Ibd

DNA fragment of interest

MATT C c ttaa g

Recombinant DNA

figure 13-2

Gaattc c ttaa )g

The restriction enzyme in this figure recognizes the sequence GAATTC on each DNA and cuts each chain between the G and A nucleotides. DNA fragments with sticky ends result.

figure 13-3

In a DNA fingerprint, DNA samples are cut, transferred to a nylon membrane, and exposed to radioactive probes. Explosing X-ray film to the membrane makes the fingerprint visible.

In a DNA fingerprint, DNA samples are cut, transferred to a nylon membrane, and exposed to radioactive probes. Explosing X-ray film to the membrane makes the fingerprint visible.

^^ Cut DNA with Run restriction fragments ^^ Transfer DNA to nylon ^^ Expose X-ray film to restriction enzyme. through gel electrophoresis. membrane. Add radioactive radiolabeled membrane.

probes that bind to complementary DNA.

^^ Cut DNA with Run restriction fragments ^^ Transfer DNA to nylon ^^ Expose X-ray film to restriction enzyme. through gel electrophoresis. membrane. Add radioactive radiolabeled membrane.

probes that bind to complementary DNA.

A DNA fingerprint can pinpoint subtle genetic differences that identify each individual uniquely. Identical twins share the same DNA fingerprint.

Comparing DNA: DNA Fingerprints

To permanently preserve the DNA fingerprint, a technician can place a positively charged nylon membrane over the gel and transfer the negatively charged DNA to the membrane. To visualize specific DNA pieces, biologists can prepare a nucleic acid complementary to the DNA of interest and label it radioactively. A sheet of X-ray film placed over the gel will be exposed only where the desired DNA is on the gel, providing a DNA fingerprint. An example of a DNA fingerprint is shown in Figure 13-4.

Accuracy of DNA Fingerprints

What makes a DNA fingerprint so powerful as an identification tool is the combined analysis of many VNTR loci. Analyzing only one VNTR locus is like having only one digit in a person's telephone number. DNA fingerprinting typically compares from five to thirteen VNTR loci. Thirteen loci, as often used by some crime labs in their criminal profiles, produce the odds that two people will share a DNA profile at about one in 100 billion. There are about 6.5 billion people on Earth, so this should be enough to remove any doubt.

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