Reassociation kinetics of deoxyribonucleic acid in antigen-stimulated mouse-spleen cells

In order to directly compare the complexity of the genome of lymphoid cells which have been antigenically stimulated, with that of non-immunized and non-lymphoid cells, DNA was pulse labeled and extracted from BALB/c mouse spleen cells at various time intervals after antigenic stimulation in vivo; the reassociation rates of these newly synthesized DNA preparations were compared with those of the total mouse spleen DNA, obtained from same sources and at the same times. DNA labeled for 60 min at 43, 53, or 72 h after antigenic restimulation, reassociated faster than the corresponding total DNA. On the other hand, the ressociation profile of DNA, labeled for 60 min during the first 24 after restimulation did not differ from that of the total DNA extracted at the same time. When labeled thymidine was available for incorporation at a constant concentration over a period of 24 h, reassociation patterns of labeled DNA were identical to those of the corresponding total DNA at all times after restimulation. Newly synthesized nuclear DNA exhibited reassociation profiles identical to those of the corresponding total nuclear DNA at all times tested. Also, no differences between the reassociation rates of nuclear and total cellular DNA were observed. It was concluded that antigenic stimualtion does not induce a major amplification of genes in the stimulated cells, and that the rapidly reassociating DNA species described represent extranuclear (cytoplasmic) DNA.