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Structure of DNA

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Structure of DNA

Post by Admin on Fri Sep 25, 2015 4:21 pm

Although the exact DNA structure was not known until 1953, its basic building
blocks had been known for many years. It had been shown that DNA is composed
of four basic molecules called nucleotides, which are identical except that each
contains a different nitrogen-containing base. Each nucleotide is made up of a
phosphate group, a sugar (of the deoxyribose type), and one of the four bases.
The four bases are adenine (A), guanine (G) ( the purines) and cytosine (C) and
thymine (T) (the pyrimidines).
In 1953 James Watson and Francis Crick were the first to succeed in putting the
building blocks together and came up with a reasonable DNA structure. They used
DNA X-ray diffraction patterns produced by Rosalind Franklin and Maurice Wilkins
and data from Erwin Chargaff. The X-ray data showed the DNA molecule to be long,
thin and helical (spiral-like) in shape.

Chargaff had established certain empirical rules about the amounts of each
component of DNA:

» The total amount of pyrimidine nucleotides (T + C) always equals the total
number of purine nucleotides (A + G).
» The amount of T always equals the amount of A, and the amount of C always
equals the amount of G. But the amount of A + T is not necessarily equal to
the amount of G + C.

The structure that Watson and Crick derived from these clues is a double helix. Each helix is a chain of nucleotides held together by phosphodiester
bonds, in which a phosphate group forms a bridge between -OH groups on two
adjacent sugar residues. The two DNA chains (helices) are running in an antiparallel
direction and are held together by hydrogen bonds between opposing bases,
thus forming a double helix. Each base pair (bp) consists of one purine and one
pyrimidine base, paired according to the following rule: G pairs with C, and A pairs
with T (Watson et al., 2008).
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