DNAs Exist as Double-Helix Duplex Structures

Most DNAs Exist as Double-Helix (Duplex) Structures 

Like most other types of biological macromolecules, nucleic acids adopt highly organized three-dimensional structures. The dominant factors that determine nucleic acid 

A body of chemical information that proved vital to understanding DNA structure came from Erwin Chargaff s analyses of the nucleotide composition of duplex DNAs from various sources (table 25.1). Although the base compositions varied over a wide range, Chargaff found that within the DNA of each source that he examined, the amount of A was very nearly equal to the amount of T, and the amount of G was very nearly equal to the amount of C. The C is present as both unmodified C and, to a lesser extent, 5-methyl-cytosine, which results from postreplicative 

Base Composition of DNAs from Different Sources 

Dimensions and hydrogen bonding of (a) thymine to adenine and (b) cytosine to guanine. Note that two hydrogen bonds are formed in the A-T base pair and three in the G-C base pair. The overall dimensions of the base pairs are the same. Consequently they fit at any position in an otherwise regular polymeric structure. (Source Adapted from S. Arnott, M. H. F. Wilkins, L. D. Hamilton, and R. Langridge, Fourier synthesis studies of lithium DNA, part III Hoogsteen models, J. Mol. Biol. I l 39l, 1965.) 

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