We operationally define two forms of SV40 minichromosomes, a 75S-form, prepared 
at low salt concentration, referred to as native minichromosomes, and a 
50S-form, obtained after treatment with 0.5 M potassium acetate, the 
salt-treated minichromosomes. Both preparations of minichromosomes serve well as 
templates for replication in vitro. Their respective replication products are 
strikingly different: replicated native minichromosomes contain a densely packed 
array of the maximal number of nucleosomes whereas replicated salt-treated 
minichromosomes carry, on average, half of the maximal number. We conclude that 
in both cases parental nucleosomes are transferred to progeny DNA, and, in 
addition, that an assembly of new nucleosomes occurs during the replication of 
native minichromosomes. This is apparently due to the presence of a nucleosome 
assembly factor as a constituent of native minichromosomes that dissociates upon 
treatment with salt. We further show that preparations of minichromosomes 
usually contain significant amounts of copurifying hnRNP particles and SV40 
virion precursor particles. However, these structures do not detectably affect 
the replication and the chromatin assembly reactions.