Molecular Cell Biology 7th Edition
Biology is a science fundamentally different from phys ics or chemistry, which deal with unchanging propics or chemistry, which deal with unchanging properties of matter that can be described by mathematical equations. Biological systems of course follow the rules of chemistry and physics, but biology is a historical science, as the forms and structures of the living world today are the results of billiom of years of euolution. Through evolution, all organisms are related in a family tree extending from primitive single-celled organisms that lived in the distant past to the diverse plants, animals, and microorganisms of the present era (Figure 1-1, Table 1-1). The great insight of Charles Darwin (Figure 1-2) was the principle of natural selection: organisms vary randomly and compete within their environment for resources: Only those that survive to reproduce are able to pass down their generic traits.
At first glance, the biological universe appears amazingly diverse-from tiny ferns to tall fir trees, from single-celled acteria and protozoans visible only under·a microscope to multicellular animals of all kinds. Yet the bewildering array of outward biological forms overlies a powerful uniformity: thanks to our common ancestry, all biological systems are composed of the same types of chemical molecules and employ similar principles of organization at the cellular level. Although the basic kinds of biological molecules have been conserved during the billions of years of evolution, the patterns in which they arc assembled to form functioning cells and organisms have undergone considerable change.
We now know that genes, which chemically are composed of deoxyribonucleic acid (DNA), ultimately define biological structure and maintain the integration of cellular function. Many genes encode proteins, the primary molecules that make up cell structures and carry out cellular activities. Alterations in the structure and organization of genes, or mutations, provide the random variation that can alter biological structure and function. While the vast majority of random mutations have no observable effect on a gene’s or protein’s function, many are deleterious, and only a few confer an evolutionary advantage. ln all organisms mutations in D’\!A arc constantly occurring, allowing over time the small alterations
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