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

DNA Testing in Forensic Science 591

DQA1). This marker looked at a genetic marker that had genetic markers used to map the human genome. With
originally been tested at the protein level at the DNA level. this large number of markers available it became possible
This type of marker looked at DNA-sequence-based dif- to pick sets of these markers to make highly discrimina-
ferences. This became the first PCR based tested to be used tory multiplexes (multiple regions amplified at the same
forensically. Other sequence-based tests were developed time). The use of fluorescent detection of DNA fragments
but they did not provide the same level of identification on automated DNA sequencers from the human genome
produced by RFLP based testing. Using the available se- project made it possible to create fluorescent multiplexes
quence based tests only allowed individualization in the with automated detection. These methodologies are now
100s to 100,000s. the methods of choice for use in the forensic testing of
The FBI established a standardized system for publicly DNA samples.
funded crime laboratories in the United States. Similar
work was going on in Canada, the United Kingdom, and I. WHAT IS DNA?
Europe. The introduction of DNA restriction fragment
length polymorphism (RFLP) technology revolutionized
DNA stands for deoxyribonucleic acid. It is the biological
the field of forensic identity testing. This is especially
blueprint of life. DNA is made up of a double-stranded
true for the area of sexual assault evidence that histori-
structure consisting of sugar (deoxyribose) and phosphate
cally has been an area of limited information. With DNA-
back bone, cross linked with two types of nucleic acids re-
based testing sperm DNA could be separated from the
ferred to as purines (adenine and guanine) and pyrimidines
victims type allowing for the first-time regular direct test-
(thymine and cytosine) (Fig. 1). The cross linking nucleic
ing of the sperm donor. Though RFLP technology has
acids always pair a purine with a pyrimidine, such that
been a tremendous aid, it has several problems. It is ex-
adenine always pairs with thymine and guanine always
pensive to implement, labor intensive, expensive to test,
pairs with cytosine.
and is limited by both quantity and quality of DNA ob-
DNA can be found in several areas of a cell. The ma-
tained. Further, because the process involved measuring
jority of DNA is located in the nucleus of cells (Fig. 2)
the movement of bands and not directly the DNA product
organized in the form of chromosomes (22 pairs of auto-
there were many statistical problems with representing the
somes and a set of sex chromosomes (X and Y)). Each
data. The technical feasibility of amplifying specificseg-
nucleated cell normally has 46 chromosomes that repre-
ments of DNA using the polymerase chain reaction (PCR)
sent the contribution from both parents. In the formation
had the potential to overcome the shortcomings of RFLP
of gametes (eggs and sperm) one chromosome of each pair
technology.
is randomly separated and placed in the gamete. The sep-
PCR-based technology is much less expensive to imple-
aration of chromosomes is referred to as segregation. The
ment, since it does not require a laboratory capable of han-
transmission of half of our chromosomes to our children in
dling radioactive isotopes. It has higher through put since
the form of gametes is the basis of Mendelian inheritance.
each worker can do more cases in the same amount of time.
This DNA is referred to as nuclear or genomic DNA. With
PCR by its nature works with smaller amounts of DNA
the exception of identical twins, no two people share the
and with DNA that has been environmentally abused. Fi-
same genomic DNA sequence.
nally, since the DNA product can be identified as different
Another source of DNA is found in the mitochondria
alternative forms the statistical manipulation of data was
in the cytoplasm of cells (Fig. 2). Unlike nuclear DNA,
similar to that for other genetic polymorphisms.
which only has two copies of each genetic region, mito-
In 1989 at the same time that the RFLP-based testing
chondrial DNA is involved in energy production within
was being converted to PCR-basedtestingcreating the first
the cell and can have between 100 and 10,000 copies per
amplified fragment length polymorphism or AFLP, new
cell. Structurally, instead of a linear arrangement of DNA
polymorphisms were being found directly using PCR. The
within chromosomes, mitochondrial DNA has a circular
converted RFLP loci which consisted of different num-
structure.MitochondrialDNAisinheritedfromthemother
bers of repeated segments, much like freight trains with
because it is found in the cytoplasm which comes from the
different numbers of box cars were referred to as “variable
egg (ova).
number of tandem repeat” or VNTR regions or loci. These
repeats consisted of 15 to 70 base pairs and were referred
A. Where Is DNA Found?
to at ”large tandem repeat loci or LTRs. The new regions
being found had much smaller repeats consisting of two, Nuclear or genomic DNA is found in all nucleated cells
three, four, or five bases in a repeat unit. These new mark- as well as in the reproductive cells (eggs and sperm). The
ers were called “short tandem repeats” or STRs for short. amount of DNA we can expect to find in different cells
The four base pair or tetra nucleotide repeats became the and types of evidence are found in Table I. DNA has been