In RNA, the other pyrimidine base is uracil (U), whereas DNA contains thymine (T).
Specific pairs of these bases hydrogen bond to each other in particularly favorable
arrangements, so they always occur in distinctive hydrogen-bonded base pairs. Adenine
and thymine (in DNA) or uracil (in RNA) each form two hydrogen bonds, so they are
always paired together; similarly, guanine and cytosine each form three hydrogen bonds,
so they always pair to each other (Figure 28-18). The resulting complementary base-
pairinglinks two strands of the polymer in a very specific order to form a double helix.
This order is the key to nucleic acid function. Figure 28-19 shows a short segment of the
double strand of DNA. In that figure, we see that adenine is paired with thymine and
guanine is paired with cytosine. In RNA, adenine forms base pairs with uracil instead of
with thymine.
DNA is one of the largest molecules known. Human DNA is estimated to have up to
three billion base pairs, resulting in a molecular weight in the tens of billions. RNA is
smaller and more variable, with molecular weights of 20,000 to 40,000. Genetic infor-
mation for each organism is stored in its DNA as a result of the locations and sequence
of the base pairs. The information is replicated when the strands unravel and new comple-
mentary strands are formed. The genetic information of the DNA is used to guide the
many syntheses that occur in a living cell. For example, for the synthesis of a specific
protein, a specific portion of the DNA double helix unwinds. Then an RNA molecule is
built using the sequence of base pairs of the DNA as a pattern. The RNA then migrates
from the cell’s nucleus to the location where the protein is to be synthesized. There the
base sequence of the RNA determines the sequence of amino acids to be used in the
protein construction. With its many possible sequences of the same base pairs to transmit
information to control synthesis of other molecules, DNA contains a vast amount of
genetic information.A stereoview of an idealized model of DNA.1136 CHAPTER 28: Organic Chemistry II: Shapes, Selected Reactions, and Biopolymers
Figure 28-18 The two kinds of hydrogen-bonded base-pairing that occur in DNA. In
RNA, adenine forms a base pair with uracil instead of with thymine.sugarsugaradeninethymineHNON NNN
NNOHHCH 3••••••sugarsugarguaninecytosineNNN NNO
NNOH•••H•••HNH•••H