
In these microscopic images of cells, the white areas indicate the presence of enzymes. The enzymes in images A and C are distributed throughout the cytoplasm because these cells were grown in the presence of purines. In contrast, the enzymes in images B and D occur in small clusters because these cells were grown in the absence of purines. Credit: Penn State Penn State scientists are the first to observe in living cells a key step in the creation of adenine and guanine, two of the four building blocks that comprise DNA. Also called purines, the two building blocks are essential for cell replication. The findings, which will be published in the 4 April 2008 issue of the journal Science, could lead to new cancer treatments that prevent cancer cells from replicating by interfering with their abilities to make purines.