Molecular Biology of The Cell

All living creatures are made of cells - small membrane-bounded compartments filled with a concentrated aqueous solution of chemicals. The simplest forms of life are solitary cells that propagate by dividing in two. Higher organisms, such as ourselves, are like cellular cities in which groups of cells perform specialized functions and are linked by intricate systems of communication. Cells occupy a halfway point in the scale of biological complexity. We study them to learn, on the one hand, how they are made from molecules and, on the other, how they cooperate to make an organism as complex as a human being. All organisms, and all of the cells that constitute them, are believed to have descended from a common ancestor cell through evolution by natural selection. This involves two essential processes: (1) the occurrence of random variation in the genetic information passed from an individual to its descendants and (2) selection in favor of genetic information that helps its possessors to survive and propagate. Evolution is the central principle of biology, helping us to make sense of the bewildering variety in the living world.

This chapter, like the book as a whole, is concerned with the progression from molecules to multicellular organisms. It discusses the evolution of the cell, first as a living unit constructed from smaller parts and then as a building block for larger structures. Through evolution, we introduce the cell components and activities that are to be treated in detail, in broadly similar sequence, in the chapters that follow. Beginning with the origins of the first cell on earth, we consider how the properties of certain types of large molecules allow hereditary information to be transmitted and expressed and permit evolution to occur. Enclosed in a

membrane, these molecules provide the essentials of a self-replicating cell. Following this, we describe the major transition that occurred in the course of evolution, from small bacteriumlike cells to much larger and more complex cells such as are found in present-day plants and animals. Lastly, we suggest ways in which single free-living cells might have given rise to large multicellular organisms, becoming specialized and cooperating in the formation of such intricate organs as the brain.

Clearly, there are dangers in introducing the cell through its evolution: the large gaps in our knowledge can be filled only by speculations that are liable to be wrong in many details. We cannot go back in time to witness the unique molecular events that took place billions of years ago. But those ancient events have left many traces for us to analyze. Ancestral plants, animals, and even bacteria are preserved as fossils. Even more important, every modern organism provides evidence of the character of living organisms in the past. Present-day biological molecules, in particular, are a rich source of information about the course of

evolution, revealing fundamental similarities between the most disparate of living organisms and allowing us to map out the differences between them on an objective universal scale. These molecular similarities and differences present us with a problem like that which confronts the literary scholar who seeks to establish the original text of an ancient author by comparing a mass of variant manuscripts that have been corrupted through repeated copying and editing. The task is hard, and the evidence is incomplete, but it is possible at least to make intelligent guesses about the major stages in the evolution of living cells.