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Index >>Ribose Nucleic Acids >>Structure of Transfer RNA - tRNA

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Ribose Nucleic Acids Comparison Between DNA and RNA Ribosomal RNA (rRNA) Characteristics of Different Ribosome Types Types of rRNA Synthesis of rRNA Messenger RNA (mRNA) Size of mRNA Structure of mRNA Stability of mRNA Synthesis of Messenger RNA - mRNA Polyadenylation Capping in mRNA Methylation in mRNA Heterogenous Nuclear RNA (hnRNA)

Structure of Transfer RNA - tRNA

	Structure of Transfer RNA - tRNA - The nucleotide sequence (primary structure) of tRNA was first worked out by Holley et al (1965) for yeast alanine tRNA. Since then the sequence of about 75 different tRNAs, ranging from bacteria to mammals, has been established. The different tRNAs are all minor variants of the same basic type of structure. Several models of the secondary structure of tRNA have been proposed, and of these the cloverleaf model of Holley is the most widely accepted. According to the cloverleaf model the single polynucleotide chain of tRNA is folded upon itself to form 5 arms. As a result of the folding the 3' and the 5' ends of the chain come near each other. An arm consists of a stem and a loop.
In the double helical stems there is internal Watson Crick base pairing which follows the A-V and G-C combinations, except for an occasional G-U base pair or a mismatch. There is no base pairing in the loops. One of the arms bas a stem but not a loop and is called the acceptor stem. The other arms are called the D arm, the anticodon arm, the variable arm and the TψC arm. The variable arm mayor may not have a stem. The acceptor stem consists of 7 base pairs and 4 unpaired nucleotide units. The latter include a constant 3' terminal -CCA sequence and a fourth nucleotide which is a variable purine (A or G). The amino acid molecule attaches to the 3' terminal of the -CCA sequence, which is' known as the amino acid binding site. The 5' end of tRNA is either guanine (G) or cytosine (C).
The second arm is called the D arm. It consists of 15-18 nucleotides with 3-4 base pairs in the stem and 7-11 unpaired nucleotides in the loop. The loop of the D arm is called Loop I or dihydrouridine (DHU) loop or the D loop. It contains two variable regions, α and β on either side of two constant guanine residues. These regions consist of 1-3 nucleotides, mostly pyrimidines, with a high proportion of DHU. The synthetase site which recognizes the amino acid activating enzyme is located on a part of the D loop and a part of the acceptor stem on the 5' side. The third arm or anticodon arm consists of an anticodon stem of 5 base pairs and a loop, called Loop II or the anticodon loop. This loop consists of 7 unpaired nucleotides of which the middle three form the anticodon.
The anticodon recognises the 3 complementary bases which constitute the codon of mRNA. On the 3' side of the anticodon is a hypermodified purine (H) while on the 5' side is U and a pyrimidine (Y). The variable arm (the "lump", the miniloop, Loop III) is of two types. In one type there is a loop containing 4-5 bases but no stem. In the other type, the arm consists of 13-21 residues, and both the stem and the loop can be distinguished. The TψC arm consists of a stem having 5 base pairs and a loop of 7 nucleotides. The outermost of the 5 pairs of the stem is C-G. The TψC loop contains a constant TψC sequence. All tRNAs have a ribosome recognition site on the TψC loop consisting of a G-T-ψ-C-R sequence.

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Difference Between Prokaryotes and eukaryotes mRNA Transfer RNA (tRNA) OR Soluble RNA (sRNA) Structure of Transfer RNA - tRNA Unusual Bases in tRNA Classification of tRNA Invariant and Semi invariant Nucleotides Tertiary Structure of tRNA Initiator tRNA Specificity of tRNA Biosynthesis of tRNA Comparison of Different Types of RNA

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