How the X shape of chromosomes is formed during cell division has been discovered at a molecular and atomic level.

It is known that chromosomes get their X shape during cell division, but the underlying mechanism of how this shape is formed has not been known. Now, an international team of scientists led by Professor Daniel Panne, of the University of Leicester, and Dr Benjamin Rowland, of the Netherlands Cancer Institute, have discovered that a protein called shugoshin, also known as SGO1, may help to lock chromosomes into an X shape.

'A chromosome actually consists of two identical long DNA threads that at first are connected along their entire length' said Dr Rowland. 'We seem to have found a universal mechanism by which cells determine the shape of DNA.... By locking cohesin at exactly the right time, as well as at the right place on the DNA, you can determine the shape of our chromosomes.'

Cells continuously divide to make new cells in a process called cell division. In this process, a cell tightly packs its DNA together and then copies it to form two identical threads. These threads are attached in the middle by a ring-shaped protein complex, called cohesin, forming X-shaped chromosomes. As the cell divides, the cohesin rings open up and the X separates in the middle releasing the identical DNA fragments to the two new daughter cells.

Dr Rowland continued: 'A host of ring-shaped cohesin molecules holds the two threads together. When a cell is about to divide, the cohesin rings open, and the arms of the DNA come apart. The rings in the middle of the DNA remain tightly closed. This is due to a protein with the exotic name shugoshin - SGO1.'

The scientists studied the cohesin complex at the molecular level using X-ray crystallography and imaging techniques, publishing their results in Nature Structural & Molecular Biology. They discovered that the protein SGO1 is responsible for keeping DNA tightly closed in the middle by locking the cohesin ring. Using a molecular key that fits precisely into a keyhole in cohesin, and only working in the centre of chromosomes, SGO1 only locks the rings in the middle, resulting in the X shape of chromosomes.

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.'

If cell division goes wrong, the new cells may receive abnormal amounts of DNA, leading to cancer or to genetic conditions such as Down's syndrome or Edwards' syndrome. These conditions are known as trisomies where there is an extra copy of a particular chromosome.