How do bacteria-sized circular chromosomes pop out of human “linear” chromosomes?

Chromosomes such as we and other higher animals have are denoted in the literature as simple linear structures containing a single linear strand of DNA. Yet, somehow, circular structures as large as bacterial chromosomes can pop out of them on a moment’s notice. Chromosomal fragments, called double minutes, are released from nuclei and can exist within cells as circular chromosomes during mitosis or micronuclei during interphase. They are usually amplified regions of the chromosome and are often associated with cancer and genetic damage. Recently, it has been learned that even normal cells eject circular DNAs (New Type of DNA Discovered?). Lagging chromosomes in anaphase become micronuclei but are not necessarily lost to the cells. In fact, they have been known to reintegrate with the main nucleus.

In humans, we have chromosomes ranging from roughly 50-250 million base pairs of DNA from which are ejected circular segments, some of which contain millions of base pairs of DNA which are roughly the size of bacterial DNA.  A base pair of DNA is roughly 340 picometers in length or 340 trillionth of a meter. A piece of DNA the size of a small bacterial chromosome or one million base pairs would be about 340 millionth of a meter or 340 microns in length. A small human chromosome of 50 million base pairs would be 50 times longer or 17,000 microns. In a human, all of this DNA has to somehow fit inside a nucleus which is roughly ten microns in diameter together with all the other chromosomes for a total of over three billion base pairs of DNA. This is equivalent to 1.02 million microns or 1.02 meters of DNA! Imagine for a moment, all of that DNA being stuffed inside a nucleus that is only ten microns in diameter. How on earth can a piece of DNA, 34 times longer than the diameter of a nucleus peel off from a piece of DNA, 1,700 times longer than the diameter of a nucleus and somehow manage to form a circle in the process? Don’t forget that all of this must happen while mixed together with a million microns of additional DNA.

How indeed!

There can be only one logical explanation. The DNA that peels off was circular to begin with. Is there any evidence for that? You bet! Take a look at this:

Circles crop

All of those structures peeling away from the remnants of the mouse nucleus shown above are circular, not linear in shape. Yet textbooks continue to blithely state that such chromosomes are all linear in shape. The geometry makes no sense, but still the dogma remains unfettered by reality. If you search the literature you will finds tons of articles about circular DNA, but none connecting the dots between circular DNA and so-called “linear” chromosomes as has been shown here. As Bernie Sanders would say: “Enough is enough”. It’s time to quit playing it safe, get with the program, and engage in a serious reality check. In others words, these circles need to be thoroughly investigated to determine exactly what they are, how they’re arranged within a chromosome, what they do, and their evolutionary significance in terms of how our chromosomes are actually put together and function.

This blog offers possible explanations for this, together with models ranging from the beginning of chromosome construction from simple viruses up to incorporation of bacterial sized DNA from prokaryotic endosymbionts and beyond.

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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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