The concept of reductionism is something I've written on before. I was spurred to add this post after listening to a podcast interview (from The Skeptics' Guide to the Universe) with the wonderful science writer Carl Zimmer.
While the interview traverses a great deal of ground, including how Carl became an award-winning writer of science books, blogs & articles, it centres on his most recent book Microcosm: E.coli and the new science of life. In the interview (& the book) he gives an excellent example of why the reductionist view, that an organism is no more than the sum of the sequence of bases in its genetic codes, is far from the truth.
Colonies of E.coli are made up of thousands - millions! - of bacterial cells that are all clones: genetically identical. So you'd expect that they'd all react in the same way to various stimuli: the presence of a particular sugar, for example. But they don't. In the presence of that sugar, some of those cells will express the genes needed to metabolise it while others - with exactly the same genes - don't. The genes are regulated differently in those individuals, or their DNA is modified epigenetically; either way the responses differ between cells. And this is something that a purely reductionist approach would never have predicted.
(I didn't think I'd like podcasts as I've always preferred reading to broadcasts for my information - but I have to say that now I'm hooked. I can exercise my brain at the gym, as well as my muscles!)