Transfer RNA

The ribonucleic acid (RNA) that is directly engaged in the translation of the sequence of nucleotides in messenger RNA to amino acid sequences for the construction of proteins is called transfer RNA or commonly tRNA. The manufacture of the tRNA itself is directed by the DNA in the cell that provides a pattern for the production of RNA by "transcription".

Below is an illustration of the composition of an example of tRNA in what is commonly called its cloverleaf form. It is a two-dimensional projection of the form of the molecule. It's actual three-dimensional shape is more complex, but this is a common way to depict it to emphasize its function of binding an amino acid to one end corresponding to the anticodon on the opposite end. This anticodon will bind to a codon consisting of three nitrogenous bases which specify an amino acid according to the genetic code. This illustration is patterned after an example in Rana. This tRNA was investigated in yeast and its structure was the culmination of seven years of work by Robert Holley of Cornell University.

When the messenger RNA blueprint for a protein reaches a ribosome for the process of building a protein by translation of that blueprint, tRNA molecules with all the required amino acids must be present for the process to proceed. Since most proteins use all twenty amino acids, all must be available, attached to appropriate tRNA molecules. In this example, the tRNA has bonded to the codon GCC on the mRNA, GCC being one of the codons corresponding to alanine. This leads to the placement of alanine in the proper place in the growing polypeptide chain that is to become a protein.

References:
Wiki tRNA

How are the amino acids correctly assigned to tRNA molecules?

The assignment of the correct amino acid to each of the forms of tRNA is crucially important to the translation process. This assignment is accomplished by twenty different enzymes called aminoacyl-tRNA synthetases (aaRS) for the twenty amino acids that are incorporated into proteins. There are thought to be 31 different tRNAs, but these 20 synthetases are capable of "charging" all of them with the correct amino acid.

How many distinct tRNAs are required?

In the genetic code, codons made of of three bases specify an amino acid. With three bases, there are 64 possible permutations. With three codons corresponding to STOP codons, this leaves 61 combinations that code for an amino acid. This would require 61 distinct tRNA species were it not for the "wobble base".