Sea urchin nuclear DNA polymerase: IV. Reversible association of DNA polymerase with nuclei during the cell cycle

Abstract

We have taken advantage of the natural synchrony of sea urchin embryos during early development to study the relationship between DNA polymerase activity and nuclear DNA replication. Nuclei were isolated from early embryos (2–16 cells) in various phases of the division cell cycles. The polymerase activity found in nuclei isolated from S phase cells was 5 to 10-fold greater than that in similar fractions isolated from mitotic or G2 embryos. As DNA synthesis is completed, the polymerase activity drops to a level slightly higher than the comparable stage before S. The ratio of DNA polymerase activity to in vivo synthesized DNA is consistently highest at the beginning of S. If mitosis is prolonged, by the isotope effect of D 2O on mitotic spindle proteins, there is a corresponding delay in the increase of nuclear polymerase activity and DNA replication. We interpret these results to mean that DNA polymerase attaches to the chromosomes as they complete mitosis, functions in DNA synthesis, dissociates from them and leaves the nucleus when DNA replication is completed. This translocation is repeated with each cell cycle.