Paul Modrich was awarded the 2015 Nobel Prize in Chemistry for showing how methyl groups attached to the DNA molecule act as signals for repairing incorrect replications of DNA. The methyl groups communicate with proteins, which then cut out the mismatched DNA base pairs, allowing DNA polymerase to fill the gap and DNA ligase to seal the strand.
Modrich’s award was part of a broader acknowledgement of success in the field for understanding how cells repair and protect our DNA, which began in the 1960s when scientists discovered that DNA was volatile rather than stable.
Modrich’s particular contribution was to show how DNA mismatch repair (MMR) works, in which mistakes (in the form of mismatched base pairs) that creep into the DNA code during chromosome replication are repaired by proteins in the MMR system. "In human cells, MMR reduces the error rate by a factor of a thousand," Modrich told the Howard Hughes Medical Institute following the receipt of the prize. If DNA errors are passed down they can lead to mutations, and defects in the MMR system are linked to Lynch syndrome, the most common form of hereditary cancer.
Modrich is the James B. Duke Professor of Biochemistry at Duke University and Investigator at the Howard Hughes Medical Institute. He received a Ph.D. in 1973 from Stanford University and a B.S. in 1968 from MIT.