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Transfer RNA

It was postulated that nucleic acids serve as a direct template for the amino acids to be ordered in a polypeptide chain. Francis Crick suggested that DNA and mRNA do not serve as templates for amino acid sequences, but carry the codes that can be translated. Hence he proposed the existence of some adapter molecules which can be used for decoding the messages. Soon adapter molecules were discovered in the soluble phase of the cytoplasm and called transfer RNA molecules.

It become known that each type of tRNA molecule is able to recognize two different types of molecules, the mRNA and the amino acid. There are  about 60 types of tRNAs, each having a molecular weight of about 25,000. In order to understand the mechanism  of recognition of amino acid and mRNA by a tRNA molecule, we need to discuss the structure of tRNA molecules.

In 1965, Robert Holly reported the structure of alanine tRNA from yeast .This molecule has 77 nucleotides, 10 of which are different and don not conform to the four standard nucleotides( A,G,C and U). In later years many more tRNA were discovered with nucleotides between 73 and 93. All tRNAs have a –C–C–A–OH sequence at the 3’ terminal which is called the amino acid acceptor terminal. The remaining part of the molecule shows considerable base pairing due to complementary regions that give rise to a clover-leaf pattern. The unusual bases contain one or more than one methyl group in the ring that do not form hydrogen bonds. the loop opposite the amino acid terminal contains the anticodon which base pairs with the mRNA triplets. The anticodon end (loop) is composed of seven bases, the middle three forming the anticodon.

In certain cases the tRNA molecule forms a three-dimensional structure which is achieved by extensive hydrogen bonding not only between complementary positions, but also between different atoms at other positions in the bases. These are tertiary tRNA structure which exist in L-shapes.
In order to hook the right kind of amino acid, tRNAs are distinguished from one another by enzymes. Since there are 20 amino acids and about 60 species of tRNA molecules, it follows that a single amino acid can be hooked up by more than one type of tRNA. Consequently, more than one triplet is available for recognizing each amino acid.

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