The Central Dogma
The miracle that we call life is so common that we tend to become blind to its beauty. We are surrounded by living things and, unless we consciously stop to examine and appreciate these organsisms, we come to take for granted their varied forms, the precision of their processes and interactions, and the elegance with which they grow, maintain themselves, reproduce, and die. In the same way we forget the complexity and precision of our own bodies, becoming aware only when something goes wrong and we become sick or crippled in some way.
It has been recognized for many centuries that living things produce offspring that are deeply similar to themselves. This tendency to inherit the qualities of the parent, whether in plants, animals, or humans, is central to our cultural understanding of living things. A central question, either implicit or explicit, has related to how is that information contained in a single cell and how is it passed on to its offspring.
Clearly it takes an enormous amount of information to guide the development and operation of even a very simple organism. And that information extends not just to the individual organism itself, but also to the processes and interactions that position that organism in the environment that supports it and the web of living things that surround it.
A Metaphorical Overview
The foundations of modern biology are provided by what has become affectionately known as the "Central Dogma." Before talking about the biological concepts included in the central dogma, it might be useful to consider some of the properties of a living cell, and by extension, a living organism in terms that are closer to our every day experience.
Imagine that we wish to construct an extraordinarily complex factory, the most beautiful, complete, and functional factory ever built. In order to do so, we will require a very comprehensive and detailed set of plans. These plans must include blueprints for every element of the structure of the factory, the design of all of the machinery that will be used in the processes carried out by the factory, and detailed instructions for the placement, assembly, and operation of each machine. The blueprints must also specify all of the control and feedback systems needed to assure that the internal processes operate in an interdependent way that maximizes efficiency and order. In order to operate efficiently over a long period of time, the complete specifications for our factory also must include detailed maintenance instructions and the capacity to repair, or to fabricate and replace, every structural and mechanical element of the building and its equipment. Finally, our factory must have a set of sensitive and responsive connections to the outside world to assure that it maintains an appropriate relationship with the environment.
You can imagine that there must be
very careful management of the information and processes in the construction and operation of our wonderful factory. We want our factory to function perfectly for a long time so the original blueprints must be very carefully preserved to assure that they are not altered, blurred, or lost. In order to be sure that this is the case, we will need to do at least two things. One is to have an ongoing repair process so that any document is corrected if a letter is changed or deleted. The other is to keep these master documents in reserve.
To saveguard the master documents we do not allow them to be used directly in day to day operations. Rather, when information is needed from the master documents, we will make copies that can be sent out to the construction and operations areas. These copies represent shop drawings and instructions. Each copy contains only to a small portion of the master blueprint, a portion relating to a particular aspect of the structure or process, a single machine or even a single part. These copies will be made out of disposable materials and are designed to be destroyed once they are used. We do not want these shop drawings to be in any way mistaken for the master documents. At the same time, we do want them to be faithful copies of the original information.
Finally, in the construction of our factory and machines, we want to be able to use materials that can be assembled in different ways to make a very wide variety of structures, machines, and parts. If we can use a simple set of materials and build various kinds of structures and machines by assembling them in different ways, we can achieve a simplicity and dependability of supply while retaining great flexibility in our maintainance and operational processes.
Oh yes. In order to be truly successful, our factory must have one more quality. Once it is operational, it needs to be able to copy all of the information present in the original master documents and send that information out into the world in a way that allows another factory, a duplicate of the original factory, to be built and to become fully functional on its own.
The Central Dogma:
DNA—Our Molecular Blueprint
The construction and operation of such a factory, particularly a factory that can replicate itself, is vastly beyond our abilities. Yet, a living cell has all of the properties of our metaphorical factory, and many others as well. We are surrounded by, and composed of, cells of astounding beauty and complexity.
An overarching question in biology relates to how our living cellular factories achieve the many goals, and more, that we have set out for our imaginary factory? The central dogma states that the instructions that determine and guide the structure, development, and internal processes of a living cell are
contained in a unique and beautiful informational molecule called "deoxyribonucleic acid," abbreviated as DNA. DNA is the molecular equivalent of our master blueprints, containing all of the information required to build and operate the cell. Beyond that, the DNA contains all of the information needed to assure that every individual cell can maintain itself in right relation to surrounding cells. Adding to this complexity, these groups of cells must be able to interact with other groups to form tissues, organs, and organisms. And, in turn, these organisms must be able to interact with and respond to their environment.
DNA provides an extraordinarily efficient and compact means of storing and replicating information. It has been estimated that the information packed into the DNA of each one of our cells is the equivalent of about 200 encyclopedia volumes! The molecular blueprint for our cellular factory is enormously complex, and complete.
There are elaborate repair mechanisms in place to assist in maintaining the quality and integrity of our DNA blueprints. And, amazingly, these DNA molecules have the ability to make precise copies of themselves allowing growth as one cell divides into two cells. As a consequence each cell retains an absolutely complete set of molecular instructions for its own processes and growth (see below—DNA structure and the molecular basis of inheritance).
RNA—Our Molecular Shop Drawings
DNA is preserved in the nucleus of the cell while most of the ongoing operations and processes of the cell occur outside the nucleus, in an area loosely termed the "cytoplasm." It is in the cytoplasm that new structural components are built and new specialized "molecular machines" are fabricated.
Information is transmitted from the nuclear blueprint to the cytoplasm by means of a second kind of nucleic acid. This second nucleic acid, termed "ribonucleic acid" (RNA) is made by "copying" a small portion of the molecular structure of DNA. These RNA molecules serve as messengers, and for that reason are termed messenger RNA, or mRNA. The mRNA molecule, while a copy of a portion of the DNA molecule, is chemically different. This assures that our mRNA "shop drawings" will never become confused with the master "blueprint" DNA that produced it.
The mRNA molecules move from the nucleus to the cytoplasm where a complex process takes over, translating the information in the mRNA into the working structures of the cell. These structures are largely, usually completely, made of protein. The assembly of these proteins, a process known as protein synthesis, is the process that converts the information contained in our blueprints and shop drawings into the three dimensional structures of the cell and all of its component parts.