Biochemistry, 4th Edition P37. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | What Sorts of Secondary Structures Can Double-Stranded DNA Molecules Adopt 323 The edges of the base pairs have a specific relationship to these grooves. The top edges of the base pairs top as defined by placing the glycosidic bond at the bottom as in Figure are exposed along the interior surface or floor of the major groove the base-pair edges nearest to the glycosidic bond form the interior surface of the minor groove. Some proteins that bind to DNA can actually recognize specific nucleotide sequences by reading the pattern of H-bonding possibilities presented by the edges of the bases in these grooves. Such DNA-protein interactions provide one step toward understanding how cells regulate the expression of genetic information encoded in DNA see Chapter 29 . Double Helical Structures Can Adopt a Number of Stable Conformations In solution DNA ordinarily assumes the familar structure we have been discussing B-DNA. However nucleic acids also occur naturally in other double helical forms. The base-pairing arrangement remains the same but the inherently flexible sugarphosphate backbone can adopt different conformations. Base-pair rotations are another kind of conformational variation. Helical twist is the rotation around the axis of the double helix of one base pair relative to the next Figure . Successive base pairs in B-DNA show a mean rotation of 36 with respect to each other. Pro-pellor twist involves rotation around a different axis namely an axis perpendicular to the helix axis Figure . Propellor twist allows greater overlap between successive bases along a strand of DNA and diminishes the area of contact between bases and solvent water. Two base pairs with 32 of right-handed helical twist the minor-groove edges are drawn with heavy shading. Propellor twist as in 2 allows greater overlap of successive bases along the same strand and reduces the area of contact between the bases and water. A-Form DNA Is an Alternative Form of Right-Handed DNA
