Klenow fragment of DNA polymerase I. Molecule model of the Klenow, or large, fragment from DNA polymerase I complexed with DNA (deoxyribonucleic acid, red and blue). 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.