Page 93 - Color Atlas of Biochemistry

P. 93

84 Biomolecules

DNA sugar and phosphate residues in the back-
bone. Along the whole length of the DNA
molecule, there are two depressions—re-
A. DNA: structure
ferred to as the “minor groove” and the “ma-
Like RNAs (see p. 82), deoxyribonucleic acids jor groove”—that lie between the strands.
(DNAs) are polymeric molecules consisting of
nucleotide building blocks. Instead of ribose,
B. Coding of genetic information
however, DNA contains 2 -deoxyribose, and
the uracil base in RNA is replaced by thymine. In all living cells, DNA serves to store genetic
Thespatial structureof the two molecules information. Specific segments of DNA
also differs (see p. 86). (“genes”) are transcribed as needed into
The first evidence of the special structure RNAs, which either carry out structural or
of DNA was the observation that the amounts catalytic tasks themselves or provide the basis
of adenine and thymine are almost equal in for synthesizing proteins (see p. 82). In the
every type of DNA. The same applies to gua- latter case, the DNA codes for the primary
nine and cytosine. The model of DNA struc- structure of proteins. The “language” used in
ture formulated in 1953 explains these con- this process has four letters (A, G, C, and T). All
stant base ratios: intact DNA consists of two of the words (“codons”) contain three letters
polydeoxynucleotide molecules (“strands”). (“triplets”), and each triplet stands for one of
Each base in one strand is linked to a comple- the 20 proteinogenic amino acids.
mentary base in the other strand by H-bonds. Thetwo strands ofDNA arenot function-
Adenine is complementary to thymine, and ally equivalent. The template strand (the (–)
guanine is complementary to cytosine. One strand or “codogenic strand,” shown in light
purine base and one pyrimidine base are gray in Fig.1) is the one that is read during the
thus involved in each base pair. synthesis of RNA (transcription; see p. 242).
The complementarity of A with T and of G Its sequence is complementary to the RNA
with C can be understood by considering the formed. The sense strand (the (+) strand or
H bonds that arepossiblebetween thediffer- “coding strand,” shown in color in Figs. 1 and
ent bases. Potential donors (see p. 6) are 2)has the same sequence as the RNA, except
amino groups (Ade, Cyt, Gua) and ring NH that T is exchanged for U. By convention, it is
groups. Possible acceptors are carbonyl oxy- agreed that gene sequences are expressed by
gen atoms (Thy, Cyt, Gua) and ring nitrogen reading the sequence of the sense strand in
atoms. Two linear and therefore highly stable the 5 3 direction. Using the genetic code
bonds can thus be formed in A–T pairs, and (see p. 248), in this case the protein sequence
three in G–C pairs. (3) is obtained directly in the reading direc-
Base pairings of this type are only possible, tion usual for proteins—i. e., from the N termi-
however, when the polarity of the two strands nus to the C terminus.
differs—i. e., when they run in opposite direc-
tions (see p. 80). In addition, the two strands
have to be intertwined to form a double helix.
Due to steric hindrance by the 2 -OH groups
of theriboseresidues, RNA is unable to form a
double helix. The structure of RNA is therefore
less regular than that of DNA (see p. 82).
The conformation of DNA that predomi-
nates within the cell (known as B-DNA)is
shown schematically in Fig. A2 and as a van
der Waals model in Fig. B1.In the schematic
diagram (A2), the deoxyribose–phosphate
“backbone” is shown as a ribbon. The bases
(indicated by lines) are located on the inside
of the double helix. This area of DNA is there-
fore apolar. By contrast, the molecule’s surface
is polar and negatively charged, due to the

88 89 90 91 92 93 94 95 96 97 98