Each consecutive triplet of mRNA bases, called codon, determines a certain amino acid in the translation procedure. Amino acids are the monomers of proteins, just as the nucleotides are the monomers of DNA and RNA molecules. In eukaryotes, the mRNA is formed of coding regions flanked by non-coding regions. Coding regions in the genic DNA are the regions (exons or parts of exons) used for the protein creation, while the non coding regions - 3' untranslated region and 5' UTR - are mostly regulatory and are not translated. Note, that along the DNA, the coding region may not be contiguous, as it might span several exons. In prokaryotes, a gene has only one coding region, flanked by the 3' UTR and the 5' UTR. The process of synthesizing a protein from an RNA molecule is accomplished by cellular constructs called ribosomes. A ribosome attaches itself to the mRNA molecule and creates an amino acid chain by repeatedly elongating it, one amino acid at a time. This is done using molecules called transfer RNA (tRNA). These molecules are small RNA molecules each serving as an adapter between mRNA and amino acids. This molecule is composed of two parts. On one part the tRNA holds an anticodon. The anticodon is a sequence of three RNA bases. On the other side, the tRNA holds an amino acid. The many-to-one mapping from anticodons to amino acids defined by tRNA molecules is the universal genetic code. The tRNAs carry the amino acids to the ribosome, where they can be attached to a growing chain of amino acids, a.k.a. a polypeptide. The Ribosome moves along the mRNA, so that successive codons are brought into position for their respective amino acids. Many ribosomes can simultaneously translate a single mRNA molecule, as shown in figure 1.4.
