Everyone has heard of DNA – the genetic material inherited from our parents, determining
our physical and biological characteristics.
What is now a relatively simple concept was once a topic of heated debate, just a century
ago.
It all began with Mendel’s theory of heritable factors, passed from parents to offspring.
According to a popular theory from the early 1900s, these heritable factors were proteins.
Let’s begin the story of how this theory was debunked, featuring bacteria.
In 1928, bacteriologist Frederick Griffith was aiming to create a vaccine for pneumonia,
a deadly lung infection caused by Streptococcus pneumoniae bacteria.
These bacteria have four types and exist as virulent S forms or harmless R forms.
S forms are protected by a polysaccharide capsule, while R forms lack this capsule and
are destroyed by the host’s immune system.
Griffith injected mice with Type IIR and Type IIIS bacteria, either living or killed by
heat.
Living S forms were deadly, as the virulent bacteria replicated and caused pneumonia.
Heat-killed S forms were harmless, as the dead bacteria couldn’t cause infection.
Living R forms were destroyed by the immune systems of the mice.
All expected results, except for one surprise – combining living R forms with heat-killed
S forms killed the mice, with living S forms discovered in their blood.
Griffith proposed that the dead S forms possessed a “transforming factor”, taken up and
incorporated by the R forms to transform into virulent bacteria.
Imagine that this yellow Kirby is a living S bacterium, and his helper is a living R
bacterium.
Upon death, S form Kirby releases the transforming factor, which is taken up by his helper, who
transforms from R to S form.
The transforming factor’s identity was a mystery, but maybe Richard Sia and Martin
Dawson could solve it.
Deciding they didn’t want to be mass mouse murderers, they performed Griffith’s experiments
in vitro in 1931.
They successfully transformed living R forms in media with S vaccines, proving that the
transforming factor wasn’t carried by the mice and transformation could occur without
a living host.
They also attempted transformation using cell-free S forms.
To break the cells, S vaccines underwent different combinations of heating at 60 degrees Celsius
and 23 rounds of freezing and thawing.
Vaccines that were heated, or heated then frozen, successfully induced transformation.
However, these vaccines contained intact cells and were not considered cell-free.
Only freezing, then heating the vaccines successfully created cell-free forms, but transformation
did not occur, as the transforming factor was likely destroyed by bacterial enzymes.
These experiments produced a few clues, but no concrete evidence.
Two possibilities were suggested: the S forms had awoken some latent transforming power
within the R forms, or had physically transferred this new ability.
Despite everything, Sia and Dawson couldn’t identify the elusive transforming factor,
and were disappointed.
Maybe it was time for everyone to give up…
Turns out their work wasn’t in vain, as not long after, James Alloway accomplished
cell-free transformation.
He obtained a cell-free extract by lysing and filtering S form bacterial cells.
This extract was successful in transforming R form bacteria.
Alloway’s proof of cell-free transformation fuelled the experiments of Oswald Avery, Colin
MacLeod and Maclyn McCarty in 1944.
They used the well-known combination of heat-killed S forms and living R forms, but with a twist.
The S forms were filtered to create cell-free extracts, containing three possible suspects:
proteins, RNA and DNA.
Enzymes that degrade these molecules were also added to the media.
In media containing protein and RNA-degrading enzymes, transformation occurred and living
S forms grew.
This eliminated proteins and RNA from the list of suspects.
Now for the solid evidence: no S forms grew in media containing DNA-degrading enzymes.
DNA, the transforming factor, was destroyed by the enzymes, so no transformation occurred
and no colonies grew.
Imagine that this pink Kirby is a living R bacterium, and this heat-killed S form’s
DNA has been destroyed by enzymes.
When Kirby attempts to transform, nothing happens, and Kirby remains as an R form.
The mystery was finally solved, with a searching for a pneumonia vaccine ending in the legendary
discoveries of bacterial transformation and DNA as the genetic material.
Using this information, scientists have built artificial genes through DNA cloning, for
research and medical purposes.
In gene therapy, people with genetic disorders are given plasmids with copies of functional
genes, greatly improving their condition.
Additionally, drug resistance spreads quickly across bacteria through transformation, making
bacteria ideal subjects for drug development studies.
This shows that besides being scientific milestones, the concepts of transformation and DNA will
continue shaping genetics and biotechnology for many years to come.