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Roll (U)
Twist (:)
Tilt (W)
Figure 1 A
. The molecular parameters describing DNA curvature are assigned to the
relative orientation of two successive dinucleotides: roll angle (U), tilt angle (W) and
twist (:). In the ideal, Watson-Crick model, U=W=0 and :=36
o
(10 basepairs per
helical turn), B DNA in solution has a twist angle :=34.3
o
(10.5 basepairs per helical
turn); for a detailed description of these and other parameters see [54].
B
.
Macroscopic curvature of an elastic rod is characterized by a deflection angle D, in the
case of DNA this is sometimes expressed in degrees per helical turn
. C
. The
experimentally determined conformation of DNA can be characterized by local roll,
tilt and twist angles, and these values can be used to reconstruct the trajectory of the
Z-axis.
1. Calculation of DNA curvature
The thinking of biologists has been profoundly influenced by the idea of local structural
polymorphism in DNA. DNA is no longer considered as a featureless polymer but rather as
a series of individual domains differing in flexibility and curvature. Unlike in the case of
helical polymorphism (e.g. B, A or Z structures), here we often deal with a localised
micropolymorphism in which the original B-DNA structure is only distorted but is not
extensively modified [9]. The deviations from ideal, straight DNA are usually expressed as
angles of deflection between adjacent base pairs (Figure 1A).
The terms “curved DNA” or “DNA curvature” are used in various contexts. For
instance, asymmetrical binding of proteins can induce both kinks and smooth bends in the
DNA trajectory. In this review we attempt to summarize another phenomenon, an inherent
structural micro-heterogeneity of DNA that occurs in the absence of bound proteins, and
