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【本文】
“We wish to suggest a structure for the salt of deoxyribonucleic acid （DNA). This structure has novel features which are of considerable biological interest.” This is how James Watson and Francis Crick announced in 1956 that they had worked out the structure of the “molecule of life”. Many scientists regard it as the most important scientific discovery of the 20th century.
The DNA molecule is like a ladder that has been twisted into a spiral like a corkscrew. Four base molecules are spread along the legs of the ladder: A （adenine）, T（thymine）, C（cytosine）， and G（guanine）. The rungs of the ladder are bonds linking the four base molecules on the ladder legs. A is always linked with T and C is always linked with G. The DNA molecule reproduces itself by unwinding the two legs of the ladder. Each leg then combines with a new leg and two identical molecules are formed. This is how living organisms are able to reproduce. Each cell in our body is about one millionth of a meter in diameter, but contains about two meters of DNA.
Every living organism, plant or animal, is determined by its DNA and the way the four base molecules are arranged. The arrangement of those four base molecules determines whether the organism is, for example, a plant or an animal, a mosquito, an elephant or a human being. And each individual within a species has its own DNA, which determines whether that individual has straight or curly hair, or fair or dark skin.
Scientists are now using DNA to study our history. In the 19th century, anthropologists found fossil remains of what they thought were early human beings. They called them “Neanderthal man” after the valley in which their remains were first found. It was thought that they were our ancestors and that we had evolve from them. However, comparisons of Neanderthal DNA with DNA of modern humans showed that while we do have an ancestor in common, we developed from this common ancestor separately. We now know that the Neanderthal line of development split from the human one about 500,000 years ago. Neanderthals and humans are different species just as chimpanzees and humans are different species. DNA studies show that all modern humans descended from one woman who lived about 200,000 years ago.
In the movie Jurassic Park, scientists bring extinct dinosaurs back to life. They use dinosaur DNA taken from preserved mosquitoes that fed on dinosaur blood. This may no longer be science fiction. The first successful extraction of DNA from an extinct animal was done in 1984. The quagga was an African animal very closely related to the zebra, with very similar DNA. The quagga used to be common in Africa, but became extinct in 1887. In 1984 scientists at The University of California extracted quagga DNA from a skin preserved in a museum. The South African Museum has set up a program to clone the quagga using zebras as host mothers. Scientists at the Australian Museum in Sydney have isolated the DNA of a Tasmanian Tiger, which became extinct in 1936, aiming to bring it back as well. The problem with bringing dinosaurs back is that they have been extinct for 65 million years. It is unlikely that any dinosaur DNA has survived.
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