DNAase I digestion of early, middle, and late S phase replicating DNA in murine leukemia L5178Y cells

Cellular DNA from a synchronized population of cultured mouse L5178Y cells was labeled with [³H]thymidine at three times during the S stage: early, middle and late S phases. The deoxyribonuclease I digestability of DNA from these three periods in the subsequent cell cycle indicated that the early S phase replicating DNA had higher enzymatic susceptibility at G1 and early S phases than at middle and late S phases, while the middle and late S phase replicating DNA demonstrated a low susceptibility throughout the cell cycle. This suggests that most, if not all, of transcriptionally active DNA is replicated very early in the S phase.