Klenow fragment of DNA polymerase I. Computer model showing the secondary (alpha-helices and beta-sheets) structures of the Klenow fragment from DNA polymerase I. DNA polymerase synthesises a new DNA strand from a complementary template strand during DNA replication. This fragment retains this activity, and the ability to cleave nucleotides form the strand in the opposite direction to replication, but has lost the ability to cleave nucleotides in the same direction as replication. This allows the enzyme to be used in a number of research applications, including preparing radioactive DNA probes.