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DNA and Chromosomes

Life depends on the ability of cells to store, retrieve, and translate the genetic instructions required to make and maintain a living organism. These instructions are stored within every living cell in its genes—the information-bearing elements that determine the characteristics of a species as a whole and of the individuals within it.

At the beginning of the twentieth century, when genetics emerged as a science, scientists became intrigued by the chemical nature of genes. The information in genes is copied and transmitted from a cell to its daughter cells millions of times during the life of a multicellular organism, and passed from generation to generation through the reproductive cells—eggs and sperm. Genes survive this process of replication and transmission essentially unchanged. What kind of molecule could be capable of such accurate and almost unlimited replication, and also be able to direct the development of an organism and the daily life of a cell? What kind of instructions does the genetic information contain? How are these instructions physically organized so that the enormous amount of information required for the development and maintenance of even the simplest organism can be contained within the tiny space of a cell?

The answers to some of these questions began to emerge in the 1940s, when it was discovered from studies in simple fungi that genetic information consists primarily of instructions for making proteins. As described in the previous chapter, proteins perform most of the cell’s functions: they serve as building blocks for cell structures; they form the enzymes that catalyze the cell’s chemical reactions; they regulate the activity of genes; and they enable cells to move and to communicate with one another. With hindsight, it is hard to imagine what other type of instructions the genetic information could have contained.

The other crucial advance made in the 1940s was the recognition that deoxyribonucleic acid (DNA) is the carrier of the cell’s genetic information. But the mechanism whereby the information could be copied for transmission from one generation of cells to the next, and how proteins might be specified by instructions in DNA, remained completely mysterious until 1953, when the structure of DNA was determined by James Watson and Francis Crick. The structure immediately revealed how DNA might be copied, or replicated, and it provided the first clues about how a molecule of DNA might encode the instructions for making proteins. Today, the fact that DNA is the genetic material is so fundamental to our understanding of life that it can be difficult to appreciate what an enormous intellectual gap this discovery filled.

In this chapter, we begin by describing the structure of DNA. We see how, despite its chemical simplicity, the structure and chemical properties of DNA make it ideally suited for carrying genetic information. We then consider how genes and other important segments of DNA are arranged in the enormously long DNA molecule that forms each chromosome in a cell. Finally, we discuss how eukaryotic cells fold these lengthy strings of DNA into compact chromosomes inside the nucleus. This packing has to be done in an orderly fashion so that the chromosomes can be distributed correctly between the two daughter cells when the cell divides. At the same time, the compact chromosomes must allow their DNA to be accessed by the large number of proteins that replicate and repair it, and that carefully regulate the activity of a bustling cell’s many genes.

This is the first of six chapters that deal with basic genetic mechanisms—the ways in which the cell maintains and makes use of the genetic information carried in its DNA. In Chapter 6, we discuss the mechanisms by which the cell accurately replicates and repairs its DNA. In Chapter 7, we consider gene expression—how genes are used to produce RNA and protein molecules. In Chapter 8, we describe how a cell controls gene expression to ensure that each of the many thousands of proteins encoded in its DNA is manufactured at the proper time and place. In Chapter 9, we discuss how present-day genes evolved, and, in Chapter 10, we consider some of the ways that DNA can be experimentally manipulated to study fundamental cell processes.

An enormous amount has been learned about these subjects over the past 80 years. Much less obvious, but equally important, is the fact that our knowledge is still incomplete: a great deal remains to be discovered about how DNA provides the instructions to build living things.