Biochemistry, 4th Edition P36. 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. | Summary 313 netically useful recombinant DNA molecules. For the isolation of even larger nucleotide sequences such as those of genes encoding large polypeptides or those of eukaryotic genes that are disrupted by large introns partial or limited digestion of DNA by restriction enzymes can be employed. However restriction endonucleases that cut only at specific nucleotide sequences 8 or even 13 nucleotides in length are also available such as Not and SfiI. Restriction Endonucleases Can Be Used to Map the Structure of a DNA Fragment The application of these sequence-specific nucleases to problems in molecular biology is considered in detail in Chapter 12 but one prominent application is described here. Because restriction endonucleases cut dsDNA at unique sites to generate large fragments they provide a means for mapping DNA molecules that are many kilobase pairs in length. Restriction digestion of a DNA molecule is in many ways analogous to proteolytic digestion of a protein by an enzyme such as trypsin see Chapter 5 The restriction endonuclease acts only at its specific sites so that a discrete set of nucleic acid fragments is generated. This action is analogous to trypsin cleavage only at Arg and Lys residues to yield a particular set of tryptic peptides from a given protein. The restriction fragments represent a unique collection of different-sized DNA pieces. Fortunately this complex mixture can be resolved by electrophoresis see the Appendix to Chapter 5 . Electrophoresis of DNA molecules on gels of restricted pore size as formed in agarose or polyacrylamide media separates them according to size the largest being retarded in their migration through the gel pores while the smallest move relatively unhindered. Figure shows a hypothetical electrophoretogram obtained for a DNA molecule treated with two different restriction nucleases alone and in combination. Just as cleavage of a protein with different proteases to generate overlapping fragments allows an .
