Living cells have DNA molecules that carry an organism’s genes. For the organism to live and develop, its DNA cannot change. But DNA molecules are not completely stable, nor are they error free- they can be damaged during the process of copying or replication, a process that is essential for cell growth and repair.
Through studies of bacterial viruses, American biochemist Paul Modrich (born 1946) showed how methyl groups (CH3 in formulas), comprising one central carbon atom attached to three hydrogen atoms, play a key role in DNA repair. They do so by attaching themselves to the DNA molecule, acting as signals for repairing incorrect replications of DNA.

In 1996, Modrich discovered the protein MutS alpha, which has the function of sensing errors in DNA when it is copied and correcting the errors. A few years later, Lorena Beese determined the structure of this protein. The following picture shows this structure. It once hung in Modrich’s laboratory until he decided to donate it to the Nobel Prize Museum. For his contribution to the “mechanistic studies of DNA repair”, Modrich was awarded the 2015 Chemistry Prize.