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we explain how scientists figured out
dna's structure to unlock its mysteries
until the early 1950s the structure of
dna remained a mystery
at cambridge university francis crick
and james watson worked on making
physical models of what dna may look
like
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meanwhile at king's college in london
maurice wilkins and rosalind franklin
were also studying dna
by examining x-ray diffraction images of
dna
in short this means that when shining
x-rays through dna molecules the x-rays
made a shadow of the molecule structure
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over time different researchers made
important but seemingly unconnected
findings about the composition of dna
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for example alexander todd discovered
that the backbone of the dna molecule
contained repeating phosphate and
deoxyribose sugar groups
linus pauling discovered the single
stranded alpha helix
prompting biologists to think of helical
forms
he also pioneered the method of model
building in chemistry
maurice wilkins and rosalind franklin
had obtained high-resolution
x-ray images of dna fibers it suggested
a helical
corkscrew-like shape franklin suspected
that all dna was helical
phoebus levine and others discovered
that dna was composed of subunits
called nucleotides a nucleotide is made
up of a sugar
a phosphate group and one of four
nitrogenous bases
these are 89 thymine guanine and
cytosine
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erwin chargaff had found that adenine
and thymine always appeared in ratios of
one to one
as did guanine and cytosine
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in the end it was watson and crick who
eventually unified all these findings to
reveal dna's structure
they used polling's method of model
building in chemistry to uncover the
structure of dna
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franklin's double helix idea and
chargaff's bindings about base pairs
were incorporated into watson and
crick's model
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this meant that matching base pairs
interlocked in the middle of the double
helix to keep the distance between the
ladder legs
or backbones constant
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finally watson and crick realized that a
always paired with t
and likewise c with g the base is
connected to the two helical sugar
phosphate backbones of dna at right
angles
so the legs retained their regular
double helix shape
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similarly the complementary pairing of
the bases meant that the backbones ran
in opposite direction to each other
one up the other down
and so watson and crick showed that each
strand of the dna molecule was a
template for the other
during cell division the two strands
separate and on each strand a new other
half is built
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this way dna can reproduce itself
without changing its structure
this discovery opened many doors in
biological research