The mechanisms by which DNA polymerases achieve their remarkable fidelity, including base selection and proofreading, are briefly reviewed. Nine proofreading models from the current literature are evaluated in the light of steady-state and transient kinetic studies of E. coli DNA polymerase I, the beststudied DNA polymerase.

Branched DNA molecules provide a challenging set of structural problems. Operationally we define branched DNA species as molecules in which double helical segments are interrupted by abrupt discontinuities, and we draw together a number of different kinds of structure in the class, including helical junctions of different orders, and base bulges (Fig. 1).

Ion pumps create ion gradients across cell membranes while consuming light energy or chemical energy. The ion gradients are used by the corresponding cell types for passive-ion transport via ion channels or carriers or for accumulation of nutrients like sugar or amino acids via cotransport systems or antiporters.

Transport of ions through channels is fundamental to a number of physiological processes, especially the electrical properties of excitable cells (Hille, 1992). To understand this process at a molecular level requires atomic resolution structures of channel proteins.

The number of globular proteins for which high resolution structures are available is rapidly increasing. In each case the particular sequence of the polypeptide appears to yield only a single, compact, biologically active structure. However, peptides with no obvious sequence similarity may form remarkably similar structures.

Most biological processes involve recognition and binding between molecules. Protein-peptide interactions are particularly common as, for example, in the action of hormones, such as oxytocin and vasopressin, or neurotransmitters including morphine-like peptides.

Rhodopsins are intrinsic membrane retinal-containing proteins composed of 7 hydrophobic a-helical transmembrane columns and hydrophilic sequences of various length connecting the helices and localized at N- and C-ends of the polypeptide. The chromophore (retinal) forms a Schiff base with a lysine residue in the middle part of the last a-helix. Absorption of a photon results in isomerization of retinal which gives rise to a conformational change in the protein moiety.

Proteins exposed to ionizing radiation suffer both reversible and irreversible effects. Reversible effects are defined as those which disappear in a short period of time after the removal of the radiation field and without further treatment of the sample. Irreversible effects are those which cause a permanent alteration in the structure of a protein.

Nonlinear dynamics of biopolymers is a new and rapidly developing field of biophysical science. It can be considered as a part of the general dynamics which deals with the internal mobility of biopolymers. Theoreticians define it also as the next (anharmonic or nonlinear) approximation after the first harmonic or linear one.

Macromolecular structure determination by X-ray crystallography and solution NMR spectroscopy has experienced unprecedented growth during the past decade.