Page 45 - Academic Press Encyclopedia of Physical Science and Technology 3rd Molecular Biology

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Encyclopedia of Physical Science and Technology EN004L-956 June 9, 2001 21:7

596 DNA Testing in Forensic Science

FIGURE 4 Ethidium bromide stained yield gel. Bottom left samples are quantitative standards. Other samples repre-
sent various samples of DNA. Upper right sample is degraded DNA. (From Schanfield, M. S. (2000). Deoxyribonucleic
Acid/Basic Principles. In “Encyclopedia of Forensic Sciences” (Siegel, J. A., Saukko, P. J., and Knupfer, G. C., eds.),
Academic Press, London, p. 484.)

membrane is exposed to a solution of denatured DNA ing and is replacing sequence-based PCR strip technology
fragments that recognizes a repeating sequence of human with fluorescent STR-based testing. Thus it is important to
or primate DNA. Pieces of DNA that recognize a specific understand the nature of PCR-based testing and the power
region of DNA are referred to as a “probe.” The probe it provides to forensic scientists.
will bind to complementary DNA fragments stuck on the
membrane. The probe has an indicator attached to it so
A. Definition and Description of PCR
that the binding of the DNA to the probe can be detected.
The unbound probe is washed off and the probe is detected The Polymerase Chain Reaction (PCR) is based on bio-
using either chemicals that change color (colorimetric de- chemical processes within cells to repair damaged DNA
tection) or chemicals that give off light (chemiluminscent and to make copies of the DNA as the cells replicate. In
detection). To be able to quantitate the amount of human the repair mode, if a single strand of DNA is damaged,
DNA present, standards with different amounts of human the damaged area is removed so that there is a single-
DNA are also placed on the membrane so that it is possible stranded section of DNA with double-stranded sections
todeterminetheapproximateamountofDNAboundtothe at either end. The polymerase enzyme fills in the missing
membrane by visual comparison to the known standards. complementary DNA. In the copy mode an entire strand
More precise quantitation can be obtained by scanning is copied during DNA replication. Figure 5 illustrates
the membrane with a scanning densitometer and deter- a polymerase enzyme copying a portion of a strand of
mining the amount of color associated with each band. DNA.
Most forensic laboratories use visual comparison. InacellaspecificgeneiscopiedortranslatedfromDNA
to RNA because the polymerase has specific start and stop
signals coded into the DNA. To copy a sequence of DNA
IV. CURRENT FORENSIC DNA TESTING in vitro, artificial start and stop signals are needed. These
signals can only be made once the sequence of the re-
At this point in time, at the beginning of the 21st century, gion to be amplified is known. Once a sequence is known,
forensic DNA testing has moved away from RFLP test- the area to be copied or amplified can be defined by a