885dc24_9222/11/043:11 PM Page922mac76mac76:385reb: Part Il Information Pathways upon which life itself is based. We might expect the for- DNA mation of phosphodiester bonds in DNA or peptide bonds in proteins to be a trivial feat for cells, given the arsenal of enzymatic and chemical tools described in Part Il. However, the framework of patterns and rules established in our examination of metabolic pathways RNA thus far must be enlarged considerably to take into account molecular information. Bonds must be formed Translation between particular subunits in informational biopoly- mers, avoiding either the occurrence or the persistence of sequence errors. This has an enormous impact on the thermodynamics, chemistry, and enzymology of the biosynthetic processes. Formation of a peptide bond re- The central dogma of molecular biology, showing the general path- quires an energy input of only about 21 kJ/mol of bonds tion. The term"dogma"is a misnomer. Introduced by Francis Crick at and can be catalyzed by relatively simple enzymes.But a time when little evidence supported these ideas, the dogma has be. to synthesize a bond between two specific amino acids come a well-established principle. at a particular point in a polypeptide, the cell invests about 125 kJ/mol while making use of more than 200 enzymes, RNA molecules, and specialized proteins. The chemistry involved in peptide bond formation does not change because of this requirement, but additional Part Ill explores these and related pi In processes are layered over the basic reaction to ensure pter 24 we examine the structure, topology, and that the peptide bond is formed between particular ackaging of chromosomes and genes. The processes amino acids. Information is expensive underlying the central dogma are elaborated in Chap- The dynamic interaction between nucleic acids and ters 25 through 27. Finally, we turn to regulation, ex- proteins is another central theme of Part Ill. with the amining how the expression of genetic information is important exception of a few catalytic RNA molecules controlled(Chapter 28) (discussed in Chapters 26 and 27), the processes that A major theme running through these chapters is make up the pathways of cellular information flow are the added complexity inherent in the biosynthesis of catalyzed and regulated by proteins. An understanding macromolecules that contain information. Assembling of these enzymes and other proteins can have practical nucleic acids and proteins with particular sequences of as well as intellectual rewards, because they form the nucleotides and amino acids represents nothing less basis of recombinant dna technology (introduced in than preserving the faithful expression of the template Chapter 9)Part III explores these and related processes. In Chapter 24 we examine the structure, topology, and packaging of chromosomes and genes. The processes underlying the central dogma are elaborated in Chap￾ters 25 through 27. Finally, we turn to regulation, ex￾amining how the expression of genetic information is controlled (Chapter 28). A major theme running through these chapters is the added complexity inherent in the biosynthesis of macromolecules that contain information. Assembling nucleic acids and proteins with particular sequences of nucleotides and amino acids represents nothing less than preserving the faithful expression of the template upon which life itself is based. We might expect the for￾mation of phosphodiester bonds in DNA or peptide bonds in proteins to be a trivial feat for cells, given the arsenal of enzymatic and chemical tools described in Part II. However, the framework of patterns and rules established in our examination of metabolic pathways thus far must be enlarged considerably to take into account molecular information. Bonds must be formed between particular subunits in informational biopoly￾mers, avoiding either the occurrence or the persistence of sequence errors. This has an enormous impact on the thermodynamics, chemistry, and enzymology of the biosynthetic processes. Formation of a peptide bond re￾quires an energy input of only about 21 kJ/mol of bonds and can be catalyzed by relatively simple enzymes. But to synthesize a bond between two specific amino acids at a particular point in a polypeptide, the cell invests about 125 kJ/mol while making use of more than 200 enzymes, RNA molecules, and specialized proteins. The chemistry involved in peptide bond formation does not change because of this requirement, but additional processes are layered over the basic reaction to ensure that the peptide bond is formed between particular amino acids. Information is expensive. The dynamic interaction between nucleic acids and proteins is another central theme of Part III. With the important exception of a few catalytic RNA molecules (discussed in Chapters 26 and 27), the processes that make up the pathways of cellular information flow are catalyzed and regulated by proteins. An understanding of these enzymes and other proteins can have practical as well as intellectual rewards, because they form the basis of recombinant DNA technology (introduced in Chapter 9). 922 Part III Information Pathways The central dogma of molecular biology, showing the general path￾ways of information flow via replication, transcription, and transla￾tion. The term “dogma” is a misnomer. Introduced by Francis Crick at a time when little evidence supported these ideas, the dogma has be￾come a well-established principle. RNA Protein Transcription Translation Replication DNA 8885d_c24_922 2/11/04 3:11 PM Page 922 mac76 mac76:385_reb: