Nucleic Acid Structure and
Properties of DNA
The genetic information in all living cells is carried in
molecules of deoxyribonucleic acid (DNA), which is pri-
marily found in chromosomes. However, DNA is also
present in cellular organelles such as mitochondria and
chloroplasts. Viruses carry genetic information in either
DNA or RNA (ribonucleic acid) molecules. When RNA
viruses infect cells, the genetic information in RNA is con-
verted to DNA prior to the replication and synthesis of new
viral particles. In the case of certain RNA viruses (retro-
viruses) such as human immunodeficiency virus (HIV),
the DNA that is copied from the infecting RNA is perma-
nently integrated into the host chromosomes and the vi-
ral genome becomes an integral part of the cells’ genetic
The structure of DNA was elucidated by James Watson
and Francis Crick in 1953. The structure they proposed
made it apparent for the first time how genetic informa-
tion in chemically stored in cells and how it is replicated
and transmitted from one generation to the next. The pro-
posed DNA structure also provided insight into the chem-
ical nature of mutations and how they might occur dur-
ing replication. Scarcely fifty years has passed since that
monumental discovery and now we are deciphering the
complete sequence of a human genome. This will lead to
a chemical understanding of thousands of genetic disor-
ders and the ability to diagnose, prevent, and treat many
inherited diseases and cancers.
In this chapter we discuss the structure and properties of
DNA. Subsequent chapters examine the functions of DNA
such as replication, mutation, recombination, repair, and
the expression of genes.
Components of Nucleic Acids
A nucleic acid is a
—a linear polymer
of nucleotides that is defined by its three components
(Figure 23-1):
1. A nitrogenous heterocyclic base (either a purine or a
pyrimidine) attached to the
'-carbon atom of the
sugar by an N-glycosidic bond. In DNA the purine
bases are adenine (A) and guanine (G) and the
pyrimidine bases are cytosine (C) and thymine (T).
The bases in RNA are the same except that uracil (U),
a pyrimidine, replaces thymine.
2. A cyclic five-carbon sugar which is ribose for RNA
and deoxyribose for DNA. The difference in structure
between ribose and
'-deoxyribose is shown in
Figure 23-1. The carbon atoms of the sugar are
numbered with a prime to distinguish them from the
carbon atoms in the base in the same nucleotide
3. A phosphate group attached to the 5'-carbon atom of
the sugar by a phosphodiester linkage. This phosphate
group is responsible for the strong negative charge of
both nucleotides and nucleic acids.
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