A study led by a Leicester University scientist explains the biology behind the distinctive structure.

Our cell’s nucleus contains two meters of DNA that must be propagated without error each time a cell divides. When cells go about their day-to-day business, their DNA is loosely distributed in the nucleus. When cells divide, the DNA becomes packed as tightly organised X-shaped chromosomes that are then evenly distributed to the two daughter cells.

These iconic X shape chromosomes become visible in the light microscope. Ever since the invention of the first light microscopes more than 100 years ago, scientists have puzzled over how chromosomes receive their X-shape during cell division.

This X-shape arrangement of chromosomes is required to ensure that each of the two daughter cells receive an identical copy of the genome. If this process goes wrong and the genome is not evenly distributed this can lead to cancer or to trisomies such as Down Syndrome.

Now, a team of researchers led by Professor Daniel Panne at the University of Leicester and Dr. Benjamin Rowland at the Netherlands Cancer Institute have determined at a molecular level how the iconic X-shape of chromosomes is generated during cell division. The work is published in the journal Nature Structural & Molecular Biology.

The team studied a key component of a ring-shaped protein complex called cohesin that is known to be important for holding chromosomes together during cell division. Using X ray crystallography the team could determine the 3D architecture of this key component at the atomic level. This allowed precision engineering of the cohesin complex in cells. Using biochemical and imaging techniques using a light microscope the team found that when they modified key amino acids, the familiar X-shaped chromosomes could not form any longer. This means that these key amino acids are required to form the X-shaped chromosome.

Professor Panne said: “It is exciting to finally understand at a molecular and atomic level how the iconic X-shape of chromosomes during cell division is generated. This has not only intrigued generations of scientists but is also important for our understanding of how this process can go wrong in disease.”