Double-helix

The unmodified and complementary oligonucleotides were also synthesized, in order to detect thermodynamic and spectroscopic differences between the double helices. Circular dichroism spectra revealed that the covalently bound anthracene does not stack in the centre of the DNA double helix. Mutagenic activity by intercalative binding of the anthracene residue is thus unlikely. Only in vitro and in vivo replication experiments with site-specifically modified... 

The furanose rings of the deoxyribose units of DNA are conformationally labile. All flexible forms of cyclopentane and related rings are of nearly constant strain and pseudorotations take place by a fast wave-like motion around the ring The flexibility of the furanose rings (M, Levitt, 1978) is presumably responsible for the partial unraveling of the DNA double helix in biological processes. 

Copper(I) tends towards a tetrahedral coordination geometry in complexes. With 2,2 -bipyr-idine as a chelate ligand a distorted tetrahedral coordination with almost orthogonal ligands results. 2,2 -Bipyridine oligomers with flexible 6,6 -links therefore form double helices with two 2,2 -bipyridine units per copper(I) ion (J. M. Lehn, 1987,1988). J. M. Lehn (1990 U. Koert, 1990) has also prepared such helicates with nucleosides, e.g., thymidine, covalently attached to suitable spacers to obtain water-soluble double helix complexes, so-called inverted DNA , with internal positive charges and external nucleic bases. Cooperative effects lead preferentially to two identical strands in these helicates when copper(I) ions are added to a mixture of two different homooligomers. 

A DNA double helix as pic tured on a 1964 postage stamp issued by Israel... 

For more about starch see The Other Double Helix— The Fascinating Chemistry of Starch in the August 2000 issue of the Journal of Chem ical Education pp 988-992... 

The structure proposed by Watson and Crick was modeled to fit crystallographic data obtained on a sample of the most common form of DNA called B DNA Other forms include A DNA which is similar to but more compact than B DNA and Z DNA which IS a left handed double helix... 

FIGURE 28 5 (a) Tube and (b) space filling models of a DNA double helix The carbohydrate-phosphate backbone is on the out side and can be roughly traced in (b) by the red oxygen atoms The blue atoms belong to the purine and pyrimidine bases and he on the inside The base pairing is more clearly seen in (a)... 

We have so far described the structure of DNA as an extended double helix The crys tallographic evidence that gave rise to this picture was obtained on a sample of DNA removed from the cell that contained it Within a cell—its native state—DNA almost always adopts some shape other than an extended chain We can understand why by doing a little arithmetic Each helix of B DNA makes a complete turn every 3 4 X 10 m and there are about 10 base parrs per turn A typical human DNA contains 10 base parrs Therefore... 

FIGURE 28 6 The effective length of DNA is reduced by coiling around the surface of histones to form nucleo somes The histone proteins are represented by the spheres and the DNA double helix by the ribbon... 

A single helix is a coil a double helix is two nested coils The tertiary structure of DNA in a nucleosome is a coiled coil Coiled coils are referred to as supercoils and are quite common... 

All of the steps from the unwinding of the original DNA double helix to the super coiling of the new DNAs are catalyzed by enzymes... 

The DNA to be copied is a double helix shown here as flat for clarity... 

Section 28 8 The most common form of DNA is B DNA which exists as a right handed double helix The carbohydrate-phosphate backbone lies on the outside the punne and pyrimidine bases on the inside The double helix IS stabilized by complementary hydrogen bonding (base pairing) between adenine (A) and thymine (T) and guanine (G) and cytosine (C)... 

Genes are constructed from sets of deoxyribonucleic acids (DNA), which in turn consist of chains of nucleotides. These chains occur in matched pairs, twisted around each other (a double helix). 

Fig. 33. Self-recognition in the self-assembly of double helixes (9,200).

Primary and Secondary Structure. The DNA double helix was first identified by Watson and Crick in 1953 (4). Not only was the Watson-Crick model consistent with the known physical and chemical properties of DNA, but it also suggested how genetic information could be organized and rephcated, thus providing a foundation for modem molecular biology. 

A double helix shortens mixing time but requires more power. The disadvantages of the higher torque requirement are freqiientlv offset bv the better mixing and heat transfer,... 

Parallel Day 1 South-facing slope is heated—single helix 2 Upslope flow on both heated slopes—double helix... 

Night 3 Downslope flow on both slopes-double helix 4 Downslope flow on both slopes—double helix... 

Figure 7.1 Schematic drawing of B-DNA. Each atom of the sugar-phosphate backbones of the double helix is represented as connected circles within ribbons. The two sugar-phosphate backbones are highlighted by orange ribbons. The base pairs that are connected to the backbone are represented as blue planks.

Notice that in B-DNA the central axis of this double helix goes through the middle of the base pairs and that the base pairs are perpendicular to the axis. 

Figure 9.14 The two domains of the POU region bind in tandem on opposite sides of the DNA double helix. Both the POU-specific domain and the POU homeodomain have a helix-turn-helix motif (blue and red) which binds to DNA with their recognition helices (red) in the major groove. The linker region that joins these domains is partly disordered. (Adapted from J.D. Klemm et al.. Cell 77 21-32, 1994.)...

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