Chargaff's Data
NOS: Addressing the problem of induction by the certainty of falsification - How Chargaff's data falsified the tetranucleotide hypothesis of DNA
- Erwin Chargaff analysed the DNA composition of different organisms during the 1930s and 1940s and made the following discoveries:
- The number of purine bases were equal to the number of pyrimidine bases
- The number of adenine bases were equal to the number of thymine bases while the number of guanine bases were equal to the number of cytosine bases
- This means that a purine base can only pair up with a pyrimidine base between the sugar-phosphate backbone, since they have different sizes
- This forms the foundation of complementary base pairing in DNA
The problem of induction
- The inductive scientific method starts with a scientist making observations and collecting raw data
- After data analysis, a hypothesis is formulated which is then tested by means of a suitably designed investigation
- This may lead to some general conclusions being drawn based on specific observations
- Using data gathered in the past to create general predictions about what will happen in the future, assumes that the future will be the same as when you gathered your data
- It is therefore impossible to prove a hypothesis generated by inductive reasoning as absolutely true, since we cannot be sure that the general observations we made in the past will hold true in the future
- This is known as the problem of induction and is the main reason why most scientific theories are considered to be tentative
- Even if several investigations supports a hypothesis, it can still be proven incorrect (falsified) in the future as new discoveries are made
- For this reason, the philosopher Karl Popper suggested that new scientific knowledge is not gained by inductive steps but rather by the falsification of existing hypotheses
Falsification of the tetranucleotide hypothesis
- The biochemist Phoebus Levene discovered the pentose sugars of DNA and RNA in the early 1900's
- He suggested that the structure of nucleic acid was a repeating tetramer unit which he called a nucleotide
- This was called the tetranucleotide hypothesis
Tetranucleotide structure diagram
The tetranucleotide structure of nucleic acid which was suggested by Levene
- At the time of his research, there were limitations to the analytic techniques available which made it difficult to determine the relative amounts of nucleotides present in nucleic acids
- The tetranucleotide hypothesis was falsified by Chargaff's data in the late 1940s, which showed the organism-specificity of nucleic acids
- When the structure of DNA was determined in the 1950s, it further proved that the repeating tetramer unit suggested by Levene would not be suitable for carrying genetic information from one generation to the next
