Circular DNA crosslinks may be the building blocks that led to your chromosomes

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.

Electron microscopy of the circular kinetoplastic DNA from Trypanosoma cruzi: Occurrence of catenated forms

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.

rubber bandfused circles

Such a fusion could be explained by tetravalent DNA strand base pairing as shown below:

Fig 40

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.

Plate X  Plate XIII

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).

Plate VI Plate Xb

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):

slide 7Circles

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.

circular arrays

69 70

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.

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About frankabernathy

I am a retired cell biologist and alumnus of Ohio State University. I became interested in chromosomes as far back as the 1960's when I wrote a term paper on the effects of radiomimetic drugs on chromosomes. I was fascinated at how they could break apart and reform new structures so easily. I became further involved in the early 1970's after taking a cytogenetics course at the University of Arkansas. I took that knowledge with me to Ohio State in 1980 where I eventually worked on my research and completed my Ph.D. dissertation, "Studies on Eukaryotic DNA Superstructure". My studies and later research suggested that the DNA within the eukaryotic chromosome is not the simple, linear molecular thread so widely suggested in all the classic textbooks published today. Instead, it may be the culmination of a geologically rapid set of endosymbiotic events where microorganisms plug into each other to create something greater than themselves. Feel free to contact me at fabernathy@sbcglobal.net.
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