Cynthia Wolberger · Transcript · · Updated
Photograph 51 explained – Transcript
Video by Cynthia Wolberger · Watch the original on YouTube
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- Rosen Franklin recorded fiber diffraction from DNA by placing long fibers of calf thymus DNA in an x-ray beam.
- The characteristic x pattern observed on the film shows that dna is in fact helical.
- The missing layer line on the fourth layer line indicates that there is more than one strand wound around one another.
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here's a brief explanation of what we learned from rosalind franklin's famous photograph 51. rosen franklin was recording fiber diffraction from dna she placed long fibers of calf thymus dna in an x-ray beam and recorded the pattern of scattered x-rays on film there's a mathematical relationship between the particular characteristics and parameters of the dna helix and the pattern of spots
when observes on the film so it's possible to just take an image like this and learn quite a bit about the nature of the helix that scattered those x-rays the first thing we learned from this photograph is that dna is in fact helical this characteristic x pattern is what one observes when one places a helix in the path of the x-rays the second thing we learned from the photograph comes from the position of this dark spot over here
from the distance from the center to the dark spot that tells us that the fundamental repeats of the helix are 3.4 angstroms apart or the dna helix it's a 3.4 angstrom rise per base pair the spacing between these sets of spots over here known as layer lines gives us the pitch of the helix and the pitch is 34 angstroms so the pitch is the distance going vertically here that the helix repeats itself
and now if you take the pitch of 34 angstroms and divide by the rise per base pair of 3.4 that tells us that there are 10 base pairs per turn in the helix now if we look closely at the layer lines you actually see that there are missing spots on this fourth layer line with a single strand of dna you would expect to see spots here as well the missing layer line tells us that there's more than one strand
wound around one another and that those strands are not symmetrically placed so they're asymmetric then that was the clue that there's a major and a minor two uh groove in the dna finally the spacing between the spots gives the helix radius and so you calculate a radius of 10 angstroms or 20 angstroms which is really exactly the diameter of bdna and you can read about all this elegant work in franklin and gosling's classic paper which was published in the