CHAPEL HILL, N.C. — For decades, biology students have learned their ATGCs — a four-letter alphabet that spells out the four chemical building blocks of DNA: adenosine, thymine, guanine and cytosine.

But DNA's alphabet expanded years ago. A fifth chemical building block was discovered in 1948, and a sixth in 2009.

Now, researchers at the University of North Carolina-Chapel Hill have discovered two new building blocks, for a total of eight. Their discovery was published Thursday in the journal Science.

Yet even the newest biology textbooks still maintain that only four chemicals form DNA's backbone.

The extra building blocks might not be widely known because they're all very similar, said Yi Zhang, a professor of biochemistry and biophysics at UNC who led the project.

The differences that distinguish the extra blocks are minor, akin to adding flourishes to a written word. They change the way the word looks, but they don't change the fundamental way it fits into a sentence — the sentence in this case being a strand of DNA.

So far, the team has found the new components only in mice, but "I would not doubt that humans have it," Zhang said. Now researchers must figure out what functions the blocks serve, he said.

"Just like any new discovery," Zhang said, "we answered less questions than the questions raised."