Emma Marris in Nature:
One of the first things any biology student learns is that DNA, the recipe for life, is written with four letters. But what if you could add extra ones? Researchers who have managed to build, and replicate, DNA with an ersatz fifth letter are on their way to finding out.
Getting the modified DNA to work requires the team to answer all sorts of basic biochemistry questions. But the ultimate hope is that a few of these artificial letters could be sprinkled into the genome of a living microbe, to track its adaptation and evolution.
The four letters that occur naturally in DNA are chemical bases called adenine (A), guanine (G), cytosine (C) and thymine (T). The sequences that code for the different proteins to be built within a cell are spelled out using these four letters.
Floyd Romesberg and his colleagues at The Scripps Research Institute in La Jolla, California, have developed a fifth base, called 3-fluorobenzene, or 3FB. The bases in a zipper of DNA pair up across the molecule’s two strands: A always pairs with T, and G always pairs with C. The 3FB pairs with itself, forming a completely new base pair.
More here.