Triplet code

For what amino acids do the following ribonucleotide triplets code ... 

An opening frame contains a series of codons (base triplets) coding for amino acids without any termination codons. There are six potential reading frames of an unidentified sequence. 

It is now known that each codon consists of a sequence of three nucleotides ie, it is a triplet code (see Table 38—1). The deciphering of the genetic code depended heavily on the chemical synthesis of nucleotide polymers, particularly triplets in repeated sequence. 

To transcribe information from DNA to mRNA, one strand of the DNA is used as a template. This is called the anticoding, or template, strand and the sequence of mRNA is complementary to that of the template DNA strand (Fig. A2.8) (i.e., C->G, G->C, T->A, and A U note that T is replaced by U in mRNA). The other DNA strand, which has the same base sequence as the mRNA, is called the coding, or sense, strand. There are 64 (4 x 4 x 4) possible triplet codes of the four bases 61 are used for coding amino acids and three for termination signals. As there are 20 amino acids for the 61 codes, some triplets code for the same amino acid. A table of the genetic code is presented in Exhibit A2.2. 

In order that tRNA molecules can function as adaptors between amino acids and mRNA, two things must be true. First, there must be a mechanism for attaching each amino acid specifically to its cognate tRNA. Doing so will create molecules known as aminoacyl-tRNAs. Secondly, each aminoacyl-tRNA must recognize the correct triplet code for that amino acid on mRNA. 1 am going to worry about the former issue first. 

Cenetic code the relationship between triplets of bases in mRNA and the corresponding amino acids in proteins (or a termination signal) for which the triplet codes. 

A set of coding rules are in action as in the translation process. First, a set of three adjacent nucleotides compose the code for each amino acid. A single amino acid can have several triplet codes or codons. Since there are four different nucleotides (or four different bases) in DNA and RNA, there exist 4 = 64 trinucleotide combinations. For instance, using U as a symbol for uracil, which is present in RNA, the triplet or code or codon UUU is specific for phenylalanine. 

Each triplet code, the codon, in the messenger RNA specifies a different amino acid, but there are more possible codons (64) than amino acids (20), and several codons can specify the same amino acid. To each codon, one molecule of transfer RNA becomes attached via a complementary anticodon. These transfer RNA molecules carry a single, specific amino acid. In this way a protein, comprising a chain of amino acids, can be built up (Fig. 6.38). 

For instance, if such a substitution is induced in the tobacco mosaic virus genome, changes in the viral protein can be detected, which correlate with the mutation. This has been made possible because the viral protein has been characterized. Thus, it is found that the amino acid threonine is replaced by isoleucine. It is also clear that several triplet codes in the nucleic acid specify the amino acid threonine, namely AC A, AGG, ACC, and ACU, emphasizing the degeneracy of the code. 

This type of mutation arises from the addition or deletion of a base to the nucleic acid molecule. This puts the triplet code of the genome out of sequence. Consequently, transcription of the information is erroneous when it is carried out distal to the mutant addition or deletion. In illustration of this, the following code makes sense when read in groups of three ... 

DNA triplet code RNA triplet code Amino acid specified... 

As any elementary textbook on molecular biology will relate, the sequences of proteins are stored in DNA in the form of a triplet code. Each amino acid is encoded by one or more triplet combinations of the four bases A, T, G, and C. For example, tryptophan is coded by the sequence TGG. The sequence of triplets is converted into a protein by a process in which DNA is first transcribed into mRNA. This message is then translated into protein on the ribosomes in conjunction with tRNA and the aminoacyl-tRNA synthetases. In prokaryotes, there is a one-to-one relationship between the sequence of triplets in the DNA. and the sequence of amino acids in the protein. In eukaryotes, the DNA often contains stretches of intervening sequences or introns which are excised from the mRNA after transcription (Chapter 1). 

OH group. (2) it must recognize the proper triplet on the messenger RNA-ribosome aggregate. Having these properties, the transfer RNA accepts or forms an intermediate transfer RNA-aminn acid thal finds its way to the polysome, complexes at a triplet coding for the activated amino acid, and allows transfer of the amino acid into peptide linkage. 

Transfer (tRNA) and ribosomal RNA (rRNA) are transcribed and used in protein synthesising processes. Messenger RNA (mRNA) codes the amino-acid sequence of proteins and 95 per cent of the total DNA transcribed is used for this purpose. In prokaryotes a single mRNA molecule may code for a single polypetide or for two or more polypeptide chains. There is a triplet code for each amino-acid 300 ribonucleotides code for a 100 amino-acid sequence. Fig. 5.All shows the relationship between the nucleotide sequence on DNA and RNA and the amino-acid sequence of protein. 

Professor Eigen s parameters of excess production, specificity, and error frequency are very useful in describing the process of change from a U-I code to the four letter triplet code we know today. 

The evolutionary change, from a purine-pyrimidine code with only the first two letters encoding sense, to a triplet code is a further evolutionary development which fits Eigen s predicted progress toward increasing specificity/efficiency and toward decreasing tolerance of errors. In the... 

Polynucleotide phosphorylase was used to produce RNA of random sequence, the composition of which reflected the mixture of nucleoside diphosphates in the reaction mixture. Mixed polynucleotides containing two bases were used in the incorporating system and shown to incorporate a pattern of amino acids consistent with a triplet code, but the observed incorporation could not define the code sequence. 

A triplet code, one made from four nucleotides taken three at a time, generates a total of 64 different triplet sequences or codons. Three of these codons, as we will see, are utilized to terminate translation and are not generally used to specify amino acids. The remaining 61 codons and the 20 amino... 

The genetic code During translation, the sequence of an mRNA molecule is read from its 5 end is a triplet code by ribosomes which then synthesize an appropriate polypeptide. Both in prokaryotes and eukaryotes, the DNA sequence of a single gene is colinear with the amino acid sequence of the polypeptide it encodes. In other words, the nucleotide sequence of the coding DNA strand, 5 to 3, specifies in exactly the same order the amino acid sequence of the encoded polypeptide, N-terminal... 

During the codon recognition phase another amino acid, for example glycyl-tRNA which has the necessary, correct triplet code moves into the aminoacyl position on the 60s ribosome and bonds with the mRNA and the 40s ribosome when the necessary co-factors, Mg2+, elF 1, elF 2, and GTP, are present. 

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