For 50 years, scientists have been well aware of catenated DNA circles (see sample reference link below). Catenated circles can be thought of as links in a metal chain. They are physically interlocked, but otherwise not really connected to one another.
However, what if there is another kind of connection between two circles? One that actually “fuses” them together. Think of two rubber bands that have been glued together at one spot. If you cut the spot where they have been glued, they become one fused circle with “dimples” marking the spot of fusion.
Such a fusion could be explained by tetravalent DNA strand base pairing as shown below:
When a portion of the hybridized region is cut out and religated, it leads to something like Fig. D. Note the dimpling effect because of continued tetravalent strand base pairing. When all four strands are cut just once and religated, the two circles remain intact and can completely fuse together as in Fig E. If DNA isolated from the core of a chromosome is treated with DNase I for as little as 60 seconds, dimples begin to form as shown in Plate X, Fig. A below: Such a structure cannot be explained in terms of a circular DNA catenation model because it is obvious this structure has two fusion components, each preceding away from the dimpled region. When such DNA is heated to melt it in formamide, the structures break up into fragments because of the DNase damage as shown in Plate XII.
Circular DNA’s not subjected to DNase treatment but melted in formamide lead to unusual daisy-like structures like the one shown in Plate VI, Fig. A below: Compare this structure to the one in Plate X, Fig. A, surrounded by yellow arrows. Note the internal circle surrounded by this dimpled DNA that may have been damaged by DNase (red arrow).
Are these fused circular structures minor components of an otherwise linear chromosome or could they be something more? When dying mouse L-1210 cells exposed to acridine orange are treated briefly with 0.1 N HCl and photographed under UV light, the results are nothing less than spectacular (see below, pics shown elsewhere in blog):
Circles of all sizes have been released from the cells. Furthermore, these structures can be labeled with tritiated thymidine, indicating the presence of DNA (See comparison below). You will have to look very hard to find any catenated circles here, if any exist at all. It is obvious the HCl broke some very basic kinds of bonds in order to release these kinds of structures. They are quite complex and evidence shown below in Figs. 69 and 70 suggests that the “pearls” within each necklace are circular structures themselves which have not be released. In other words, circles are composed of other smaller circles.
Fig. 69 Fig. 70
So the obvious question is this: How can all of these circular structures be locked up inside a simple linear chromosome? How indeed! Perhaps a better explanation is in order. One that requires a considerable amount of additional investigations by many many laboratories for years and years to come.