Cells and Genomes

The surface of our planet is populated by living things curious, intricately

organized chemical factories that take in matter from their surroundings and

use these raw materials to generate copies of themselves. The living organisms

appear extraordinarily diverse in almost every way. What could be more

different than a tiger and a piece of seaweed, or a bacterium and a tree? Yet

our ancestors, knowing nothing of cells or DNA, saw that all these things had

something in common. They called that something "life," marveled at it,

struggled to define it, and despaired of explaining what it was or how it

worked in terms that relate to nonliving matter.

The discoveries of the twentieth century have not diminished the

marvel quite the contrary. But they have lifted away the mystery

surrounding the nature of life. We can now see that all living things are made

of cells, and that these units of living matter all share the same machinery for

their most basic functions. Living things, though infinitely varied when viewed

from the outside, are fundamentally similar inside. The whole of biology is a

counterpoint between the two themes: astonishing variety in individual

particulars; astonishing constancy in fundamental mechanisms. In this first

chapter we begin by outlining the features that are universal to all living

things. We then survey, briefly, the diversity of cells. And we see how, thanks

to the common code in which the specifications for all living organisms are

written, it is possible to read, measure, and decipher these specifications to

achieve a coherent understanding of all the forms of life, from the smallest to

the greatest.

The Universal Features of Cells on Earth

It is estimated that there are more than 10 million perhaps 100

million living species on Earth today. Each species is different, and each

reproduces itself faithfully, yielding progeny that belong to the same species:

the parent organism hands down information specifying, in extraordinary

detail, the characteristics that the offspring shall have. This phenomenon of

heredity

processes, such as the growth of a crystal, or the burning of a candle, or the

formation of waves on water, in which orderly structures are generated but

without the same type of link between the peculiarities of parents and the

peculiarities of offspring. Like the candle flame, the living organism must

consume free energy to create and maintain its organization; but the free

energy drives a hugely complex system of chemical processes that is specified

by the hereditary information.

Most living organisms are single cells; others, such as ourselves, are vast

multicellular cities in which groups of cells perform specialized functions and

are linked by intricate systems of communication. But in all cases, whether we

discuss the solitary bacterium or the aggregate of more than 10

form a human body, the whole organism has been generated by cell divisions

from a single cell. The single cell, therefore, is the vehicle for the hereditary

information that defines the species (Figure 1-1). And specified by this

information, the cell includes the machinery to gather raw materials from the

environment, and to construct out of them a new cell in its own image,

complete with a new copy of the hereditary information. Nothing less than a

cell has this capability.is a central part of the definition of life: it distinguishes life from other13 cells that