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Introduction:
The secondary structure of DNA is actually very similar to
the secondary structure of proteins. The protein single alpha
helix structure held together by hydrogen bonds was discovered
with the aid of X-ray diffraction studies. The X-ray diffraction
patterns for DNA show somewhat similar patterns.
In addition, chemical studies by E. Chargaff indicate several
important clues about the structure of DNA. In the DNA of all
organisms:
a) The concentration of adenine equals that of thymine.
b) The concentration of guanine equals that of cytosine.
Chargaff's findings clearly indicate that some type of heterocyclic
amine base pairing exists in the DNA structure. X-ray diffraction
data shows that a repeating helical pattern occurs every 34 Angstrom
units with 10 subunits per turn. Each subunit occupies 3.4 Angstrom
units which is the same amount of space occupied by a single
nucleotide unit. Using Chargaff's information and the X-ray data
in conjunction with building actual molecular models, Watson
and Crick developed the double helix as a model for DNA.
The double helix in DNA consists of two right-handed
polynucleotide chains that are coiled about the same axis. The
heterocyclic amine bases project inward toward the center so
that the base of one strand interacts or pairs with a base of
the other strand. According to the chemical and X-ray data and
model building exercises, only specific heterocyclic amine bases
may be paired.
DNA Double Helix - Chime
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Credit: The structure of the 'Dickerson Dodecamer'
was originally reported in: Drew, H. R., Wing, R. M., Takano,
T., Broka, C., Tanaka, S., Itakura, K. & Dickerson, R. E.
(1981). Structure of a B-DNA dodecamer: conformation and dynamics.
Proc. Natl. Acad. Sci. USA 78, 2179-2183. and the coordinates
were acquired from the Brookhaven Protein Data Bank. The filename
is 1BNA.
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