The Chemical Nature Of Genes

A capsule is an outer coat covering a bacterial cell. Unlike a slime layer, it is not easily washed off. Although capsules are not essential for bacterial growth and reproduction in laboratory conditions, they can make the difference between life and death in natural situations. For example, Streptococcus pneumoniae (a member of the pneumococci, the group of pneumonia-causing bacteria used in Griffith's experiment; see text) has non-capsulated and capsulated strains. Those lacking a capsule are easily destroyed by the host and do not cause disease. However, the capsulated strain kills mice quickly. The capsule helps the bacterium resist phagocytosis by host cells. It contains a great deal of water, protecting the bacterium from desiccation; it keeps out detergents which could destroy the cell surface membrane; and it helps bacteria attach to host cells.

We know today that DNA is the chemical in which information is from parent to offspring. This spread looks at how researchers established this link between DNA and inheritance. In the 1860s, nearly 100 years before Watson and Crick's work on the structure of DNA, Gregor Mendel established that inheritance depends on factors that are transmitted from parents to offspring. In 1909 it was found that patterns of inheritance were reflected in the behaviour of chromosomes. Wilhelm Johannsen referred to these factors as genes. Genes were assumed to be located on the chromosomes because genes that are inherited together (linked genes) were found to be carried on the same chromosome. However, the chemical composition of genes was not known.

Protein or DNA: which is the genetic material?

Chromosomes were known to contain both protein and DNA. Most biologists assumed that proteins, with their highly complex and infinitely variable structure, were the inherited material. The nucleic acids were thought to be too simple to carry complex genetic information. This view was reinforced by the work of Phoebus Aaron Levene. Levene made major contributions to the chemistry of nucleic acids but believed, mistakenly, that DNA was a very small molecule, probably only four nucleotides long.

In 1928 Fred Griffith, an English medical bacteriologist, published a paper describing experiments on pneumococci. His results set the stage for the research that finally showed that DNA is the genetic material.

Griffith's experiment: transformation of pneumococci.

Pneumococci are bacteria that cause pneumonia. They occur in two strains: a disease-causing smooth strain (strain S), and a harmless rough strain (strain R). Strain S has a capsule on its cell surface; this capsule is absent from the harmless strain R (see Fact of life).

Griffith found that mice injected with live strain S soon died, but those injected with live strain R survived. Mice injected with dead strain S bacteria (killed by heat) аll survived. The results of this series of experiments were as expected. However, the results of Griffith's next series of experiments were thoroughly baffling: mice injected with a mixture of heat-killed strain S and live strain R died. Moreover, Griffith recovered live strain-S bacteria from the dead mice.

After many careful experiments, Griffith concluded that hereditary material had passed from the dead bacteria to the live bacteria. This changed harmless strain R bacteria into virulent strain S pathogens. This process is called transformation.

Avery's experiment: DNA was the transforming agent.

In the 1940s, Oswald T. Avery, Colin MacLeod, and Maclyn McCarty showed that DNA was responsible for transformation.

§ They used enzymes that hydrolysed polysaccharide, DNA, RNA, and protein on samples of the disease-causing strain-S pneumococci.

§ Different samples had different parts of their cells destroyed by these enzymes.

§ The researchers then exposed strain-R pneumococci to the treated samples of strain S.

§ The transformation of strain R to strain S was blocked only when the DNA in the sample was destroyed.

These results provided strong evidence that DNA carried genetic information for transformation. However, many scientists remained unconvinced.

Hershey and Chase: the role of DNA on the T2 phage life cycle

In 1952, Alfred D. Hershey and Martha Chase performed several experiments with T2 bacteriophage, a virus that infects bacteria. Their results convinced even the sceptics that DNA, and not protein, was the genetic material.

Electron micrographs indicate that T2 bacteriophage infects Escherichia coli by injecting its DNA into the bacterium while leaving its protein coat on the outside. The phage takes over the genetic machinery of the host cell to make new phages. Eventually, the bacterial cell bursts (a process called lysis), releasing new phages to infect other bacteria (figure 1).

Hershey and Chase wanted to test the hypothesis that only the viral DNA entered the bacterium. They made use of the fact that DNA contains phosphorus but not sulphur, whereas protein contains sulphur but not phosphorus.

§ With some T2 phages, they labelled the viral DNA with a radioactive isotope of phosphorus (³²P). With other T2 phages, they labelled the viral protein coat with a radioactive isotope of sulphur (³⁵S).

§ They added the viruses to a culture of E. coli and gave them enough time to infect their host cells (but not enough time to reproduce).

§ The viral coats were then separated from the infected bacteria by shaking the mixture vigorously in a blender.

§ When E. coli was infected with a T2 phage containing ³⁵S (labelled Protein), little radioactivity occurred within the bacterial cells.

§ With a T2 phage containing ³²P (labelled DNA), the bacterial cells were radioactive. Moreover, when the bacterial cells burst open, the new viruses that emerged were radioactively labelled with ³²P. When the protein was labelled, new viruses were only slightly radioactive.