The Structure of DNA
DNA is composed of four kinds of nucleotides, each of which consists of:- a five-carbon sugar (deoxyribose),
- a phosphate group, and
- one of four nitrogen containing bases -adenine (A), guanine (G), thymine (T), cytosine (C).
Edwin Chargaff in 1949 showed that the amount of adenine relative to guanine differs among species. However, the amount of adenine always equals the amount of thymine and the amount of guanine always equals the amount of cytosine or A=T and G=C. Because they occurred in equal amounts, this indicated that these bases were always paired with each other.
This animation (Audio - Important) describes DNA subunits.
Rosalind Franklin was an expert in a technique called x-ray crystallography (or X-ray diffraction). She used this technique to examine DNA fibers and produce images of DNA molecules. She concluded that DNA was some sort of helix.
Watson and Crick deduced the actual double helix structure of DNA. DNA exists as a long, thin molecule of uniform diameter. It consists of two nucleotide strands twisted into a double helix with the strands running in opposite directions. The strands are held together by hydrogen bonds between bases. A binds with T and C with G. This base pairing is constant for all species. Nucleotides are joined along the molecule's length with the sugar-phosphate linkages forming a sort of "backbone." The sequence of bases in a nucleotide strand is different from species to species.
These animations (Audio - Important) describe the structure of DNA:
DNA structure 1.
DNA structure 2.
REVIEW: Which is not a nucleotide base in DNA?
a. adenine
b. guanine
c. uracil
d. thymine
e. cytosine
REVIEW: James Watson and Francis Crick
a. established the double-stranded nature of DNA.
b. established the principle of base pairing.
c. explained how DNA's structure permitted it to be replicated.
d. proposed the concept of the double-helix.
e. all of these
REVIEW: What are the base-pairing rules for DNA?
REVIEW: In the pairing of two nucleotides within the double helix
a. hydrogen bonds are used.
b. adenine and thymine bind together.
c. purines bind with pyrimidines.
d. double-ring nitrogenous bases connect to single-ring bases.
e. all of these
REVIEW: From X-ray diffraction data, which of the following was determined about DNA?
a. The molecule had uniform diameter.
b. The molecule was long and narrow.
c. Part of the molecule repeated itself often.
d. The shape of the molecule could be spiral.
e. all of these
REVIEW: In the bonding of nitrogenous bases
REVIEW: Each DNA strand has a backbone that consists of alternating
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