 Grete Kellenberger's major scientific contribution was the discovery that recombination is due to a physical exchange of DNA, and not to selective replication.An article on this subject authored by Grete Kellenberger, Maria Ludovica Zichichi, and Jean Weigle was published in the same issue of the Proceedings of the National Academy of Sciences (PNAS) as the article from Meselson and Weigle on the topic.However, although the data for Grete's article were obtained using a more original approach and were ready months before experiments were concluded in Meselson's laboratory, Grete's article appeared after Meselson's.