In the 1950s, a young American biologist, James Watson, teamed up
with British graduate student Francis Crick at Cambridge University in
England to try to determine the structure of DNA. By 1953, they had put
together a model for the structure of DNA as shown in Figure 10-4. They
proposed that DNA is made of two chains that wrap around each other in
the shape of a double helix, a shape similar to a winding spiral
staircase. Their final model was correct and was remarkable because it
explained how DNA could replicate.
Watson and Crick relied on other scientists’ work to develop their
DNA model. Part of that work was X-ray diffraction photographs of DNA
crystals. The photographs and crystals were produced by researchers
Rosalind Franklin and Maurice Wilkins, at King’s College in London.
In 1962, Watson, Crick, and Wilkins received the Nobel Prize in
Medicine for their work on DNA. Rosalind Franklin died in 1958 and so
could not be named in the award. However, an important genetics
institute in Cambridge now bears her name, and her contribution is
recognized around the world.DNA NUCLEOTIDES
DNA is a nucleic acid made of two long chains (also called strands)
of repeating subunits called nucleotides. Each nucleotide consists of
three parts: a five-carbon sugar, a phosphate group, and a nitrogenous
base. The five-carbon sugar in a DNA nucleotide is called deoxyribose.
The phosphate group consists of a phosphorus (P) atom bonded to four
oxygen (O) atoms. The nitrogenous base contains nitrogen (N) atoms and
carbon (C) atoms and is a base (accepts hydrogen ions).
The DNA double helix is similar to a spiral staircase. The
alternating sugar and phosphate molecules form the side “handrails” of
the staircase. Nucleotides along each strand are connected by covalent
bonds between the sugar of one nucleotide and the phosphate group of the
next nucleotide. Each full turn of the DNA helix has 10 nucleotide
The nitrogenous bases (called “bases” for short) face toward the
center of the DNA molecule. The bases on one strand of DNA face—and form
bonds called hydrogen bonds with—the bases on the other strand.
Nitrogenous bases are bonded in pairs between the two strands by two or
three hydrogen bonds. The base pairs form the “steps” of the staircase.
The base pairs are of uniform width because, in each pair one base has a
two-ring structure and the other base has a single-ring structure.
Hydrogen bonds between the bases help hold the two chains of the DNA double helix together, as shown the figuer below.Nitrogenous Bases
The sugar and phosphate group are identical in all DNA nucleotides.
However, the nitrogenous base may be any one of four different
kinds—thymine, cytosine, adenine, or guanine . The nitrogenous bases and
their chemical structures, called rings. The nitrogenous bases are
often represented by the first letter of their name—T (thymine), C
(cytosine), A (adenine), and G (guanine). Nitrogenous bases that have a
double ring of carbon and nitrogen atoms, such as adenine and guanine,
are called purines. Nitrogenous bases that have a single ring of carbon
and nitrogen atoms, such as cytosine and thymine, are called
In 1949, American biochemist Erwin Chargaff observed that the
percentage of adenine equals the percentage of thymine, and the
percentage of cytosine equals that of guanine in the DNA of a variety of
organisms. This observation was key to understanding the structure of
DNA because it meant bases pair by base-pairing rules—in DNA, cytosine
on one strand pairs with guanine on the opposite strand, and adenine
pairs with thymine. These pairs of bases are called complementary base
pairs. Notice that each complementary base pair contains one
double-ringed purine and one single-ringed pyrimidine.
Because of the base-pairing rules, the order of the nitrogenous bases
on the nucleotides in one chain of the DNA molecule is complementary to
the order of bases on the opposite chain. For example, if a DNA chain
has the sequence ATTC, then the other chain must have the complementary
sequence TAAG. The order of nitrogenous bases on a chain of DNA is
called its base sequence. Complementary base pairing is important in DNA
structure and function for two reasons. First, the hydrogen bonds
between the base pairs help hold the two strands of a DNA molecule
together. Second, the complementary nature of DNA helps explain how DNA
replicates before a cell divides. One strand of a DNA molecule can serve
as a template for making a new complementary strand.DNA Models
The structure of DNA is often simplified when it is drawn or modeled.
For example, the DNA double helix is often illustrated as a straight
ladder. The ugarphosphate “handrails” are drawn as a straight line so
that the basepair “steps” between the DNA strands are easier to see.
Notice that simplifying the DNA structure highlights the complementary
base pairs in each of the DNA nucleotides. In some cases the structure
of DNA is simplified even more by just writing the first letter of each
of the nitrogenous bases in the DNA nucleotides. For example, the DNA in
figure above would be represented by: