Investigating CSI – Background material Table of Contents I ...
Investigating CSI – Background material Table of Contents I ...
Investigating CSI – Background material Table of Contents I ...
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with thymine and guanine with cytosine), so these nucleotides are said to<br />
be complementary.<br />
To locate a specific VNTR sequence on a single stranded DNA fragment,<br />
you can find it by simply:<br />
Making a DNA probe out <strong>of</strong> a DNA sequence complementary to<br />
that <strong>of</strong> a VNTR locus<br />
Labeling the probe with a radioactive compound<br />
Letting the probe bind to like DNA sequences on the membrane<br />
Using the radioactive tag to find where the probe has attached<br />
6. Once the radioactive probe is stuck to its target on the membrane, you<br />
can take a picture <strong>of</strong> it using special X-ray film. You don't need a camera<br />
or other machinery to accomplish this feat --all you have to do is place the<br />
membrane against a special sheet <strong>of</strong> film for a short period <strong>of</strong> time! How<br />
does this work? In a regular camera, the film has a special coating that<br />
undergoes chemical changes when it absorbs the energy <strong>of</strong> a photon <strong>of</strong><br />
light (to learn more, see How Photographic Film Works). When you take a<br />
picture with a camera, the light you let in by opening the shutter forms an<br />
image on the film. X-ray film, on the other hand, picks up radiation emitted<br />
from the natural decay <strong>of</strong> the isotope used in your probe. What you see on<br />
the film is a darkened band that indicates the places on the membrane<br />
where the probe has bound to DNA containing the VNTR sequence.<br />
15