DNA, double-helix

During the 1940s, biologists determined that the bases in DNA from a variety of organisms had a specific relationship the amount of adenine (A) was equal to the amount of thymine (T), and the amount of guanine (G) was equal to the amount of cytosine (C). Eventually, scientists determined that adenine is always paired (1 1) with thymine, and guanine is always paired (1 1) with cytosine. 

Number of purine molecules = Number of pyrimidine molecules Adenine (A) = Thymine (T) 

FIGURE 17.6 An atomic model of a DNA molecule shows the double helix as the characteristic shape of DNA molecules. 

FIGURE 17.7 Hydrogen bonds between complementary base pairs hold the polynucleotide strands together in the double helix of DNA. 

Write the complementary base sequence for the following DNA segment —ACGATCT— 

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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. 

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

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)... 

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... 

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... 

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. 

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.)...

FIGURE 1.5 The DNA double helix. Two complementary polynucleotide chains running in opposite directions can pair through hydrogen bonding between their nitrogenous bases. Their complementary nucleotide sequences give rise to structural complementarity. 

The DNA isolated from different cells and viruses characteristically consists of two polynucleotide strands wound together to form a long, slender, helical molecule, the DNA double helix. The strands run in opposite directions that is, they are antiparallel and are held together in the double helical structure through interchain hydrogen bonds (Eigure 11.19). These H bonds pair the bases of nucleotides in one chain to complementary bases in the other, a phenomenon called base pairing. 

Because of the double helical nature of DNA molecules, their size can be represented in terms of the numbers of nucleotide base pairs they contain. For example, the E. coli chromosome consists of 4.64 X 10 base pairs (abbreviated bp) or 4.64 X 10 kilobase pairs (kbp). DNA is a threadlike molecule. The diameter of the DNA double helix is only 2 nm, but the length of the DNA molecule forming the E. coli chromosome is over 1.6 X 10 nm (1.6 mm). Because the long dimension of an E. coli cell is only 2000 nm (0.002 mm), its chromosome must be highly folded. Because of their long, threadlike nature, DNA molecules are easily sheared into shorter fragments during isolation procedures, and it is difficult to obtain intact chromosomes even from the simple cells of prokaryotes. 

FIGURE 12.13 (here and on the facing page) Comparison of the A-, B-, and Z-forms of the DNA double helix. The distance required to complete one helical torn is shorter in A-DNA than it is in B-DNA. The alternating pyrimidine-pnrine sequence of Z-DNA is the... 

The first one consists of 11-12 water molecules per nucleotide unit, which are coordinated directly to sites of the DNA double helix. Two of these water molecules are bound very tightly to the ionic phosphate residue and cannot be removed without completely destroying the structure of DNA. There are four other water molecules... 

E Transcription is the process by which RNA is produced to carry genetic information from the nucleus to the ribosomes. A short segment of the DNA double helix unwinds, and complementary ribonucleotides line up to pro-... 

The rate law for a reaction is a window into the changes that take place at the molecular level in the course of the reaction. Knowing how those changes take place provides answers to many important questions. For example, what controls the rate of formation of the DNA double helix from its individual strands What molecular events convert ozone into oxygen or turn a mixture of fuel and air into carbon dioxide and water when it ignites in an engine ... 

Self-Tesi 13.11 A Consider the following mechanism for the formation of a DNA double helix from its strands A and B ... 

FIGURE 19.29 The bases in the DNA double helix fit together by virtue of the hydrogen bonds that they can form, as shown on the left. Once formed, the AT and CC pairs are almost identical in size and shape. As a result, the turns of the helix shown on the right are regular and consistent. 

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