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Figure 9.10 A Nucleotide Microarray The colored dots indicate the positions on the array to which labeled nucleic acid fragments hybridized. Different samples were labeled with distinct fluorescent markers, allowing the researcher to compare hybridization patterns.The arrays are often used to study gene expression.

screen a single sample for a vast range of different sequences simultaneously.

■ What is the significance of DNA fragments from two different sources hybridizing?

■ Why would a disease-causing bacterium express different genes when inside the body?

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Chapter 9 Biotechnology and Recombinant DNA

PERSPECTIVE 9.1 Science Takes the Witness Stand

Fingerprints have been used to identify criminals and solve crimes for more than 100 years. More recently, DNA fingerprinting is gaining prominence for its role in criminal investigations. If a criminal leaves a blood drop or semen at the scene of a crime, the DNA can be extracted and compared with the DNA from the blood of a suspect.

It is not feasible to compare the entire nucleotide sequence of two samples; instead, restriction fragment length polymorphisms (RFLPs) are compared.The probes that are used hybridize to regions of the chromosome that vary greatly among individuals.These regions are located between genes and consist of a repetitive core sequence, a tandem repeat.The number of times this sequence is repeated in various chromosomal locations differs from one individual to the next. For example, in given chromosomal location, one individual may have a series of 8 tandem repeats, whereas another individual may have 10.The relative number of repeats is reflected in the size and number of DNA fragments generated by digestion with a restriction enzyme, giving rise to the RFLPs.These variations can be detected by Southern blot hybridization using the core sequence as a probe (figure 1).

DNA fingerprinting compares the hybridization pattern of DNA in samples from different sources. By comparing DNA taken from a sample found at a crime scene with that of DNA from a suspect, it is possible to determine the likelihood that the DNA samples came from the same person.

The results of DNA testing must be interpreted with caution. For example, it is difficult to state with certainty that two bands are identical in size. Further, during electrophoresis DNA fragments of the same size can migrate at different rates because of variations in the preparation of the gel, concentration of the DNA, or other unknown variables. Even with these technical problems, however, there seems little doubt that molecular biology will play an increasingly important role in criminal investigations as science takes the witness stand.

Suspect's blood

Figure 1 DNA Fingerprinting (Typing)

The probe recognizes sequences in DNA that vary in number among people.

Suspect's blood

A restriction enzyme is used to cleave the DNA into specific-sized fragments.

Electrophoresis process. The DNA fragments are separated according to their length, and end up as bands.

The band patterns are transferred to a nylon membrane.

A probe labeled with a radioactive isotope binds to the target fragments.

Autoradiography is used to determine the location of the labeled probe. The result is an array of patterns that resemble a retail bar code.

X-ray film-

3. Suspect's blood

DNA fingerprinting (typing). The probe recognizes sequences in DNA that vary in number among people.

X-ray film-

1. Evidence #1 (victim's blood)

2. Evidence #2 (unidentified blood)

3. Suspect's blood

Evidence is gathered. Work begins on matching evidence with the victim and the suspect.

DNA is extracted from victim's blood, the unidentified blood, and the suspect's blood.

A restriction enzyme is used to cleave the DNA into specific-sized fragments.

Electrophoresis process. The DNA fragments are separated according to their length, and end up as bands.

The band patterns are transferred to a nylon membrane.

A probe labeled with a radioactive isotope binds to the target fragments.

Autoradiography is used to determine the location of the labeled probe. The result is an array of patterns that resemble a retail bar code.

A comparison can then be made between resulting patterns from the suspect and from the unidentified blood found at the scene of the crime. In this example, they match, indicating that the suspect was at the scene of the crime.

DNA fingerprinting (typing). The probe recognizes sequences in DNA that vary in number among people.

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