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X-ray diffraction photos of DNA helped to solve the puzzle of reproduction

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Two Cambridge University scientists have published their answer to one of the most fundamental questions of biology - how do living things reproduce themselves?

In an article published today in Nature magazine, James D Watson and Francis Crick describe the structure of a chemical called deoxyribonucleic acid, or DNA.

DNA is the material that makes up genes which passhereditarycharacteristics from one parent to another.

In short, it consists of a double helix of two strands coiled around each other. The strands are made up of complementary elements that fit together and when uncoiled can produce two copies of the original.

Momentous discovery

This special property for accurate self-replication allows DNA to duplicate the genes of an organism during the nuclear divisions for growth and the production of germ cells for the next generation.

They began their article with the modest statement: "We wish to suggest a structure for the salt of deoxyribose nucleic acid (DNA). This structure has novel features which are of considerable biological interest."

On 28 February, Mr Crick walked into a Cambridge pub with Mr Watson to celebrate the fact that they had unravelled the structure of DNA, saying: "We have discovered the secret of life!"

The momentous discovery was the culmination of research by Medical Research Council scientists Maurice Wilkins and Rosalind Franklin in London, who produced X-raydiffractionphotographs and other evidence. 

The US Embassy in Tehran was seized by Islamic militants on 3 November 1979

President Carter announced the disastrous mission in a broadcast to the nation earlier today.

"I ordered this rescue mission prepared in order to safeguard American lives and protect America's national interests, and to reduce the tensions in the world that have been caused among many nations as this crisis has continued," he said.

He took full personal responsibility for the operation, and its cancellation, but did not rule out another attempt.

It was the first the American public, or the wider world, had heard of the mission, although it had been planned since shortly after the US embassy in Iran was seized last November by Islamic militants. They have held 53 US citizens hostage there ever since.

The dramatic attempt to free the hostages began yesterday when six Hercules C130 transport planes set off torendezvouswith a group of nine helicopters at a remote desert airstrip, south-east of Tehran.

But the mission ran into trouble almost as soon as it had started.

From farce to tragedy

Two helicopters went down with engine trouble, and a third was diverted to help.

Then another helicopter was damaged as it landed on the airstrip, leaving only five workable helicopters. The mission had become impossible.

President Carter ordered the operation to abort. It was then that the farce became a tragedy.

As the aircraft took off again, another helicopter crashed into one of the C130 aircraft and burst into flames. Eight soldiers died, and another four men suffered burns.

"An act of war"

In Tehran there were jubilant scenes as thousands of people celebrated the failure of the mission.

The Foreign Minister, Sadeq Qotbzadeh, condemned the rescue effort as "an act of war".

In Europe, there was shock and surprise that the mission had taken place without advance consultation of America's allies.

EEC governments have recently agreed to threaten sanctions against Iran in the hope of preventing the use of force.

hereditary:passing down from inheritance(世襲的，遺傳的)

diffraction:change in the directions and intensities of a group of waves after passing by an obstacle or through an aperture（衍射）

rendezvous:a prearranged meeting place, especially an assembly point for troops or ships （指定集合地點）