8 October 2012 Last updated at 09:58 ET By James Gallagher Health and science reporter, BBC News

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British scientist John Gurdon told a news conference he still keeps a bad report given to him by his school science teacher

Two pioneers of stem cell research have shared the Nobel prize for medicine or physiology.

John Gurdon from the UK and Shinya Yamanaka from Japan were awarded the prize for changing adult cells into stem cells, which can become any other type of cell in the body.

Prof Gurdon used a gut sample to clone frogs and Prof Yamanaka altered genes to reprogramme cells.

The Nobel committee said they had "revolutionised" science.

The prize is in stark contrast to Prof Gurdon's first foray into science when his biology teacher described his scientific ambitions as "a waste of time".

"I believe Gurdon has ideas about becoming a scientist; on his present showing this is quite ridiculous; if he can't learn simple biological facts he would have no chance of doing the work of a specialist, and it would be a sheer waste of time, both on his part and of those who would have to teach him."

When a sperm fertilises an egg there is just one type of cell. It multiplies and some of the resulting cells become specialised to create all the tissues of the body including nerve and bone and skin.

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Stem cell experts win Nobel prize