2. DNA is the genetic material

1.2 DNA is the genetic material

Key terms defined in this section

Avirulent mutants of a virus have lost the capacity to infect a host cell productively, that is, to make more virus.
Transfection of eukaryotic cells is the acquisition of new genetic markers by incorporation of added DNA.
Transforming principle is DNA that is taken up by a bacterium and whose expression then changes the properties of the recipient cell.

The idea that genetic material is nucleic acid had its roots in the discovery of transformation in 1928. The bacterium Pneumococcus kills mice by causing pneumonia. The virulence of the bacterium is determined by its capsular polysaccharide. This is a component of the surface that allows the bacterium to escape destruction by the host. Several types (I, II, III) of Pneumococcus have different capsular polysaccharides. They have a smooth (S) appearance.

Each of the smooth Pneumococcal types can give rise to variants that fail to produce the capsular polysaccharide. These bacteria have a rough (R) surface (consisting of the material that was beneath the capsular polysaccharide). They are avirulent. They do not kill the mice, because the absence of the polysaccharide allows the animal to destroy the bacteria.

Figure 1.2 The transforming principle is DNA.

When smooth bacteria are killed by heat treatment, they lose their ability to harm the animal. But inactive heat-killed S bacteria and the ineffectual variant R bacteria together have a quite different effect from either bacterium by itself. Figure 1.2 shows that when they are injected together into an animal, the mouse dies as the result of a Pneumococcal infection. Virulent S bacteria can be recovered from the mouse postmortem.

In this experiment, the dead S bacteria were of type III. The live R bacteria had been derived from type II. The virulent bacteria recovered from the mixed infection had the smooth coat of type III. So some property of the dead type III S bacteria can transform the live R bacteria so that they make the type III capsular polysaccharide, and as a result become virulent (Griffith, 1928).

The component of the dead bacteria responsible for transformation was called the transforming principle. It was purified by developing a cell-free system, in which extracts of the dead S bacteria could be added to the live R bacteria before injection into the animal. Purification of the transforming principle in 1944 showed that it is deoxyribonucleic acid (DNA) (Avery et al., 1944).

The next step was to demonstrate that DNA provides the genetic material in a quite different system. Phage T2 is a virus that infects the bacterium E. coli. When phage particles are added to bacteria, they adsorb to the outside surface, some material enters the bacterium, and then ~20 minutes later each bacterium bursts open (lyses) to release a large number of progeny phage.

Figure 1.3 The genetic material of phage T2 is DNA.

Figure 1.3 illustrates the results of an experiment in 1952 in which bacteria were infected with T2 phages that had been radioactively labeled either in their DNA component (with ³²P) or in their protein component (with ³⁵S). The infected bacteria were agitated in a blender, and two fractions were separated by centrifugation. One contained the empty phage coats that were released from the surface of the bacteria; these consist of protein and therefore carried the ³⁵S radioactive label. The other fraction consisted of the infected bacteria themselves.

Most of the ³²P label was present in the infected bacteria. The progeny phage particles produced by the infection contained ~30% of the original ³²P label. The progeny received very little Xless than 1% Xof the protein contained in the original phage population. This experiment therefore showed directly that the DNA of parent phages enters the bacteria and then becomes part of the progeny phages, exactly the pattern of inheritance expected of genetic material (Hershey and Chase, 1952).

A phage (virus) reproduces by commandeering the machinery of an infected host cell to manufacture more copies of itself. The phage possesses genetic material whose behavior is analogous to that of cellular genomes: its traits are faithfully reproduced, and they are subject to the same rules that govern inheritance. The case of T2 reinforces the general conclusion that the genetic material is DNA, whether part of the genome of a cell or virus.

Figure 1.4 Eukaryotic cells can acquire a new phenotype as the result of transfection by added DNA.

When DNA is added to populations of single eukaryotic cells growing in culture, the nucleic acid enters the cells, and in some of them results in the production of new proteins. When a purified DNA is used, its incorporation leads to the production of a particular protein. Figure 1.4 depicts one of the standard systems.

Although for historical reasons these experiments are described as transfection when performed with eukaryotic cells, they are a direct counterpart to bacterial transformation. The DNA that is introduced into the recipient cell becomes part of its genetic material, inherited in the same way as any other part. Its expression confers a new trait upon the cells (synthesis of thymidine kinase in the example of the figure). At first, these experiments were successful only with individual cells adapted to grow in a culture medium. Since then, however, DNA has been introduced into mouse eggs by microinjection; and it may become a stable part of the genetic material of the mouse (see 17 Rearrangement of DNA).

Such experiments show directly not only that DNA is the genetic material in eukaryotes, but also that it can be transferred between different species and yet remain functional.

The genetic material of all known organisms and many viruses is DNA. However, some viruses use an alternative nucleic acid, ribonucleic acid (RNA), as the genetic material. Although its chemical formula is slightly different from that of DNA, in these circumstances RNA exercises the same role. The general principle of the nature of the genetic material, then, is that it is always nucleic acid; in fact, it is DNA except in the RNA viruses.

Research

Avery, O. T., MacLeod, C. M., and McCarty, M. (1944). Studies on the chemical nature of the substance inducing transformation of pneumococcal types. J. Exp. Med. 98, 451-460.

Griffith, F. (1928). The significance of pneuomococcal types. J. Hyg. 27, 113-159.

Hershey, A. D. and Chase, M. (1952). Independent functions of viral protein and nucleic acid in growth of bacteriophage. J. Gen. Physiol. 36, 39-56.