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Biosynthesis of Cellular Components Biosynthesis of Capsular Materials Peptidoglycan Biosynthesis Antibiotics Affecting Cell Wall Synthesis Teichoic Acid Gram Negative Bacterial Cell Envelope Cytoplasmic Membrane Informational Macromolecules Deoxyribonucleic Acid (DNA) Ribonucleic Acid (RNA) Properties of RNA

Breaking the Genetic Code

	Breaking the Genetic Code - Once it was clear that m-RNA on ribosomes guides the amino acid insertion, it was of interest to understand the sequence of nucleotides that code for each amino acid. This aspect of breaking the genetic code was first reported by Marshall Nirenberg and J.H. Mathaei in 1961 who found that ill extracts of E. coli, the natural m-RNA could be replaced by synthetic m-RNA of known sequences. By doing this, the sequence of nucleotides that code for amino acids was revealed. The first synthetic polynucleotide used was polyuridylic acid (poly U) which was found to guide the polymerization of phenylalanine into polyphenylalanine. These experiments were extended by Khorana and others using synthetic messengers of known sequences and the sequence of nucleotides that direct the insertion of every known amino acid was established.
The general features of the code as understood today are: (i) The code is nonover lapping, i.e. during the translation of m-RNA, a sequence of three nucleotides are read without an overlap. (ii) The code is universal, that is, a set of nucleotides that code for an amino acid in one system does so in all biological systems tested so far, and (iii) the code is ambiguous
Thus, it was concluded that a change in the amino acid sequence occurs as a consequence of a mutation and replacement of one amino acid by another occurs as a result of replacing one base by another. The amino acid replacement in a variety of proteins from bacteria, yeast and animals have now been attributed to changes in the codon sequence.The codons (a codon is a set of three nucleotides that codes for a amino acid) arrangement is based on a aminoacyl t-RNA-synthetic m-RNA-ribosome binding technique first developed by Nirenberg and his colleagues. By 1972. the complete nucleotide sequence of a gene that codes for the coat protein of the bacterial virus MS2 (single stranded RNA virus) and the complete amino acid sequence of the protein that it directs was reported. The data was in full agreement with the evidence derived by the use of synthetic nucleotides.
The discovery of the m-RNA as an intermediate in the flow of information raised questions as to how the nuc1eotides in the m-RNA guide the amino acids into peptides. It was soon found that there are RNA molecules which are specific for different amino acids and are responsible for the transfer of the amino acid to the ribosomes. These molecules are known as the transfer RNA (t-RNA) molecules and these act as adapters for fitting the correct amino acids in the peptide directed by the m-RNA. The amino acids which are first attached to the t-RNA are then arranged in the correct order into the peptide under the influence of several other protein factors and enzymes.
	The discovery of t-RNA and the reaction by which an amino acid becomes attached to it came from M.B. Hoagland. He found that each amino acid first reacts with A TP, catalysed by a specific activating enzyme , to give an activated amino acid. This activated amino acid does not separate from the enzyme but immediately reacts with a molecule of t-RNA that is specific for that particular amino acid. This reaction is also catalysed by the same enzyme (aminoacyl-t-RNA synthetase) to give an aminoacyl-t-RNA The sequence and the structure of t-RNAs is now well established. All have a clover leaf structure and the anticodon for a particular amino acid is always found in the central loop. The other loops contain sequences common to all t-RNAs suggesting common function in binding the t-RNA ribosome-m-RNA complex. Interestingly, once the amino acid is linked to the 3 OH end of adenine, t-RNA recognizes its correct triplet on the m-RNA.

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DNA Replication Enzymology of DNA Replication Properties of E.Coli DNA Polymerases Biosynthesis of RNA Transcription Proteins Protein Structure Protein Synthesis Amino Acids Genetic Code Breaking the Genetic Code List of Genetic Code

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