1.8 CONFORMATION OF NUCLEIC ACIDS

Critical thinking Questions 1. How can curls be induced in hair? 2. Suppose that a protease is synthesized by the solid-phase method from D rather than L amino acids. How would the sedimentation, electrophoretic and circular dichroism properties of this enzyme compare with those of the native form? 3. Proteins that span biological membranes often contain alpha helices. Why is an alpha-helix particularly suited to exist in a hydrophobic environment? 4. What is the essential condition for a protein to be identical to another protein? 5. In sickle cell anemia, are all of the structural levels of the protein modified?

We have learnt in previous section about the Nucleic acids and their structure. Deoxyribonucleic acid (DNA) contains all the information required to build the cells and tissues of an organism. The exact replication of this information in any species assures its genetic continuity from generation to generation and is critical to the normal development of an individual. The information stored in DNA is arranged in hereditary units, now known as genes that control identifiable traits of an organism. In the process of transcription, the information stored in DNA is copied into ribonucleic acid (RNA), which has three distinct roles in protein synthesis. Discovery of the structure of DNA in 1953 and subsequent elucidation of how DNA directs synthesis of RNA, which then directs assembly of proteins—the so-called central dogma—were monumental achievements marking the early days of molecular biology. However, the simplified representation of the central dogma as DNA  RNA  protein does not reflect the role of proteins in the synthesis of nucleic acids. Moreover, as discussed in later chapters, proteins are largely responsible for regulating gene expression, the entire process whereby the Figure 1.8.1 The chemical structure shows a hydroxyl group information encoded in DNA is decoded into the proteins that characterize various cell at the 3’ end and a phosphate group at the 5’ end. Note also that two phosphoester bonds link adjacent nucleotides; this two-bond linkage commonly is referred to as a types. phosphodiester bond. By convention, a polynucleotide We have seen in detail the structure of nucleic acids, and hence we shall directly sequence is always written unless otherwise indicated. in the 5n3 direction (left to right) poop into the further details of its polymerization. Native DNA Is a Double Helix of Complementary Antiparallel Strands. The modern era of molecular biology began in 1953 when James D. Watson and Francis H. C. Crick proposed that DNA has a double-helical