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Modern Genetic Analysis Chapter 2. The Structure of Genes and Genomes
Introduction

The minimal unit of life is a cell. Organisms can be unicellular, such as bacteria and many algae and fungi, or multicellular, such as plants and animals. The overall characteristics of an organism are determined by the structure and function of its constituent cells. Therefore to understand how genes dictate the fundamental characteristics of species and how they contribute to variation within a species, it is necessary to place genes and gene action within the cellular context. In other words we need to know the nature of the genetic material, its location within cells, and the way it performs its functions in the cellular setting. This chapter provides an introduction to the nature and cellular organization of the genetic material in several categories of organisms. Then Chapter 3 considers details of the function of the genetic material.

How is DNA organized in a cell? A complete set of the DNA of an organism is called a genome. A genome is composed of long DNA molecules, which are, in turn, the main components of chromosomes. Each chromosome contains one DNA molecule carrying many genes. As we saw in Chapter 1, genes are regions of chromosomal DNA that can be transcribed into RNA. The genomes of most prokaryotic organisms consist of only one chromosome, whereas the genomes of eukaryotes consist of several to many chromosomes. In eukaryotes most of the chromosomes are in the nucleus, but mitochondria and chloroplasts each contain one unique type of chromosome.

We also need to consider DNA molecules that are not part of the genomes of organisms. Cells of many prokaryotes and some eukaryotes contain accessory DNA molecules called plasmids. These plasmid DNA molecules contain genes necessary for their own propagation, so they are (somewhat loosely) called plasmid genomes. Furthermore viruses, which are not organisms, nevertheless contain their own sets of genes, and these are also referred to as genomes.

Since the genomes of all organisms, all plasmids, and most viruses are composed of DNA, our discussion will begin with the structure of this molecule, which dictates the inherent properties of the living world.

© 1999 by W. H. Freeman and Company. All rights reserved.