Two different ways of representing metabolic pathways. (a) The 'old view' in which the metabolites hold 'center stage'. The names of enzymes (in yellow boxes) are written above reaction arrows that show the chemical transformation of reactants (red circles; representing one or more co-reactants) to new metabolites. These can often be detected, characterized, and quantified by physical and chemical techniques, most notably in recent years by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. (b) The modern 'genome-centric view' of metabolism in which the enzymes (gene products themselves) hold 'center stage'. Note that the metabolic pathway is represented as a string of enzymes (E1 to En), with the metabolites entering and leaving above the arrows. The tools of genomics include the polymerase chain reaction (PCR) for gene amplification and thence sequencing, and identification of the code with that of a particular protein, and DNA sequencing, which makes genome-genome comparisons almost commonplace.