Uncovering the Structure of DNA


It was in 1952 that Alfred Hershey and Martha
Chase performed an experiment that determined the genetic material of a cell. Until this
point in time, it was unclear if protein or DNA was the genetic material of a cell. Hershey
and Chase worked with the T2 bacteriophage virus because it was simply composed of a
protein coat and viral DNA. It was already known that viruses insert their genetic material
into bacterial cells to replicate, so they used this knowledge in their experiment. Hershey
and Chase performed two variations of their experiment. The first uses radioactive sulfur-35
to grow viruses that incorporate this radioactive element into their protein coats. This allowed
them to follow the protein coat through the next steps. The second variation uses radioactive
Phosphorus-32 to grow viruses that incorporate the phosphorus into their DNA. Both radioactively
labeled viruses were then allowed to infect bacteria separately. Then a blender was used
to separate the non-genetic factor from the bacterial cells, and the liquid was centrifuged.
The heavy, solid pellet at the bottom contained the bacterial cells and the genetic component.
The less dense supernatant contained the non-genetic component of the virus. Hershey and Chase
found that radioactive sulfur was in the supernatant and radioactive phosphorus was in the pellet.
This confirmed that DNA was the genetic material, and the protein coat was not. It was in fact
the DNA that entered the bacterial cells and the protein coat was left outside the cells.
Once DNA was confirmed as the genetic material, the race to uncover it’s structure began.
Rosalind Franklin’s experiments using X-ray diffraction provided the experimental evidence
to support the structure of DNA proposed by Watson and Crick. Photo 51, Franklin’s X-ray
diffraction photograph of DNA, elucidated key structural features of DNA. Interpretation
of the photograph is not straightforward, but Rosalind was the foremost expert in the
field at the time. The cross in the center confirmed that the DNA molecule was helical
in shape. The angle of the cross indicates the steepness of the angle of the helix. The
distance between the dark horizontal bars indicates that a twist of the helix occurs
every 3.4 nanometers. And the distance from the center to the top indicates that the distance
between nitrogen bases is 0.34 nanometers. It’s important to note that there are some
differences between DNA in prokaryotes and eukaryotes. For example, DNA in prokaryotes
is naked, the simple strand of DNA that you’re probably used to seeing. In eukaryotes, DNA
is always associated with histone proteins. The histones are used to tightly pack DNA
in the nucleus. Eukaryotes have 50 times more DNA than prokaryotes, so there is a lot to
pack in. Eukaryotic DNA is bundles into nucleosomes, which consist of 8 histones that have DNA
wrapped around it twice. An H1 histone binds the DNA to the core histones and the DNA between
nucleosomes is called linker DNA. The nucleosomes can coil even tighter to form chromatosomes
and solenoids and eventually coil into chromosomes for division. But over winding will not allow
for transcription, so the coils need to be unwound to allow for transcription. The over
or under-winding of a DNA strand is called supercoiling. Thanks for watching this episode
of Teacher’s Pet, don’t forget to like and subscribe and follow me on twitter @SciencePet

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