Scientists believe they have resolved one of the great mysteries of life – how chromosomes get their X shape.

Chromosomes are long DNA molecules which contain the genetic material of an organism. These structures were discovered in the late 1800s after the light microscope was invented.

They have an X shape, but the reason behind this has always been a mystery. It is known that they get their shape during cell division but researchers wanted to know how that happened.

Scientists led by Professor Daniel Panne, of the University of Leicester, and Dr Benjamin Rowland, of the Netherlands Cancer Institute, think the answer lies in a protein called shugoshin which “locks” the chromosomes into X shapes.

When a cell divides, it copies the DNA and splits it equally between the two new cells. DNA in each cell is about two metres long, so that has to be made into compact parcels to make it fit.

The cells leave the two copies attached in the middle until they divide. Under the microscope, such a parcel looks like an X. Just before the cell divides, the X is released in the middle and each arm of the X goes to a separate cell.

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Dr Rowland said: “A chromosome actually consists of two identical long DNA threads that at first are connected along their entire length.

“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 researchers discovered that the protein, also known as SGO1, locks the cohesin rings giving chromosomes their X shape.

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When cells start dividing, “molecular scissors” cut the rings loose, separating the DNA.

Prof 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.

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“This has not only intrigued generations of scientists but is also important for our understanding of how this process can go wrong in disease.”

The research is published in the journal Nature Structural & Molecular Biology.

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