Molecular Biology 3rd Edition

Book Preface

Before tackling the complex details of biology at the molecular level, we need to get familiar with the subject of our investigation—the living world. First, we will consider what it means to be alive and then we shall survey a range of cells and organisms that are often studied by molecular biologists.

Life is impossible to define exactly, but a general idea is sufficient here. Living things consist of cells—some consist of a single cell, whereas others are made from assemblies of many million cells. Whatever the situation, living cells must grow, divide, and pass on their characteristics to their offspring. Molecular biology focuses on the details of growth and division. In particular, we are interested in how division is arranged so that each descendent can inherit their parents characteristics.

Scientists have devoted much effort in investigating certain favored organisms. In some cases this is a matter of convenience—bacteria, yeast, and other single-celled microorganisms are relatively easy to investigate. In other cases it is due to self-interest. Mice—and some other animals—reveal much about humans, plants provide our food, and viruses make us sick

What Is Life?

Although there is no definition of life that suits all people, everyone has an idea of what being alive means. Generally, it is accepted that something is alive if it can grow and reproduce, at least during some stage of its existence. Thus, we still regardadults who are no longer growing and those individuals beyond reproductive age as being alive. We also regard sterile individuals, such as mules or worker bees as being alive, even though they lack the ability to reproduce. Part of the difficulty in defining life is the complication introduced by multicellular organisms. Although a multicellular organism as a whole may not grow or reproduce some of its cells may still retain these abilities.

The basic ingredients needed to sustain life include the following:
● Genetic information. Biological information is carried by the nucleic acid molecules, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The units of genetic information are known as genes, and each consists physically of a segment of a nucleic acid molecule. DNA is used for long-term storage of large amounts of genetic information (except by some viruses—see Chapter 24: Viruses, Viroids, and Prions). Whenever genetic information is actually used, working copies of the genes are carried on RNA. The total genetic information possessed by an organism is known as its genome. DNA genomes are maintained by replication, a process where a copy of the DNA is produced by enzymes and then passed on to the daughter cells.
● Mechanism for energy generation. By itself, information is useless. Energy isneeded to put the genetic information to use. Living creatures must all obtain energy for growth and reproduction. Metabolism is the set of processes in which energy is acquired, liberated, and used for biosynthesis of cell components, then catabolized and recycled. Living organisms use raw material from the environment to grow and reproduce.
● Machinery for making more living matter. Synthesis of new cell components requires chemical machinery. In particular, the ribosomes are needed for making proteins, the macromolecules that make up the bulk of all living tissue. These tiny subcellular machines allow the organism to grow and maintain itself.
● A characteristic outward physical form. Living creatures all have a material body that is characteristic for each type of life-form. This structure contains all the metabolic and biosynthetic machinery for generating energy and making new living matter. It also contains the DNA molecules that carry the genome. The form has levels of organization from single cells to tissues, organs, and organ systems of multicellular life-forms.
● Identity or self. All living organisms have what one might call an identity. The term self-replication implies that an organism knows to make a copy of itself—not merely to assemble random organic material. This concept of “self” versus “non-self” is very evident in the immune systems that protect higher animals against disease. But even primitive creatures attempt to preserve their own existence.

● Ability to reproduce. The organism uses energy and raw materials to make itself, and then uses the same materials to produce offspring. Some organisms simply reproduce with asexual reproduction (making offspring without creating gametes) and other organisms use sexual reproduction (two gametes fuse to form a new organism).
● Adaptation. The most important characteristic of a living creature is the ability to adapt to its current environment. This concept also encompasses evolution or the adaptations that get passed from generation to generation.