DNA A-form

Z-DNA a form of DNA that is twisted into a left-handed spiral named for its zigzag conformation which is slimmer than B-DNA Z-scheme a mechanism whereby electrons flow between PSII and PSI during photosynthesis... 

A-DNA a form of a DNA double helix characterized by having fewer residues per turn and major and minor grooves with dimensions that are more similar to each other than those of B-DNA (9.3)... 

Z-DNA a form of DNA that is a left-handed helix, which has been seen to occur naturally under certain circumstances (9.3)... 

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... 

Some oligonucleotides adopt an A-form helical stmcture (Fig. 2a) (5). The average stmctural parameters have been found consistent with the fiber diffraction model, but, as for B-form DNA, considerable variation is apparent among iadividual base pairs. 

Nucleic acids in the DNA contain a high number of nucleophilic sites that can be attacked by electrophilic intermediates (metabolites) of chemical compounds. DNA adducts formed may cause alterations in the expression of a critical gene in the cell and thus lead to cell death. For example, modification of p53 tumor suppressor gene may inactivate the functions of the p53 protein and render cells sensitive to malignant transformation. Also, formation of RNA adducts may inhibit key cellular events because RNA is essential for protein synthesis. 

The unique characteristic of each protein is the distinctive sequence of amino acid residues in its polypeptide chain(s). Indeed, it is the amino acid sequence of proteins that is encoded by the nucleotide sequence of DNA. This amino acid sequence, then, is a form of genetic information. By convention, the amino acid sequence is read from the N-terminal end of the polypeptide chain through to the C-terminal end. As an example, every molecule of ribonucle-... 

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. 

An alternative form of the right-handed double helix is A-DNA. A-DNA molecules differ in a number of ways from B-DNA. The pitch, or distance required to complete one helical turn, is different. In B-DNA, it is 3.4 nm, whereas in A-DNA it is 2.46 nm. One turn in A-DNA requires 11 bp to complete. Depending on local sequence, 10 to 10.6 bp define one helical turn in B-form DNA. In A-DNA, the base pairs are no longer nearly perpendicular to the helix axis but instead are tilted 19° with respect to this axis. Successive base pairs occur every 0.23 nm along the axis, as opposed to 0.332 nm in B-DNA. The B-form of DNA is thus longer and thinner than the short, squat A-form, which has its base pairs displaced around, rather than centered on, the helix axis. Figure 12.13 shows the relevant structural characteristics of the A- and B-forms of DNA. (Z-DNA, another form of DNA to be discussed shortly, is also depicted in Figure 12.13.) A comparison of the structural properties of A-, B-, and Z-DNA is summarized in Table 12.1. 

A graph of the fraction of single-stranded DNA reannealed c/cq) as a function of CqI on a semilogarithmic plot is referred to as a c t (pronounced cot ) curve (Figure 12.20). The rate of reassociation can be followed spectrophoto-metrically by the UV absorbance decrease as duplex DNA is formed. Note that... 

HSFl necessary for regulating its non-DNA binding state. The non-DNA binding form of HSFl is suggested to be a globular protein resembling a horseshoe conformation. 

---