How do we know genes are made of DNA

The concept of the gene as an inherited physical entity determining some aspect of an organism's phenotype dates back to the earliest days of genetics. The question of what genes are actually made of was a major concern of molecular biologists (not that they would have described themselves as such!) in the first half of the 20th century. Since it was recognised by this time that genes must be located on chromosomes, and that chromosomes (in eucaryotes) comprised largely protein and DNA, the reasonable assumption was made that genes must be made up of one of these substances. In the early years, protein was regarded as the more likely candidate, since, from what was known of molecular structure at the time, it offered far more scope for the variation which would be essential to account for the thousands of genes that any organism must possess. The road to proving that DNA is in fact the 'stuff of life' was a long and hard one, which can be read about elsewhere; we shall mention below just some of the key experiments which provided crucial evidence.

In 1928 the Englishman Fred Griffith carried out a seminal series of experiments which not only demonstrated for the first time the phenomenon of genetic transfer in bacteria (a subject we shall consider in more detail later in this chapter), but also acted as the first step towards proving that DNA was the genetic material. As we shall see, Griffith showed that it was possible for heritable characteristics to be transferred from one type of bacterium to another, but the cellular component responsible for this phenomenon was not known at this time.

Attempts were made throughout the 1930s to isolate and identify the transforming principle, as it became known, and in 1944 Avery, MacLeod and McCarty published a paper, which for the first time, proposed DNA as the genetic material. Avery and his colleagues demonstrated that when DNA was rendered inactive by enzymatic treatment, transforming ability was lost from a cell extract, but if proteins,

A gene is a sequence of DNA that usually encodes a polypeptide.

Box 11.1 Hershey and Chase: the Waring blender experiment

In 1952 Alfred Hershey and Martha Chase provided further experimental evidence that DNA was the genetic material. In their experiment, the bacteriophage T2 was grown with E. coli cells in a medium with radiolabelled ingredients, so that their proteins contained 35S, and their DNA 32P. The phages were harvested, and mixed with a fresh culture of E. coli. They were left long enough for the phage particles to infect the bacteria, (but not long enough to produce new phage particles and lyse the cells). The culture was then subjected to mechanical agitation in a Waring blender, which, it was hoped, would remove the 'shell' of the phages from the outside of the bacteria, but leave the injected genetic material inside.

Separate phage ghosts from bacterial cells

Ghosts Bacteria labelled Ghosts labelled Bacteria unlabelled with 32P with 35S unlabelled

From Reece, RJ: Analysis of Genes and Genomes, John Wiley & Sons, 2003. Reproduced by permission of the publishers

The bacteria were sedimented by centrifugation, leaving the much lighter phage 'shells' in the supernatant. When solid and liquid phases were analysed for 32P and 35S, it was found that nearly all the 32P was associated with the bacterial cells, while the great majority of the 35S remained in the supernatant. The conclusion drawn from these results is that it was the 32P-labelled DNA which had been injected into the bacteria, and which was therefore the genetic material.



Figure 11.1 DNA replication is semiconservative. Following replication, each new DNA molecule comprises one strand from the parent DNA and one newly synthesised strand

carbohydrates or any other cellular component was similarly inactivated, the ability was retained. In spite of this apparently convincing proof, the pro-protein lobby was not easily persuaded. It was to be several more years before the experimental results of Alfred Hershey and Martha Chase (Box 11.1) coupled with Watson and Crick's model for DNA structure (Figure 2.23) finally cemented the universal acceptance of DNA's central role in genetics.

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