The Griffith and the Hershey/Chase experiments proved that DNA was the hereditary material so that the next questions scientists
asked was, ?What is the chemical nature of DNA??
DNA was actually recognized as far back as 1869. Over the ensuing years, a number of things were discovered about the molecule.
First, DNA consists of nucleotides. Each nucleotide in turn consists of a sugar (deoxyribose), a phosphate group, and one of four
possible nitrogenous bases (A, C, G, T) (Image 3). The nitrogenous base is attached to the number 1 carbon of the sugar and the
phosphate group is attached to the number 5 carbon (Image 5). Because there are four bases, there are four different nucleotides. The
nucleotides are hooked together by 3?-5? phosphodiester linkages into long, thin molecules (Image 6).
As the 20th century progressed, scientists realized that understanding the structure of DNA was a key to understanding how it
functioned. Mirsky showed that all cells of a given species contain equal amounts of DNA except for gametes (eggs and sperm)
which contain only one-half as much. This observation makes sense because you get half of your genetic make-up from your father
and half from your mother. Also, Chargaff found that the proportion of nitrogenous bases varies from one species to another, but that
in any given cell, the amount of A = T, and G = C. As an example, pretend that we extracted the DNA from an aardvark. The
proportions of the various DNA bases from this species are as follows:
A = 20
G = 30
C = 30
T = 20
Now, let?s extract the DNA from a hummingbird. We get the following results:
A = 40
G = 10
C = 10
T = 40
Although the proportions of bases differ between the species, within a given species A = T and C = G.
In 1953 Watson and Crick published a paper which described the three-dimensional structure of DNA. They based their results on
the facts that DNA consisted of nucleotides, that DNA was a long, thin molecule, that the DNA molecule was probably helical, and
that A = T and C = G. They concluded that DNA was a double helix.
Think of a ladder. The ladder has two parts, the side pieces or rails and the cross pieces or rungs. The rails of the DNA ladder
consist of alternating sugar and phosphate molecules (Image 7). The rails are antiparallel, which means that whereas one rail is