Size Distribution and Molecular Polarity of Newly Replicated DNA in Escherichia coli

Newly synthesized DNA, in E. coli lysogenic for the phage lambda, was labeled by short pulses of [(3)H]-thymidine, isolated, and separated on the basis of size by alkaline sucrose density gradient centrifugation. The molecular polarity of this DNA was determined by hybridization with each of the separated strands of lambda DNA. The results show that, in the 3' to 5' direction, replication proceeds by synthesis of short chains that are subsequently joined to long DNA. This is true for both a polA(+) and a polA(-) strain. (The polA locus codes for DNA polymerase I.) In the 5' to 3' direction, replication proceeds continuously, by addition of nucleotides to long DNA, for the polA(+) strain. In the polA(-) strain, however, replication in the 5' to 3' direction is also discontinuous, but the discontinuities are 1-40 times less frequent than in the other direction.