DNA, Gene Structure, and DNA Replication - Insect Molecular Genetics - page 11

respectively. The individual nucleotides are linked together with 3 ′ to 5 ′ phos-
phodiester bonds. RNA is typically single-stranded, making it less stable than
double-stranded DNA, although it can form complex structures (such as hairpins)
or become double-stranded.
1.7 The Double Helix
The discovery, by Watson and Crick (1953) , that DNA is a double helix of antipar-
allel polynucleotides ranks as one of the most important discoveries in biology
because it provided a hypothesis as to how DNA replication could be achieved
with a minimal error rate. Nitrogenous bases are located inside the double helix,
with the sugar and phosphate groups forming the backbone of the molecule on
the outside ( Figure 1.6 ). The nitrogenous bases of the two polynucleotides inter-
act by hydrogen bonding , with an A pairing to a T and a G to a C.
Hydrogen bonds are weak bonds in which two negatively charged atoms
share a hydrogen atom between them. Two hydrogen bonds form between
A and T and three hydrogen bonds form between G and C. Bonding between
G and C is thus stronger, and more energy is required to break it. The hydrogen
Figure 1.6 Two representations of the double-helix structure of DNA. The model on the left shows
the hydrogen-bonding between nitrogenous bases that holds the two antiparallel strands together.
The model on the right shows the relative sizes of the atoms in the molecule.