LEHNINGER PRINCIPLES OF BIOCHEMISTRY - VOL. 3. INFORMATION PATHWAYS - 2017

PART III. INFORMATION PATHWAYS

The third and final part of the book focuses on the biochemical mechanisms that govern the transmission of hereditary information and the evolution of living organisms.

What is the Molecular Basis of genetic material? How is Genetic information passed down from generation to generation with such high fidelity? How do rare alterations arise in genetic material to serve as the raw material for evolution? How is genetic information translated into the Amino acid sequences of protein molecules?

Our modern understanding of metabolic information pathways emerged at the intersection of genetics, physics, and chemistry—the foundations of contemporary biochemistry. In 1953, James Watson and Francis Crick proposed the double-helical Structure of DNA (see Fig. 8-15 in Vol. 1). Genetic theory helped establish METABOLISM/2.html">THE CONCEPT OF information encoding within genes, while groundbreaking discoveries in physics revealed the molecular STRUCTURE OF THE Gene. Chemistry contributed to this theory by defining the composition of DNA. The ultimate value of the Watson-Crick hypothesis lay in its ability to synthesize diverse observations from various scientific fields.

The revolution in our understanding of Introduction/20.html">DNA Structure inevitably raised questions about its function. The double-stranded architecture inherently suggests a Replication mechanism, enabling encoded information to be passed across generations. The discovery of messenger and Transfer RNA, along with the deciphering of The Genetic Code, provided crucial insights into how DNA is converted into functional Proteins.

These and other breakthroughs led to the formulation of the Central dogma of molecular biology, which outlines the three Primary processes of genetic information Processing within The Cell. The first process is replication, in which DNA is copied, using a parent DNA strand to generate daughter DNA molecules with identical sequences. The second process is Transcription, whereby a portion of the genetic information encoded in DNA is transcribed into RNA molecules. The third process is Translation, during which genetic information encoded in RNA is delivered to Ribosomes and translated into a polypeptide with a specific Amino Acid Sequence.

The central dogma of molecular biology dictates the direction of Information Flow within the cell: from replication to transcription and translation. The term "dogma" is somewhat inaccurate and has persisted merely for historical reasons. It was introduced by Francis Crick at a time when there was little evidence to support the proposed ideas, which later developed into a well-established theory.

Class="center">The central concept (dogma) of molecular biology, outlining the main metabolic information pathways—replication, transcription, and translation. Calling it a "dogma" is not entirely correct, as Francis Crick proposed this concept when empirical evidence supporting these ideas was scarce, long before they evolved into a well-supported scientific theory.

Part III discusses these and other processes related to information transfer. Chapter 24 explores the structure, topology, and packaging of Chromosomes and genes. The core processes underlying the central dogma are examined in Chapters 25-27. Finally, Chapter 28 addresses the Regulation of Gene Expression.

A critical question running through all these chapters concerns the complex Biosynthesis of informational macromolecules. The assembly of NUCLEOTIDES and Amino Acids into precise sequences of Nucleic Acids and Proteins serves to preserve and accurately copy the template—the very foundation of life. One might assume that forming phosphodiester bonds in DNA or peptide bonds in proteins is a trivial task for Cells equipped with the arsenal of enzymatic and chemical tools described in Part II. However, accounting for the mechanisms of information storage and transfer requires a significant expansion of the perspective we developed from analyzing metabolic pathways. Chemical bonds must form specifically between the correct subunits of informational Biopolymers with a minimal probability of introducing and propagating errors. This requirement imposes stringent thermodynamic, chemical, and enzymological constraints on biosynthetic processes. While forming a peptide bond requires an energy input of approximately 21 kJ/mol and can be catalyzed by relatively simple Enzymes, synthesizing a bond between two specific amino acids at a precise position in a polypeptide requires about 125 kJ/mol and involves over 200 enzymes, RNA molecules, and specialized proteins. The basic chemical process of peptide bond formation is identical, but additional mechanisms are recruited to guarantee that the bond forms strictly between designated amino acids. Information comes at a high cost.

Another key theme in Part III is the dynamic interaction between nucleic acids and proteins. With the rare exception of catalytic RNA molecules (discussed in Chapters 26 and 27), metabolic processes involving information transfer are catalyzed and regulated by proteins. The Study of these enzymes and other proteins holds both fundamental and applied significance, enabling their use in recombinant DNA technologies (see Chapter 9, Vol. 1).

Returning to the theme of evolution, many of the processes examined in Part III originated billions of years ago, and some can be traced all the way back to the Last Universal Common Ancestor (LUCA). Ribosomes, virtually the entire translational machinery, and certain elements of the transcriptional apparatus are shared by All living organisms on Earth. Genetic information can be viewed as a molecular clock that allows scientists to determine the evolutionary relationships among species. Shared information pathways connect humans to every living Organism on Earth, as well as to all extinct species. Studying these pathways helps researchers pull back the curtain on the opening act of the play of life on Earth.

Supercoiling means much more to DNA than just a mechanical constraint; it confines an otherwise sprawling, unruly DNA molecule within the cramped quarters of the intracellular environment.

— Nicholas Cozzarelli, Harvey Lectures, 1993