An international team of scientists has managed, for the first time in history, to measure the mass of human chromosomes. To do this, they have used a powerful X-ray source at the UK’s national synchrotron scientific facility, Diamond Light Source, to determine the individual masses of the 46 chromosomes in human cells.
Chromosomes weigh more than expected
Each human cell normally contains 23 pairs of chromosomes, or 46 in total representing 22 pairs of numbered chromosomes (autosomes) and one pair of sex chromosomes. Chromosomes prevent DNA from falling apart, which helps maintain its structure during the process of cell replication. Within these are four copies of 3.5 billion base pairs of DNA. Thus, using a method called phase-sensitive X-ray optics to determine the number of electrons in a stretch of the 46 human chromosomes, they found that human chromosomes were about 20 times heavier than the DNA they contained, a mass much more Bigger than previously expected, so it is clear that we still have things to discover.
Measurements suggest that the 46 crosomomas weigh 242 picograms (trillionths of a gram), an unexplained excess mass on chromosomes , the scientists say.
The implications of understanding chromosome mass could be important in the medical field, as they contain the instructions for life in almost every cell in our body.
“Scientists have investigated chromosomes for 130 years, but there are still parts of these complex structures that are not well understood ,” explains Ian Robinson, leader of the work.
Understanding precisely how chromosomes work, including their mass, is essential for medical laboratories, since “a large number of studies of chromosomes are carried out in laboratories to diagnose cancer from patient samples. Therefore, any improvement in our ability to obtain images of chromosomes would be very valuable “ , clarifies Archana Bhartiya, co-author of the work.
How have they calculated the mass of the chromosomes?
The researchers used a technique known as X-ray optiography to construct a highly sensitive 3D reconstruction, which involves stitching together the diffraction patterns that occur when the X-ray beam travels through chromosomes. Because the beam used was billions of times brighter than the Sun, exquisite resolution could be achieved (that is, there were an exceptionally large number of photons passing through it at any one time). Chromosomes were photographed in metaphase, just before cell division, when the 46 chromosomes within each cell have tightly packed the DNA inside. Thus they were able to determine the density of electrons on the chromosome.
“We know the mass of DNA from the Human Genome Project , but this is the first time that we have been able to accurately measure the masses of chromosomes that include this DNA,” said biophysicist Ian Robinson of University College London.
Referencia: A. Bhartiya et al. 2021. X-ray Ptychography Imaging of Human Chromosomes After Low-dose Irradiation. Chromosome Res 29, 107-126; doi: 10.1007/s10577-021-09660-7
