) binds at or near the A site whereas RF3/GTP binds elsewhere on the ribosome. The release factors cause the peptidyl transferase to transfer the polypeptide to a water molecule instead of to aminoacyl-tRNA, effectively cleaving the bond between the polypeptide and tRNA in the P site. The polypeptide, now leaves the ribosome, followed by the mRNA and free tRNA, and the ribosome dissociates into 30S and 50S subunits ready to start translation afresh. 

Termination Termination codons (UAG, UAA, or UGA) recognized by RF-1, RF-2, and RF-3 release factors Termination codons (UAG, UAA, or UGA) recognized by eRF release factor... 

N-terminal analysis of the peptide gave IGRTWGSADK (measured molecular mass = 1,091.8 Da theoretical = l,091.2Da). Therefore, the C-terminus of the protein has been extended with the peptide WGSADK (molecular mass = 663.3 Da). This sequence matches perfectly with the translated cloned DNA sequence, before another in-frame termination codon (UAA) was met (Figure 5). 

It is of great interest that the Hb-Tak chain is produced in amounts similar to that of most p chain variants. The presence of a threonyl residue in position 147 is unexpected if one assumes that the formation of this p chain is due to a mutation of a terminating codon UAA or UAG. The study of this hemoglobin might well be of importance for a better insight into the mechanism of normal chain termination and in the nature of the DNA between two genes. The presence of Hb-Tak is not associated with any major hematological disorder. 

When synthesis of the polypeptide is completed, a termination codon (UGA, UAG, or UAA) causes the polypeptide chain to be released. 

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