Introduction

By the late 1800’s, investigators like Gregor Mendel, the Austrian Monk who demonstrated that physical traits were inherited as a result of discrete “particles” of information passed from generation to generation.  These ‘particles’ soon came to be known as ‘genes.’

Let us take a closer look at how scientists discovered the all-important DNA molecule.

Learning

Despite the information supporting the fact that characteristics were inherited through one’s genes, scientists at this time had no idea what ‘genes’ were or what they were made of. Around the start of the 1900’s, scientists armed with more powerful microscopes were able to identify small packages contained within the nucleus of dividing cells. These packages were called ‘chromosomes’ and it soon became clear that genes were actually parts of these structures.

Biochemistry allowed scientists in the late 1920’s to determine that chromosomes were composed of only two biological molecules: protein and DNA.  Therefore, one of these components must, it was reasoned, be responsible for housing the code of inheritance.  But which one of these was the ‘molecule of heredity’?

Early on in this debate, most scientists actually believed that protein must be responsible for encoding the genetic information of a cell.  This was for several reasons, including:

• DNA was comprised of only 4 different subunits – nucleotides (Guanine, Adenine, Thymine and Cytosine) while protein was comprised of 20 different subunits – amino acids.
• DNA is very simplistic in structure (a chain) while protein is very complex in structure (multiple, interacting chains)

Given the complexity of life forms on our planet, it was a reasonable assumption at the time to consider protein as the molecule of heredity.  However, this had to be proved.

Let us look at the work of several investigators which determined, once and for all, that DNA – and not protein – is the molecule of heredity.

Miescher Studies

Johann Friedrich Miescher (born 1844) wanted to determine the chemical composition of cells. Then he purified and analyzed the compounds prevalent in white blood cells since these cells had nuclei that were the largest component of the cells. There he identified a material that was slightly acidic in nature. Since this was most likely derived from the large nucleus, he termed this compound ‘nucleic acid’.

It was later determined that there are two types of nucleic acids in cells:

• DNA (Deoxyribonucleic Acid)
• RNA (Ribonucleic Acid)

In addition, it was also determined that material in the nucleus contained an abundance of protein. This led to the next logical question – which molecule, Nucleic Acid OR Protein was responsible for carrying genetic information? Let us keep addressing this key question.

The Griffith Experiments

Griffith in 1928, worked with a virulent strain of Streptococcal pneumonia. In this experiment, bacteria from the III-S strain were killed by heat, and their remains were added to II-R strain bacteria. While neither harmed the mice on their own, the blend of the two was able to kill mice. Griffith concluded that the R-strain bacteria must have taken up what he called a “transforming principle” from the heat-killed S bacteria, which allowed them to “transform” into smooth-coated bacteria and become virulent. But what was it? Griffith was not able to identify the chemical nature of this “transforming principle” beyond the fact that it was able to survive heat treatment.

We now know that DNA can survive heat treatment and be transformed (inserted) into bacteria.

Avery, McCarty, MacLeod Experiment

A team of researchers in 1944, named Oswald Avery, Colin MacLeod and Maclyn McCarty used a process of elimination to identify the identity of the transforming principle.

They were able to prove that the DNA component and NOT the protein component was responsible for transforming bacteria into ‘virulent’ strains. Therefore, by destroying the DNA component of the extract, the remaining protein component was NOT capable of transforming the R-strain bacteria into the disease-causing S-strain form. This provided the strongest evidence to date that DNA, and not protein, must be the ‘transformation principle’ and therefore the molecule of heredity.

The Hershey-Chase Experiments

The Hershey–Chase experiments were a series of experiments conducted in 1952 by researchers Alfred Hershey and Martha Chase.  Their observations helped to confirm that DNA is the genetic material.

In their landmark experiments, Alfred Hershey and Martha Chase showed that when bacteriophages (viruses which are composed almost entirely of DNA and protein) infect bacterial cells, it is their DNA which enters the host bacterial cell while their protein does not.

Summary

• Not until the early 1900s did scientists begin to recognize the relationship between inheritance and tiny physical structures in cells (chromosomes).
• Chromosomes were determined to contain DNA and protein.
• Scientists initially believed that it was protein that contained the genetic code.
• Miescher identified the acidic compounds in the nucleus and called them “nucleic acid”
• Griffith discovered that there was a “transforming principle”
• Avery, McCarty, MacLeod was able to prove that the DNA component was the ‘transforming’ bacteria to become more ‘virulent.’
• Hershey and Chase proved definitively, that DNA was the molecule of heredity.

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Sources:

“Historical Basis of Modern Understanding.” By OpenStax Biology 2e. Retrieved from https://openstax.org/books/biology-2e/pages/14-1-historical-basis-of-modern-understanding Licensed under: CC-BY: Attribution

 “The Structure of DNA.” By Concepts of Biology. Retrieved from https://openstax.org/books/concepts-biology/pages/9-1-the-structure-of-dna/_Licensed under: CC-BY: Attribution