Scientists say human chromosomes are much heavier than they should be and they don’t know why

Scientists have found that human chromosomes, which contain the instructions for life in nearly every cell of our bodies, weigh 20 times more than the DNA they house, an advance which suggests that there could be missing components of the genome yet to be discovered.

A typical human cell normally contains 23 pairs of chromosomes, or 46 in total, and within these are four copies of 3.5 billion molecule base pairs of DNA.

Along with DNA, chromosomes also consist of proteins that serve a variety of functions such as reading the DNA, regulating cell division, and tightly packaging and coiling up two-meter strands of DNA into each of our cells.

In the research, scientists, led by those from University College London (UCL), used a powerful X-ray beam at the UK’s national synchrotron facility – Diamond Light Source – to determine the number of electrons in a spread of 46 chromosomes, a measure which they then used to calculate mass.

To estimate the number of electrons, the researchers used a technique called X-ray ptychography which involves stitching together the patterns that occur as the X-ray beam diffracts and passes through the chromosomes.

They viewed the chromosomes during the metaphase part of the cell division cycle – just before they were about to split into two daughter cells, at a stage when packaging proteins wind up the DNA into very compact, precise structures.

The findings, published in the journal Chromosome Research, revealed that the 46 chromosomes in each of our cells weigh 242 picograms – trillionths of a gram – and about 20 times heavier than the DNA they contained.

“This is heavier than we would expect, and, if replicated, points to unexplained excess mass in chromosomes,” study senior author Ian Robinson from the London Centre for Nanotechnology at UCL said in a statement.

Based on the results, the scientists believe there might be missing components of chromosomes that are yet to be discovered.

“Chromosomes have been investigated by scientists for 130 years but there are still parts of these complex structures that are poorly understood,” Robinson said.

“The mass of DNA we know from the Human Genome Project, but this is the first time we have been able to precisely measure the masses of chromosomes that include this DNA,” he added.

According to the researchers, further studies are needed to unravel the source of this mysterious mass and they may have important implications for human health.

“A vast amount of study of chromosomes is undertaken in medical labs to diagnose cancer from patient samples. Any improvements in our abilities to image chromosomes would therefore be highly valuable,” Archana Bhartiya, a PhD student at the London Centre for Nanotechnology at UCL and lead author of the paper, said in a statement.