Unifying Themes in Biology

 Cell theory describes the organization of living systems

 As was stated at the beginning of this chapter, all organ-isms are composed of cells, life's basic units . Cells were discovered by Robert Hooke in England in 1665, using one of the first microscopes, one that magnified 30 times. Not long after that, the Dutch scientist Anton van Leeuwenhoek used microscopes capable of magnifying 300 times and discovered an amazing world of single-celled life in a drop of pond water.
 In 1839, the German biologists Matthias Schneider and Theodor Schwann, summarizing a large number of observations by themselves and others, concluded that all living organisms consist of cells. 
Their conclusion has come to be known as the cell theory Later biologists added the idea that all cells come from preexisting cells. The cell theory, one of the basic ideas in biology, is the foundation for understanding the reproduction and growth of all organisms.

 The molecular basis of inheritance explains the continuity of life

 Even the simplest cell is incredibly complex—more intricate than any computer. The information that specifies what a cell is like—its detailed plan—is encoded in deoxyribonucleic add (DNA), a long, cablelike molecule. Each DNA molecule is formed from two long chains of building blocks, called nucleotides, wound around each other . Four different nucleotides are found in DNA, and the sequence in which they occur encodes the cell's information. Specific sequences of several hundred to many thousand nucleotides make up a gene, a discrete unit of information. 
The continuity of life from one generation to the next—heredity—depends on the faithful copying of a cell's DNA into daughter cells. The entire set of DNA instructions that specifies a cell is called its genome. The sequence of the human genome, 3 billion nucleotides long, was decoded in rough draft form in 2001, a triumph of scientific investigation. 

The relationship between structure and function underlies living systems

 One of the unifying themes of molecular biology is the relationship between structure and function. Function in molecules, and larger macro molecular complexes, is dependent on their structure.

Although this observation may seem trivial, it has far-reaching implications. We study the structure of molecules and macro molecular complexes to learn about their function. When we know the function of a particular structure, we can infer the function of similar structures found in different con-texts, such as in different organisms. 

Biologists study both aspects, looking for the relation-ships between structure and function. On the one hand, this allows similar structures to be used to infer possible similar functions. On the other hand, this knowledge also gives clues as to what kinds of structures may be involved in a process if we know about the functionality. 

For example, suppose that we know the structure of a human cell's surface receptor for insulin, the hormone that con-trols uptake of glucose. We then find a similar molecule in the membrane of a cell from a different species—perhaps even a very different organism, such as a worm. We might conclude that this membrane molecule acts as a receptor for an insulin-like molecule produced by the worm. In this way, we might be able to discern the evolutionary relationship between glucose uptake in worms and in humans. 

The diversity of life arises by evolutionary change 

The unity of life that we see in certain key characteristics shared by many related life-forms contrasts with the incredible diver-sky of living things in the varied environments of Earth. The underlying unity of biochemistry and genetics argues that all life has evolved from the same origin event. The diversity of life arises by evolutionary change leading to the present biodiversity we see. 

Biologists divide lifers great diversity into three great groups, called domains: Bacteria, Archaea, and Eukarya. The domains Bacteria and Archaea are composed of single-celled organisms with little internal structure (termed prokaryotes), and the domain Eukarya is made up of organisms composed of a complex, organized cell or multiple complex cells (termed eukalyotes). Within Eukarya are four main groups called kingdoms. Kingdom Protista consists of all the unicellular eukaryotes except yeasts (which are fungi), as well as the multi-cellular algae. Because of the great diversity among the protists, many biologists feel kingdom Protista should be split into several kingdoms.