RECOMBINANT DNA TECHNIQUES: CLONING AND MANIPULATION OF DNA - A Guide to Methods in the Biomedical Sciences - page 67

Any two humans differ by about 0.1 % of their genome, representing
about 3 million of the 3 billion base pairs in the human genome. To
compare the DNA of a suspect with DNA obtained at a crime scene, the
investigator must use methods that take advantage of these differences
to generate a unique DNA profile for comparing samples.
RFLP
One method that can be used is RFLP (restriction fragment length
polymorphisms; see Chapter 2) analysis. By comparing 4 or 5 known
RFLPs, a forensic scientist can readily exclude or include a suspect with
DNA obtained at a crime scene. RFLP requires a significant amount of
DNA, more than normally found at a crime scene in “pristine” condition.
PCR
Because RFLP cannot be used reliably, PCR techniques are more
commonly employed, since this allows an investigator to amplify and an-
alyze small amounts of DNA. The major problem with PCR is that even
minute amounts of material that might have contaminated the sample
during its identification or collection can raise doubt about the conclu-
sions of the investigator.
Short tandem repeat (STR)
Short tandem repeat (STR) analysis is a very common method of
analyzing specific loci in genomic DNA. STRs commonly are composed
of repeats of 2 to 5 nucleotides in a “head to tail” manner (for exam-
ple, gatagatagata represents 3 gata repeats). STRs can be found on
different chromosomes and vary significantly between individuals. The
CODIS ( CO mbined D NA I ndex S ystem) database, which was estab-
lished by Congress in 1994 for the use of the FBI and local and state
governments to identify sex offenders, uses a core of 13 STRs to distin-
guish between individuals. The odds that any two individuals are identical
at the 13 STRs are about 1 in a billion.
Other markers
Two other sets of markers are useful in identifying the DNA profile of
an individual. These include mitrochondrial DNA analysis and Y chro-
mosome analysis. Mitochondrial DNA is often used when nuclear DNA,
which is required for RFLP and STR analysis, cannot be extracted with
confidence. The mitochondria, bacteria-like organelles within each cell