Chapter 10: Lipids: Storage, Membrane Structure, Signaling, and Biological Functions

Section 10.1 details the three major forms of DNA—A-DNA, B-DNA, and Z-DNA—highlighting their helical parameters, groove dimensions, and biological significance. It emphasizes the dynamic nature of DNA, which can undergo conformational shifts influenced by sequence, hydration, and supercoiling. Section 10.2 introduces supercoiling as a method of DNA compaction, particularly in circular DNA like plasmids, and explains the role of topoisomerases in regulating DNA topology. It outlines linking number (Lk), twist (Tw), and writhe (Wr), and how changes in these parameters influence DNA superhelicity. Section 10.3 explores higher-order DNA organization in eukaryotic chromosomes, describing nucleosomes as repeating units of chromatin in which DNA wraps around histone octamers. It introduces the 30-nm fiber and hierarchical packaging into metaphase chromosomes. Section 10.4 shifts focus to RNA structure and highlights the functional versatility of RNA due to its ability to fold into elaborate tertiary structures, including stem-loops, bulges, pseudoknots, and ribozymes. The chapter also discusses riboswitches and non-coding RNAs, which regulate gene expression through structural conformations. Section 10.5 covers the physical properties of nucleic acids, such as UV absorption at 260 nm, melting curves, and thermal denaturation. It discusses how these properties are used to determine Tm and sequence complexity. The chapter concludes by emphasizing the interplay between sequence, structure, and function in nucleic acids and sets the stage for the molecular mechanisms of DNA replication, transcription, and repair discussed in later chapters.