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A Few Things You Should Know...
Isotopes:
Different atomic forms of the same element, which have the
same proton number, but a different neutron number.
The mass number of the
14 15
7N `heavier' isotope changes due
to different numbers of
neutrons. The atomic/proton
number stays the same.
7N
Ultracentrifuge:
This machine is so advanced and sensitive, that it can separate
out molecules that contain Nitrogen-14 and Nitrogen-15.
It does this by spinning vessels containing the molecules in a
way that causes them to collect in different areas, depending
on their densities.…read more

Slide 3

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Ultracentrifuge
A sample containing the molecules being tested are placed in
tubes.
The ultracentrifuge will then spin them at great speed, causing
molecules of different densities (weight, if you like) to separate
at different bands in the tube.
Heavier molecules separate out in lower bands than light
molecules.…read more

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Let's go back to 1953...…read more

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Elucidation of the Structure of DNA
On April 25, 1953, James Watson and Francis Crick released a paper in
which they described the structure of:
Deoxyribose Nucleic Acid
The paper probably stands as one of the most important documents
ever to be released in the field of Molecular Biology.
Towards the end of the paper, the duo
commented:
`It has not escaped out notice that the
specific pairing we have postulated
immediately suggests a possible
copying mechanism for the genetic
material.'
Their assumption would be followed up
by other scientists though.…read more

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3 Possible Copying Mechanisms?
Immediately after Watson and Crick delivered their paper on DNA
structure, scientists began the race to find out how the molecule
copied itself.
Matthew Meselson and Franklin Stahl were a pair of scientists who
decided to be unbiased, and focus on the 3 accepted, possible
copying mechanisms at the time.
The experiment they carried out is one of the most famous in
Biology.
They were the first to work out which
of the 3 mechanisms (next slide) was
correct.
They achieved their goal in 1958…read more

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