If we believe that chromosomes contain genes (see post 21.10), then genes must be made of either DNA or a type of protein called a histone – because that’s what chromosomes are made of. The question is then – are genes made of DNA or histones?
In the early 1940s, most people believed that genes must be made of histones. Their reason was that histones were made of 20 different amino acids, similar to the number (26) of letters used in the English language (A, B, C ……X, Y, Z). If sequences of 26 letters could be used to write everything that had ever been written in the English language, they thought that sequences of 20 amino acids could be used to code genetic information. In contrast, the only variability in a DNA sequence was whether a base was adenine (A), guanine (G), thymine (T) or cytosine (C). If DNA were the genetic material, then a vast amount of information would need to be expressed using only four letters – this seemed unlikely.
This reason for deciding that DNA was not the genetic material seems crazy in the twenty-first century. We are used to the idea of coding all information, as well as the instructions, for manipulating it, into a string of binary digits (or bits) that can have only two values 0 or 1. We now regard the person who developed this idea, Turing, as being one of the most influential scientists of the twentieth century. But in the 1940s very few scientists had heard of him or his ideas.
So the hypothesis was that DNA is too simple to be the genetic material. However, three American biologists, Oswald T Avery (1877-1955), Colin M Macleod (1909-1972) and Maclyn McCarty (1911-2005), performed an experiment that showed that this hypothesis was false. Their experiments used the bacterium Streptococcus pneumoniae that has a non-infectious strain and a strain that that cause pneumonia. They extracted and purified DNA from the infectious strain and added it to the non-infectious strain. The colony of bacteria grew (see post 16.5), producing infectious bacteria. So, DNA can convert non-infectious bacteria into infectious bacteria – it can carry genetic information.
Avery and his colleagues had shown that the hypothesis stated at the beginning of the last paragraph is false. But many people still believed that the genetic material was protein. One criticism was that they had not proved that DNA is always the genetic material. But an experiment can never prove that a hypothesis is true – only that it is false (see post 16.3). So, Avery and his colleagues were following the standard way of doing science – they did an experiment to test a hypothesis and showed that the hypothesis was false. Some people believed that, even if DNA could work as the genetic material, proteins must be involved too. They had unnecessarily complicated a simple model – that DNA is the genetic material. Following Occam’s razor, we reject the more complicated hypothesis and conclude that DNA is the genetic material.
Our current hypothesis (see post 16.2), that genes are made of DNA, has revolutionised biology and medicine.