DISSERTATION

______________________________________________________________________ Introduction
(adenine and guanine). Adenine binds only to thymine, forming two hydrogen bonds, and
cytosine binds exclusively to guanine, forming three hydrogen bonds.
Figure 1.3. Scheme of the primary DNA structure.
Two complementary DNA strands (single-stranded DNA, ssDNA) coil around the same axis
forming a double helix (double-stranded DNA, dsDNA). This way hydrophobic bases are inside
the helix and the hydrophilic sugar-phosphate backbone is orientated outside (Figure 1.4). Both
DNA strands store the same biological information, that is, the nucleobase sequence. However,
they can bind only in an antiparallel disposition, which means that the strands are orientated in
opposite directions, one in the direction 3' to 5' and the other from 5' to 3'. The 5' end has a
terminal phosphate group, while the 3' end, a terminal hydroxyl group. DNA can form three
conformations – A, B and Z. B-DNA is a right-handed conformation, where each full turn in
the helix is 3.4 nm long and consists of ten bases (thus with 0.34 nm between adjacent bases).
A-DNA is also right-handed and it occurs when the humidity is lower than 75 %. In this
conformation the base pairs are not perpendicular to the helix axis, resulting in a wider and
1.2 DNA 7