Differential replication of satellite and main band DNA during early stages of callus formation in carrot root tissue

Abstract

DNA synthesis during the early stages of callus formation was examined in carrot root slices cultured on an agar medium containing 2,4-D. During the first 12 hr of culture, only a low level of 3H-thymidine was incorporated into DNA, after which the incorporation rapidly increased and reached a maximum at about 48 hr, then gradually decreased. CsCl density centrifugation of total tissue DNA indicated that a satellite DNA with a buoyant density of about 1.712 g/ml replicates in the early phase of the DNA synthetic period, then the main band DNA with a buoyant density of 1.695–1.700 g/ml starts its replication and continues to be produced throughout the synthetic period. The labeled satellite DNA, as well as the labeled main band DNA, was localized mainly in the subcellular fraction of 1,000 × g sediments obtained from tissue homogenates. The patterns of cellular localization were not modified by the addition of Triton X-100 to the initial homogenates. A considerable portion of the labeled satellite DNA was found in nuclease-resistant chromatin subunits after limited digestion of the isolated chromatin with micrococcal nuclease.