Hydrogen-bonded single-stranded

Fig. 6. Electron micrographs of hydrogen-bonded single-stranded circles formed when purified linear heavy AAV single polynucleotide chains are exposed to annealing conditions. The arrows point to the projections observed on such single-stranded circles...

A. Side view of channel spanning the lipid layer of a planar lipid bilayer, The structure is comprised of two monomers, each in a left-handed, single stranded p -helical conformation, and joined together at the head or formyl end by means of six, intermolecular hydrogen bonds. The two formyl protons are seen at the center of the structure in this view. Replacement of these protons by methyls destabilizes the conducting dimer as shown with N-acetyl desformyl Gramicidin A (Fig. 3D). 

The two strands, in which opposing bases are held together by hydrogen bonds, wind around a central axis in the form of a double helix. Double-stranded DNA exists in at least six forms (A-E and Z). The B form is usually found under physiologic conditions (low salt, high degree of hydration). A single turn of B-DNA about the axis of the molecule contains ten base pairs. The distance spanned by one turn of B-DNA is 3.4 nm. The width (helical diameter) of the double helix in B-DNA is 2 nm. 

Figure 35-7. Diagrammatic representation of the secondary structure of a single-stranded RNA molecule in which a stem loop, or "hairpin," has been formed and is dependent upon the intramolecular base pairing. Note that A forms hydrogen bonds with U in RNA.

During the rephcation of DNA, there must be a separation of the two strands to allow each to serve as a template by hydrogen bonding its nucleotide bases to the incoming deoxynucleoside triphosphate. The separation of the DNA double hehx is promoted by SSBs, specific protein molecules that stabihze the single-stranded structure as the rephcation fork progresses. These stabi-... 

All multicellular life starts as a single cell. Copies of the DNA in that cell must eventually occupy almost every one of the trillions of cells in a human body. For that to happen, the DNA in the original cell must replicate itself many times. The key to this replication is the famous double helix. When two strands of DNA— let s call them X and Y—separate, each strand can assemble the other. X builds a new Y, forming a fresh double helix. Y does the same thing. This doubles the number of DNA molecules. This mechanism depends on the two strands of DNA being able to hold together under normal conditions, yet unwind easily. That is where hydrogen bonds come in. 

A recently discovered subset of triple-stranded /l-helices from bacteriophage tail proteins (alternatively termed triple-stranded /1-solenoids ) represents another distinct group of /1-fibrous folds (Fig. 3B). In these structures, three identical chains related by threefold rotational symmetry wind around a common axis. These chains form unusual parallel /1-sheets with no intra- and only intermolecular -structural hydrogen bonding. Kajava and Steven (this volume) survey the distinguishing structural features of the known triple-stranded /1-solenoids, also documenting their notable diversity and differences in comparison to the single-stranded /1-solenoids. 

Fig. 4. New structural models for amyloid and prion filaments with the parallel and in-register arrangement of //-strands in the //-sheets. //-Strands are denoted by arrows. The filaments are formed by hydrogen-bonded stacks of repetitive units. Axial projections of single repetitive units corresponding to each model are shown on the top. Lateral views of the overall structures are on the bottom. (A) The core of a //-helical model of the //-amyloid protofilament (Petkova et al., 2002). Two such protofilaments coil around one another to form a //-amyloid fibril. (B) The core of a //-helical model of the HET-s prion fibril (Ritter et al., 2005). The repetitive unit consists of two //-helical coils. (C) The core of a superpleated //-structura l model suggested for yeast prion Ure2p protofilaments and other amyloids (Kajava et al., 2004).

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