Helix, double, in DNA

DNA is a linear polymer of covalently joined deoxyribonucleotides, of four types deoxyadenylate (A), deoxyguanylate (G), deoxycytidy-late (C), and deoxythymidylate (T). Each nucleotide, with its unique three-dimensional structure, can associate very specifically but non-covalently with one other nucleotide in the complementary chain A always associates with T, and G with C. Thus, in the double-stranded DNA molecule, the entire sequence of nucleotides in one strand is complementary to the sequence in the other. The two strands, held together by hydrogen bonds (represented here by vertical blue lines) between each pair of complementary nucleotides, twist about each other to form the DNA double helix. In DNA replication, the two strands separate and two new strands are synthesized, each with a sequence complementary to one of the original strands. The result is two double-helical molecules, each identical to the original DNA. 

Another model we have examined is based on Eichhom s observation that cis-Pt(NH2)enPtCl2 pack in the solid so that the square planes are V 3.4 1 apart, whereas this is not true for trans-Pt(NH3)2Cl2. Because the distance between base pairs along the double helix in DNA is also 3.4 A, Eichhom suggested the cis-Pt(NH3)Cl2 molecules might "costack" (28). 

One proposed mechanism for the formation of a double helix in DNA is given by... 

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. 

Figure 3.10 Stniciural details of the bridging units between pairs of bases in separate strands of the double helix of DNA (a) the thymine-adenine pair (b) the cytosine-guanine pair.

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. 

Bacterial promoters are approximately 40 nucleotides (40 bp or four turns of the DNA double helix) in length, a region small enough to be covered by an E coli RNA holopolymerase molecule. In this consensus promoter region are two short, conserved sequence elements. Approximately 35 bp upstream of the transcrip-... 

DNA replication differs between the leading strand and the lagging strand of the DNA double helix. In cells, replication Af the lagging strand involves the formation of short RNA primers by action of an enzyme called RNA primase (or primase for short). Such RNA primers are made at intervals on the lagging strand and are then removed and replaced with DNA by DNA polymerase. 

The curvature effect of the double helix of DNA, caused by the binding of dimers of active receptors to the HRE sequences, has been obtained by means of experiments of transfection of lineal DNA structures to cells that previously did not express the gene under study. The reality of the cells in vivo must be... 

The Beato group has studied in depth the influence of the nucleosome structure in response to glucocorticoids (Beato 1989). Nucleosomes are formed by segments of 120 nucleotides of the double helix of DNA that make two twists around an octamer of histone. There are 200 nucleotides between two consecutive nucleosomes, so that a gene normally has tens of nucleosomes. 

In forming the double-helix polymeric DNA structure, the two sugar-phosphate backbones twist around the central stack of base pairs, generating a major and minor groove. Several conformations, known as DNA polymorphs, are possible. 

DNA codes for its own synthesis at the time of cell division. Thus, DNA acts as the agent of inheritance. As is developed below, DNA is a double-stranded helical molecule—the famous double helix—in which the two strands are complementary. DNA is the repository of information that is expressed in synthesis of the proteins of the cell. Therefore, DNA acts as the determinant of the biochemical personality of the cell. ... 

The most famous stracture in aU chemistry is the Watson-Crick double helix for DNA (figure 12.3). The discovery of this structure by James Watson and Francis Crick in 1953 was the beginning of molecular biology. An amazing number of insights about the nature of life have been derived from this structure. 

Intercalation the insertion of a molecule between adjacent base pairs in the double helix of DNA. 

Klug, A. and Travers, A.A. (1989) The helical repeat of nucleosome-wrapped DNA. Cell 56, 10-11. White, J.H. and Bauer, W.R. (1989) The helical repeat of nucleosome-wrapped DNA. Cell 56, 9-10. Germond, J.E., Hirt, B., Oudet, P., Gross-Bellard, M., and Chambon, P. (1975) Folding of the DNA double helix in chromatin-like structures from simian virus 40. Proc. Natl. Acad. Sci. USA 72, 1843-1847. 

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