4.8.2 Structure of nucleic acids
Primary structure
Nucleotides join together in DNA and RNA by forming a phosphate ester
bond between the 5
0 -phosphate group on one nucleotide and the 3
0 -hydroxyl
group on the sugar (ribose or 2
0 -deoxyribose) of another nucleotide. In the
nucleic acids, these phosphate ester links provide the nucleic acids with a
long unbranched chain with a ‘backbone’ of sugar and phosphate units with
heterocyclic bases sticking out from the chain at regular intervals. One end
of the nucleic acid polymer has a free hydroxyl at C-3
0 (the 3
0 -end), and the
other end has a phosphate at C-5
0 (the 5
0 -end).
The structure of nucleic acids depends on the sequence of individual
nucleotides. The actual base sequences for many nucleic acids from various
species are available to date. Instead of writing the full name of each
nucleotide, abbreviations are used, e.g. A for adenine, T for thymidine, G
for guanosine and C for cytidine. Thus, a typical DNA sequence might be
presented as TAGGCT.
O
O
Base
O
P
O
O
O
O
O
Base
P O
O
Generalized structure of DNA
5'
3'
5'-end
3'-end
Secondary structure: base pairing
The base sequence along the chain of a DNA contains the genetic information.
Samples of DNA isolated from different tissues of the same species have the
same proportions of heterocyclic bases, but the samples from different species
often have different proportions of bases. For example, human thymus DNA
comprises 30.9% adenine, 29.4% thymine, 19.9% guanine and 19.8%
cytosine, while the bacterium Staphylococcus aureus contains 30.8% adenine,
29.2% thymine, 21% guanine and 19% cytosine. In these examples, it is clear
that the bases in DNA occur in pairs. Adenine and thymine are usually present
in equal amounts; so are cytosine and guanine. In the late 1940s, E. Chargaff
pointed out these regularities and summarized as follows.
4.8 NUCLEIC ACIDS
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