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Text 3.
Watson and Crick Describe Structure of DNA.
I.
First thoughts.
Can you tell what DNA is?
II.Study these words. Make sure you know them.
nucleic acid, chain, ribose, humidity, to deduce,
protein, x-ray.
III. Read the text and do the tasks following it.
What is DNA?
DNA stands for deoxyribonucleic acid. This is a substance
that is present in every living cell. However, each living thing
has a different kind of DNA. The kind of DNA you have in your
cells gives you all the features you inherit from your parents.
In the late nineteenth century, a German biochemist found
that the nucleic acids, long-chain polymers of nucleotides, were
made up of sugar, phosphoric acid, and several nitrogen-
containing bases. Later it was found that the sugar in nucleic
acid can be ribose or deoxyribose, giving two forms: RNA and
DNA. In 1943, American Oswald Avery proved that DNA
carries genetic information. He even suggested DNA might
actually be the gene.
In 1948, Linus Pauling discovered that many proteins take
the shape of an alpha helix, spiraled like a spring coil. In 1950,
biochemist Erwin Chargaff found that the arrangement of
nitrogen bases in DNA varied widely, but the amount of
certain bases always occurred in a one-to-one ratio. These
discoveries were an important foundation for the later
description of DNA.
In the early 1950s, the race to discover DNA was on. At
Cambridge University, graduate student Francis Crick and
research fellow James Watson (b. 1928) had become
impressed especially by Pauling’s work. Meanwhile at King’s
College in London, Maurice Wilkins (b. 1916) and Rosalind
Franklin were also studying DNA. The Cambridge team’s
approach was to make physical models to narrow down the
possibilities and eventually create an accurate picture of the
molecule. The King’s team took an experimental approach,
looking particularly at x-ray diffraction images of DNA.
In 1951, Watson attended a lecture by Franklin on her
work to date. She had found that DNA can exist in two forms,
depending on the relative humidity in the surrounding air. This
had helped her deduce that the phosphate part of the molecule
was on the outside. Watson returned to Cambridge with a
rather muddy recollection of the facts Franklin had presented,
though clearly critical of her lecture style and personal
appearance. Based on this information, Watson and Crick
made a failed model. It caused the head of their unit to tell
them to stop DNA research. But the subject just kept coming
up.
IV. Comprehension check.
1. Find English equivalents in the text for:
обнаружить ______________
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