Chargaff’s Rule is the most basic rule of DNA

Well before the structure of the DNA double helix was revealed by Watson and Crick (with the help of Franklin and Gosling!), biochemists first had to figure out the chemical make-up of deoxyribonucleic acid.

Identifying the bases and how they were linked together with a phosphate backbone was the work of Phoebus Levene.

He was the first to tease out the chemical makeup of ‘desoxypentose nucleic acid,’ which we lovingly refer to today as DNA!

However, because only 4 bases existed, Levene triumphantly declared that DNA was far too simple to be considered the chemical that stored the genetic information of organisms.

He fell into the same trap as most everyone else at the time, and believed that the genetic material was protein.

Levene proposed the ‘tetranucleotide hypothesis’ which stated that DNA was made up of repeats of the same 4 nucleotides and these nucleotides were present in equal amounts.

We now know that this was totally wrong and part of the reason why Levene thought that nucleotides were present in equal amounts was because the analytical methods used in 1928 were not accurate enough.

It took another 22 years, and the invention of much more sensitive paper based chromatography techniques to tease out the true chemical nature of DNA.

But the first cracks in the hypothesis that ‘proteins are the genetic material’ appeared in 1944 when Avery, Macleod and McCarty showed in a series of experiments that DNA, and not protein, could ‘transform’ the bacteria pneumococcus.

While the majority of scientists at the time ignored this paper (because the results were contrary to popular belief) Erwin Chargaff of Columbia University was intrigued.

He saw Avery’s results as the first evidence that maybe the field had it all wrong about DNA, and that it wasn’t just a random jumble of nucleotides but something much more important.

So, he did what he did best, got in the lab, extracted a bunch of DNA from a bunch of different organisms and calculated in exacting detail how much of each base was present in every sample he tested.

A good number of these papers are in German, but the one that I’m pulling from today was published in Nature in 1950.

The figure above shows the nucleotide content of human sperm and human thymus.

~28% of human DNA is made up of Adenine, 18% Guanine, 18% Cytosine, and 30% Thymine.

These results show that the nucleotides aren’t all present in the same amounts as Levene had hypothesized, but that Adenine and Thymine, and Guanine and Cytosine are found in equal concentrations.

This was a key discovery on the path to showing how these seemingly complementary nucleotides bound together in the structure of DNA.