Codon chain termination

The terms first, second, and third nucleotide refer to the individual nucleotides of a triplet codon. U, uridine nucleotide C, cytosine nucleotide A, adenine nucleotide G, guanine nucleotide Term, chain terminator codon. AUG, which codes for Met, serves as the initiator codon in mammalian cells and encodes for internal methionines in a protein. (Abbreviations of amino acids are explained in Chapter 3.)... 

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)... 

Termination. When a chain termination codon is reached, no aminoaeyl tRNA is available that can fill the A site, so chain elongation stops. Since the polypeptide chain is still attached to the tRNA oeeupying the P site, release of the protein is... 

Hemoglobins Havingan Extended a-Globin Chain Due to Mutation of the Chain Termination Codon... 

The answer is e. (Murray, pp 452-467. Schver, pp 3-45. Sack, pp 245-257. Wilson, pp 151—180.) Chain termination is determined by three codons UAA, UAG, and UGA. Aside from chain termination codons, each group of three bases in a sequence codes for an amino acid. The next three bases specify another amino acid. Thus, the genetic code is nonoverlap-... 

The amino acid symbols are given in the table entitled Structures of Common Amino Acids A chain-initiating codon is indicated by init and a chain-terminating codon by term. 

Since this sequence specifies the carboxyl end of the peptide chain, it must contain one or more of the chain-termination codons UAA, UAG, or UGA. UAA and UAG occur in tandem in the sequence, so we can infer the reading frame. The mapping of the amino acid residues to the mRNA is as follows ... 

The molecular defect has recently been linked [7] to a single base substitution, an A—transversion, in the penultimate 3 nucleotide of the third intron of the Apo E gene. This leads to a loss of the correct 3 splice site, thus giving rise to two abnormally spliced mRNA forms. The smaUer form contains 53 nucleotides and the larger one, the entire third intron of the gene. Since both mRNA species contain chain termination codons within the intronic sequence, only short Apo E peptides not detectable by standard gel electrophoretic techniques are produced. Apo E deficiency is, therefore, the result of a molecular error which gives rise to shorter, nonfunctional forms of Apo E. In contrast to Apo B, where the mechanism is posttranslational, here it is clearly pretranslational. 

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