Methods for fragmenting and labeling DNA in a single reaction volume and incubation step using a uracil DNA glycosylase, an apurinic/apyrimidinic endonuclease, and a terminal transferase are disclosed. In a preferred embodiment the UDG, AP and TdT activities are first mixed together to form an enzyme mixture and then the enzyme mixture is mixed with the uracil containing DNA. The fragmentation and labeling reactions thus take place simultaneously as part of the same reaction. The methods may be used in a variety of applications where fragmenting and end-labeling single or double stranded DNA is desired.