A-DNA B-DNA Z-DNA
In a DNA molecule, the two strands are not parallel, but intertwined with each
other. Each strand looks like a helix. The two strands form a "
double
helix" structure, which
was first discovered by
James D. Watson and Francis Crick in 1953.
In this structure, also
known as the
B form, the helix makes a turn every 3.4 nm, and the distance between two
neighboring base pairs is 0.34 nm. Hence, there are about 10 pairs per turn.
The
intertwined strands make two grooves of different widths, referred to as the
major
groove and the
minor groove, which may facilitate binding with
specific proteins.
In a solution with higher salt concentrations or with alcohol added, the DNA structure
may change to an
A form, which is still right-handed, but every 2.3 nm makes
a turn and there are 11 base pairs per turn.
Another DNA structure is called the
Z
form, because its bases seem to zigzag. Z DNA is left-handed. One turn spans 4.6 nm, comprising 12
base pairs. The DNA molecule with alternating G-C sequences in alcohol or
high salt solution tends to have such structure.