DNA is the genetic material of bacteria

KEY TERMS:

• Transformation of bacteria is the acquisition of new genetic material by incorporation of added DNA.
• Avirulent mutants of a bacterium or virus have lost the capacity to infect a host productively, that is, to make more bacterium or virus.
• The transforming principle is DNA that is taken up by a bacterium and whose expression then changes the properties of the recipient cell.
• Deoxyribonucleic acid (DNA) is a nucleic acid molecule consisting of long chains of polymerized (deoxyribo)nucleotides. In double-stranded DNA the two strands are held together by hydrogen bonds between complementary nucleotide base pairs. 

KEY CONCEPTS:

• Bacterial transformation provided the first proof that DNA is the genetic material. Genetic properties can be transferred from one bacterial strain to another by extracting DNA from the first strain and adding it to the second strain.

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.

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 1.3 shows that when they are jointly injected 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).

Figure 1.4 shows the identification of the component of the dead bacteria responsible for transformation. This 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, MacLeod, and McCarty, 1944).