Our DNA is organized into distinct structures known as chromosomes. In these, the strand of DNA is tightly wound around tiny proteins called histones, which act like a spool to the DNA thread. These then form the chromosomes, of which almost every cell has 46. While it has been thought that the majority of chromosomes are made up of DNA and histones (together known as chromatin), it now seems that this might not be the case.
Researchers from the University of Edinburgh have used advanced imaging techniques to produce detailed 3D models of all 46 human chromosomes. Known as 3D-CLEM, it combines light and electron microscopy with modeling software. This allowed them to see in unprecedented detail the intimate structure of chromosomes, and revealed that almost half of their entire structure is not actually made up of DNA, but instead made of a “mysterious” sheath surrounding them.
“The imaging technique we have developed to study chromosomes is truly groundbreaking,” explains Dr Daniel Booth, who co-authored the study published in Molecular Cell, in a statement. “Defining the structure of all 46 human chromosomes for the first time has forced us to reconsider the idea that they are composed almost exclusively of chromatin, an assumption that has gone largely unchallenged for almost 100 years.”
Known as the “chromosome periphery”, the function of this sheath is little understood, but it has been found to change in volume depending on what stage the cell is in. The mysterious casing was found to account for between 30 and 47 percent of the chromosome at any one time, meaning that the actual genetic material only makes up between 53 and 70 percent of the structure.
The changes in volume might give hints as to its role, as the team suspect that the sheath may be there to keep the individual chromosomes separate from each other during cell division and prevent errors from occurring. This means it may play a role in preventing cancer and other associated diseases.
This new information is revelatory, as it overturns nearly a hundred years of thinking. Added to the fact that the vast majority of our DNA is what is known as “junk DNA”, in that it does not directly code proteins, it seems that even less of what is found in our nucleus is actually what makes you specifically you.