1.1 Genes as determinants of the inherent properties of species

What is the nature of genes, and how do they perform their biological roles? Three fundamental properties

are required of genes and the DNA of which they are composed.

1. Replication. Hereditary molecules must be capable of being copied at two key stages of the life cycle

(Figure 1-2). The first stage is the production of the cell type that will ensure the continuation of a

species from one generation to the next. In plants and animals, these cells are the gametes: egg and

sperm. The other stage is when the first cell of a new organism undergoes multiple rounds of division

to produce a multicellular organism. In plants and animals, this is the stage at which the fertilized egg,

the zygote, divides repeatedly to produce the complex organismal appearance that we recognize.

2. Generation of form. The working structures that make up an organism can be thought of as form or substance, and DNA has the essential “information” needed to create form.

3. Mutation. A gene that has changed from one allelic form into another has undergone mutation—an event that happens rarely but regularly. Mutation is not only a basis for variation within a species, but also, over the long term, the raw material for evolution.

We will examine replication and the generation of

form in this section and mutation in the next.

DNA and its replication

An organism’s basic complement of DNA is called its genome. The somatic cells of most plants and animals contain two copies of their genome (Figure 1-3); these

organisms are diploid. The cells of most fungi, algae, and bacteria contain just one copy of the genome; these organisms are haploid. The genome itself is made up of one or more extremely long molecules of DNA that are organized into chromosomes. Genes are simply the regions of chromosomal DNA that are involved in the cell’s production of proteins. Each chromosome in the genome carries a different array of genes. In diploid cells, each chromosome and its component genes are present twice. For example, human somatic cells contain two sets of 23 chromosomes, for a total of 46 chromosomes. Two chromosomes with the same gene array are said to be homologous. When a cell divides, all its chromosomes

(its one or two copies of the genome) are replicated and then separated, so that each daughter cell receives the full complement of chromosomes.

To understand replication, we need to understand the basic nature of DNA. DNA is a linear, double-helical structure that looks rather like a molecular spiral staircase. The double helix is composed of two intertwined chains made up of building blocks called nucleotides. Each nucleotide consists of a phosphate group, a deoxyribose sugar molecule, and one of four different nitrogenous bases: adenine, guanine, cytosine, or thymine. Each of the four nucleotides is usually designated by the first letter of the base it contains: A, G, C, or T. Each nucleotide chain is held together by bonds between the sugar and phosphate portions of the consecutive nucleotides, which form the “backbone” of the chain. The two intertwined chains are held together by weak bonds between bases on opposite chains (Figure 1-4).

There is a “lock-and-key” fit between the bases on the opposite strands, such that adenine pairs only with thymine and guanine pairs only with cytosine. The bases that form base pairs are said to be complementary. Hence a short segment of DNA drawn with arbitrary nucleotide sequence might be

· · · ·CAGT· · · ·

· · · ·GTCA· · · ·

For replication of DNA to take place, the two strands of the double helix must come apart, rather like

the opening of a zipper. The two exposed nucleotide chains then act as alignment guides, or templates, for the deposition of free nucleotides, which are then joined together by the enzyme DNA polymerase to form a new strand. The crucial point illustrated in Figure 1-5 is that because of base complementarity, the two daughter DNA molecules are identical with each other and with the original molecule.

Generation of form

If DNA represents information, what constitutes form at the cellular level? The simple answer is “protein” because the great majority of structures in a cell are protein or have been made by protein. In this section, we trace the steps through which information becomes form.

The biological role of most genes is to carry information specifying the chemical composition of proteins or the regulatory signals that will govern their production by the cell. This information is encoded by the sequence of nucleotides. A typical gene contains the information for one specific protein. The collection of proteins an organism can synthesize, as well as the timing and amount of production of each protein, is an extremely important determinant of the structure and physiology of organisms.

A protein generally has one of two basic functions, depending on the gene. First, the protein may be a structural component, contributing to the physical properties of cells or organisms. Examples of structural proteins are microtubule, muscle, and hair proteins. Second, the protein may be an active agent in cellular processes—such as an active-transport protein or an enzyme that catalyzes one of the chemical reactions of the cell.

The primary structure of a protein is a linear chain of amino acids, called a polypeptide. The sequence of amino acids in the primary chain is specified by the sequence of nucleotides in the gene. The completed primary chain is coiled and folded—and in some cases, associated with other chains or small molecules—to form a functional protein. A given amino acid sequence may fold in a large number of stable ways. The final folded state of a protein depends both on the sequence of amino acids specified by its gene and on the physiology of the cell during folding.