Elizabeth PennisiThe genetic code in mammals may have gained another letter—or at least a significant footnote. Textbooks speak of four DNA building blocks, whose order specifies genes.
But researchers in the field of epigenetics have also identified four chemically modified versions of these nucleoside bases, which affect how genes are expressed.
Three of them are altered versions of cytosine, one of the original four bases. The last, methylated adenine, is mired in controversy about whether it exists in mammals.Now, researchers at Yale University have not only identified plenty of this modified base in mouse embryonic cells, but also found it plays a key role in the development of the placenta.