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
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