Towards the theoretical bases of the folding of the 100-A nucleosome filament.

This paper attempts to model DNA packaging at the various stages of ever-increasing DNA folding, from the 100-A nucleosome filament to various further stages leading up to the metaphase chromosome. We have assumed that a phase transition has induced chromatin into a condensed mode. The mean-field model allows the simultaneous discussion of chromatin with packing ratio eta and DNA replication at various stages of folding. We derive a formula correlating (during the S phase of the cell cycle) the DNA polymerase velocity rf (measured in nucleotides per minute) in a relation of inverse proportionality with the degree of DNA packaging: rf = lambda eta-1/2. This model suggests that in the heterochromatic regions of chromatin there is reduced activity of DNA polymerases. We discuss the possible relevance of our model to late replicating telomeres in yeast and several higher eukaryotes.