Chapter 19: Nucleic Acids

The structure of DNA is explored in detail, focusing on the double helix model where two antiparallel strands twist together in a right-handed spiral, stabilized by complementary base pairing between adenine and thymine and between guanine and cytosine. The hydrogen bonds between bases and stacking interactions between adjacent base pairs provide thermodynamic stability, with the strength of this stability varying based on guanine-cytosine content. The chapter explains DNA supercoiling, a topologically important feature where circular DNA molecules form compact coiled structures to maintain stability, a process regulated by topoisomerase enzymes. RNA is presented as typically single-stranded but capable of folding into complex secondary structures, with discussion of four major functional classes including ribosomal RNA, transfer RNA, messenger RNA, and small regulatory RNAs. The chapter then addresses the critical challenge of packaging DNA within cellular compartments through nucleosome formation, where DNA wraps around histone protein octamers, and through higher-order chromatin structures that enable efficient storage while remaining accessible for genetic processes. Chemical modification of histone proteins, particularly acetylation, is highlighted as a regulatory mechanism controlling gene expression. The chapter concludes with an examination of nuclease enzymes that catalyze phosphodiester bond hydrolysis, restriction endonucleases that recognize specific DNA sequences as tools for molecular biology, and practical applications of restriction enzymes in molecular cloning, DNA fingerprinting, and recombinant DNA technology.