Datasheet

By complementarity, we mean that a thymine (T) on one strand is always
facing an adenine (A) (and vice versa) — and guanine (G) is always facing a
cytosine (C). These couples, A-T and G-C, although not linked by a chemical
bond, have a strict one-to-one reciprocal relationship. When you know the
sequence of nucleotides along one strand, you can automatically deduce the
sequence on the other one. This amazing property — and not the stylish
helical structure — is the Rosetta Stone that explains everything about DNA
T G
5' 3'
3'5'
A C T
ACTGA
Figure 1-6:
The two
comple-
mentary
strands of
a complete
DNA
molecule.
19
Chapter 1: Finding Out What Bioinformatics Can Do for You
The IUPAC code for DNA sequences
The following table lists the one-letter codes
(IUPAC codes) used to work with DNA sequences.
Official IUPAC codes, from the International Union
of Pure and Applied Chemistry, are defined for all
possible two- and three-way ambiguities. The
table shows only the ones most frequently used.
Most Common Letters Used for DNA Nucleotide Sequences
1-Letter Code Nucleotide Name Category
A Adenine Purine
C Cytosine Pyrimidine
G Guanine Purine
T Thymine Pyrimidine
N Any nucleotide (any base) (n/a)
R A or G Purine
Y C or T Pyrimidine
-- ----- None (gap)
05_089857 ch01.qxp 11/6/06 3:52 PM Page 19