Chapter 34: Nucleic Acid Structure & Function

Deoxyribonucleic acid (DNA) is presented as a polymeric molecule composed of four distinct monomeric units—deoxyadenylate, deoxyguanylate, deoxycytidylate, and thymidylate—interconnected by 3-prime, 5-prime phosphodiester bonds. This structure possesses a distinct polarity, typically written in the 5-prime to 3-prime direction, and organizes into an antiparallel double helix where two strands are held together by specific hydrogen bonds between purine and pyrimidine bases. The stability of this double helix, primarily the B form under physiological conditions, is maintained by Watson-Crick base pairing—adenine with thymine and guanine with cytosine—as well as van der Waals and hydrophobic stacking interactions. A significant portion of the discussion is dedicated to the physical properties of DNA, such as its ability to undergo denaturation, or "melting," when subjected to heat or low salt concentrations. This process is influenced by the genetic sequence, where regions rich in guanine and cytosine are more resistant to separation due to the presence of three hydrogen bonds compared to the two found in adenine-thymine pairs. The chapter further details how DNA can exist in relaxed or supercoiled forms, a state managed by enzymes known as topoisomerases which relieve the torsional stress essential for processes like replication and transcription. The narrative shifts to contrast the chemical nature of ribonucleic acid (RNA) with DNA, highlighting key differences such as the presence of ribose sugar, the substitution of uracil for thymine, and its typically single-stranded existence. Despite being single-stranded, RNA molecules can fold into complex secondary structures, like hairpins, enabling diverse functional roles. The major classes of RNA are described in detail: messenger RNA (mRNA), which undergoes extensive post-transcriptional processing including 5-prime capping and 3-prime polyadenylation to transport genetic code from the nucleus to the cytoplasm; transfer RNA (tRNA), which acts as an adapter molecule in a cloverleaf configuration to align amino acids during protein synthesis; and ribosomal RNA (rRNA), which provides the structural and catalytic framework for the ribosome. The chapter also introduces modern discoveries in regulatory biology, including microRNAs (miRNAs) and small interfering RNAs (siRNAs) that modulate gene expression through silencing mechanisms, alongside long noncoding RNAs (lncRNAs) and circular RNAs (circRNAs) that impact cellular function. Finally, the role of nucleases is discussed, categorizing enzymes like endonucleases and exonucleases that degrade or edit nucleic acids, including the revolutionary CRISPR-Cas systems and restriction enzymes that have become indispensable tools in modern molecular genetics and medical research.