The "mortal coil" referred to in Shakespeare's Hamlet really does exist - and could hold the key to controlling the ageing process, scientists say.

The coil is a coiled piece of DNA which becomes detached and replicates inside a cell until the cell is overwhelmed and dies.

Scientists at the Massachusetts Institute of Technology, Cambridge in the US, discovered the phenomenon in ageing yeast cells. They believe the same mechanism may be at work in cells of higher organisms - possibly even humans.

Blocking it may one day provide a way of holding back or halting the ageing process.

The breakthrough is the culmination of a series of important discoveries about ageing to emerge from studies of yeast by Massachusetts researchers.

They found that during a yeast cell's life, whenever a particular coiled piece of DNA breaks off a chromosome it begins to replicate. The coiled fragments, known as ERC (extrachromosomal ribosomal DNA), multiply within the very heart of the cell, the nucleolus.

The nucleolus is a vital structure which produces the raw material for the cell's protein-making machinery.

Eventually the nucleolus is so stuffed with ERC coils that the cell's ability to replicate DNA breaks down and the cell dies.

The findings are published in the journal Cell.

The scientists, led by Professor Leonard Guarente, wrote: "It is remarkable that this mechanism of ageing in mother yeast cells is so simple at a molecular level.

"It is conceivable that inhibitors of this [ageing] process can be found, and if so, such strategies might eventually prove useful in forestalling ageing in yeast and, perhaps, in higher organisms."

In his famous soliloquy, Hamlet speaks of shuffling off "this mortal coil" in a reference to death.

The researchers said that their data suggested accumulation of the cellular 'mortal coils' may be like a time bomb. When a certain time is reached - with enough coils replicated - the cell is killed.

Professor Guarente said: "The best part is, it's obvious it's a clock. Set the clock early and the alarm rings early."

Damage to the cell may set the clock ticking, suggesting that, ironically, production of the fatal ERCs could result from the cell's attempt to repair itself. But the mechanism also appears to be inherited.

The researchers thought it likely that ERCs could form in 'stem' or 'progenitor' cells in mammals. These are primitive cells from which mature cells form and are found in organs like the skin, kidney, and liver, as well as the bone marrow and blood.

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