Genes: DNA's instructions
Assembling the protein chain
The genetic code is read by 'adaptor' molecules called transfer RNAs (tRNAs). These deliver amino acids to the ribosomes according to the sequence of the mRNA.
Each tRNA molecule is attached specifically to one of the 20 amino acids and three critical bases recognize the complementary codon in the mRNA. As the tRNAs bind and release, the amino acids on adjacent tRNAs are joined to form a growing amino acid chain.
Each codon on the mRNA molecule is read, one at a time. For each codon, the tRNA molecule with the complementary anticodon temporarily binds to the mRNA. The amino acid that is joined to the end of the tRNA molecule is brought in line with the growing polypeptide chain, and the amino acid links to the end of that chain. Once their amino acid is added, the tRNAs disengage from the mRNA molecule, leaving the next codons on the mRNA molecule to be 'read'.
Folding: Putting proteins to work
Proteins carry out most of active functions of a cell. From the DNA, we can recognize and read off the amino acids that make up the proteins in our bodies. However, we still only understand a little about how the chain of amino acids becomes a working protein. The three dimensional (3D) structure of a protein (such as the structure of the hormone, insulin, at right) is difficult to predict just from the DNA sequence.
Many proteins are enzymes; these biological catalysts enable or speed up chemical reactions in the cell. Each enzyme has a region into which two or more chemicals fit snugly. This region is called the 'active site'. reactions between components occur through binding first to the active site of the enzyme. Once the reaction occurs, the shapes of the components change so they vacate the active site, leaving the enzyme free to catalyse another reaction.
