Investigating CSI – Background material Table of Contents I ...

with thymine and guanine with cytosine), so these nucleotides are said to
be complementary.
To locate a specific VNTR sequence on a single stranded DNA fragment,
you can find it by simply:
Making a DNA probe out of a DNA sequence complementary to
that of a VNTR locus
Labeling the probe with a radioactive compound
Letting the probe bind to like DNA sequences on the membrane
Using the radioactive tag to find where the probe has attached
6. Once the radioactive probe is stuck to its target on the membrane, you
can take a picture of it using special X-ray film. You don't need a camera
or other machinery to accomplish this feat --all you have to do is place the
membrane against a special sheet of film for a short period of time! How
does this work? In a regular camera, the film has a special coating that
undergoes chemical changes when it absorbs the energy of a photon of
light (to learn more, see How Photographic Film Works). When you take a
picture with a camera, the light you let in by opening the shutter forms an
image on the film. X-ray film, on the other hand, picks up radiation emitted
from the natural decay of the isotope used in your probe. What you see on
the film is a darkened band that indicates the places on the membrane
where the probe has bound to DNA containing the VNTR sequence.
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