Transfer Rna
Transfer RNA (tRNA) is often referred to as the "Rosetta Stone" of genetics, as it translates the instructions encoded by DNA, by way of messenger RNA (mRNA), into specific sequences of amino acids that form proteins and polypeptides. This class of small globular RNA is only 75 to 90 nucleotides long, and there is at least one tRNA for every amino acid. The job of tRNA is to transport free amino acids within the cell and attach them to the growing polypeptide chain. First, an amino acid molecule is attached to its particular tRNA. This process is catalyzed by an enzyme called aminoacyl--tRNA synthetase that binds to the inside of the tRNA molecule. The molecule is now charged. The next step, joining the amino acid to the polypeptide chain, is carried out inside the ribosome. Each amino acid is specified by a particular sequence of three nucleotide bases called codons. There are four different kinds of nucleotides in mRNA. This makes possible 64 different codons (43). Two of these codons are called STOP codons; one of these is the START codon (AUG). With only 20 different amino acids, it is clear that some amino acids have more then one codon.
This is referred to as the degeneracy of the genetic code. On the other end of the tRNA molecule are three special nucleotide bases called the anticodon. These interact with three complimentary codon bases in the mRNA by way of hydrogen bonds. These weak directional bonds are also the force that holds together the double strands of DNA.
In order to understand how this happens it was necessary to first understand the three dimensional structure (conformation) of the tRNA molecule. This was first attempted in 1965, where the two-dimensional folding pattern was deduced from the sequence of nucleotides found in yeast alanine tRNA. Later work (1974), using x-ray diffraction analysis, was able to reveal the conformation of yeast phenylalanine tRNA. The molecule is shaped like an upside-down L. The vertical portion is made up of the D stem and the anti-codon stem, and the horizontal arm of the L is made up of the acceptor stem and the T stem. Thus, the translation depends entirely upon the physical structure. At one end of each tRNA is a structure that recognizes the genetic code, and at the other end is the particular amino acid for that code. Amazingly, this unusual shape is conserved between bacteria plants and animals.
Another unusual thing about tRNA is that it contains some unusual bases. The other classes of nucleic acids can undergo the simple modification of adding a methyl (CH3-) group. However, tRNA is unique in that it undergoes a range of modifications from methylation to total restructuring of the purine ring. (There are two types of bases, purines and pyrimidines.) These modifications occur in all parts of the tRNA molecule, and increase its structural integrity and versatility.
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