by Richard Saferstein, Ph.D.
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[This article is the
second of two parts. Click for Part 1.]
one describes DNA in the context of a criminal investigation, it's assumed that
the subject of attention is the DNA found within the nucleus of a cell. Actually,
a human cell contains two types of DNA - nuclear and mitochondrial. The first
constitutes the 23 pair of chromosomes contained within the nuclei of our cells.
Each parent contributes to the genetic makeup of these chromosomes. On the other
hand, mitochondrial DNA (mtDNA) is found outside the nucleus and is inherited
solely from the mother.
Mitochondria are cell
structures found in all our cells. They are the power plants of our body, providing
about 90% of the energy that our body needs to function. What's important from
the forensic science perspective is that a single mitochondrion contains several
loops of DNA all of which are involved in energy generation. Further, since
each cell in our body contains hundreds to thousands of mitochondria, this effectively
means that there are hundreds to thousands of mtDNA copies in a human cell.
This compares to just two copies of nuclear DNA located in that same cell. Thus,
forensic scientists are offered enhanced sensitivity and the opportunity to
characterize mtDNA in situations where nuclear DNA is significantly degraded,
such as in charred remains, or may be present in small quantity (i.e., hair
shaft). Interestingly, in situations where authorities cannot obtain a reference
sample from an individual who may be long deceased or missing, a mtDNA reference
sample can be obtained from any maternally related relative. However, all individuals
of the same maternal lineage will be indistinguishable by mtDNA analysis.
While mtDNA analysis
is significantly more sensitive than nuclear DNA profiling, it must be noted
that forensic analysis of mtDNA is more rigorous, time consuming, and costly
when compared to nuclear DNA profiling. For this reason, at this time, only
a handful of public and private forensic laboratories receive evidence for this
type of determination.
As was previously
discussed, nuclear DNA is composed of a continuous linear strand of nucleotides
(A,T,G,C's). On the other hand, mtDNA is constructed in a circular or loop configuration.
Each loop contains a sufficient number (approximately 16,569) of A,T,G, and
C's to comprise 37 genes involved in mitochondrial energy generation. Two regions
of mtDNA have been found to be highly variable in the human population. These
two regions have been designated Hypervariable Region I (HV1) and Hypervariable
Region II (HV2). As indicated above, the process for analyzing HV1 and HV2 is
tedious. It involves generating many copies of these DNA regions by PCR and
then determing the order of the ATGC bases constituting the hypervariable regions.
This process is known as sequencing. The FBI laboratory, the Armed Forces
DNA Identification Laboratory, and other laboratories have collaborated to compile
a mtDNA population database containing the base sequences from Hypervariable
Regions l and ll.
Once the sequences
of the hypervariable regions from a case sample is obtained, most laboratories
will simply report out the number of times these sequences appear in the mtDNA
database. Currently, however, the mtDNA data base is too small to allow for
reasonable estimates of the frequency of occurrence of mtDNA sequences in the
population. However, even under the best of circumstances, mtDNA typing will
not approach STR analysis in it's discrimination power. Thus, mtDNA analysis
is best reserved for samples for which nuclear DNA typing is simply not possible.
DNA Typing The current approach for the examination of hair specimens still
includes an examination of morphological characteristics. However, recent major
breakthroughs in nuclear DNA (deoxyribonucleic acid) typing has extended this
technology to the individualization of human hair. Forensic hair examiners can
link human hair to a particular individual by characterizing the nuclear DNA
present in the hair root or in follicular tissue adhering to the root When pulled
from the head, many hairs will be found with a follicular tag-- a translucent
piece of tissue surrounding the hair's shaft near the root. This has proven
to be the richest source of DNA associated with hair.
the FBI initiated a program to compare human head and pubic hairs through mitochondrial
DNA. Importantly, there are many more copies of mitochondrial DNA located in
our cells as compared to nuclear DNA. For this reason, the success rate of finding
and typing mitochondrial DNA is much greater from samples, such as hair, which
have limited quantities of nuclear DNA.
AND PRESERVATION OF BIOLOGICAL EVIDENCE FOR DNA ANALYSIS
Since the early 1990s, the
advent of DNA profiling has vaulted biological crime scene evidence to a stature
of importance that is only eclipsed by the fingerprint. In fact, the high sensitivity
of DNA determinations has even changed the way police investigators define biological
evidence. In the past, crime laboratories were usually able to extract some
useful information from a blood or semen stain, or from a hair deposited at
the crime scene. Today, the sensitivity of PCR means that 1 nanogram (one billionth
of a gram) or less of DNA can yield sufficient information to individualize
evidence. With this technology in-hand, the horizon of the criminal investigator
extends beyond the traditional dried blood or semen stain to include stamps
and envelopes licked with saliva, a cup that has come in-contact with a person's
lips, or a bedsheet containing dead skin cells. Table II illustrates the power
of DNA as a creator of physical evidence.
evidence collector must handle all body fluids and biologically stained materials
with a minimum amount of personal contact. All body fluids must be assumed to
be infectious; hence, wearing disposable latex gloves while handling the evidence
is required. Latex gloves will also significantly reduce the possibility that
the evidence collector will contaminate the evidence. These gloves should be
changed frequently during the evidence collection phase of the investigation.
Safety considerations and the avoidance of contamination also call for the wearing
of face masks, shoe covers, and possibly coveralls.
Blood has great evidential
value when a transfer between a victim and suspect can be demonstrated. For
this reason, all clothing from both victim and suspect should be collected and
sent to the laboratory for examination. This procedure must be followed even
when the presence of blood on a garment does not appear obvious to the investigator.
Laboratory search procedures are far more revealing and sensitive than any that
can be conducted at the crime scene. In addition, blood should also be searched
for in the less than obvious places. For example, the criminal may have wiped
his or her hands on materials not readily apparent to the investigator. Efforts
must be made to find towels, handkerchiefs, or rags that may have been used
and then hidden. Attention should be given to examining floor cracks or other
crevices that may have trapped a quantity of blood.
Location of DNA on the Evidence
|baseball bat or similar weapon
|hat, bandanna, or mask
Nose or ear
|facial tissue, cotton swab
|mucus, blood, sweat, semen,
|stamp or envelope
|tape or ligature
|bottle, can, or glass
or rectal cells
| blanket, pillow, sheet
semen, urine, saliva
|"through and through" bullet
|fingernail, partial fingernail
Source: National Institute
of Justice- U.S. Dept. of Justice
The packaging of biological
evidence in plastic or airtight containers must always be avoided, because the
accumulation of residual moisture could contribute to the growth of DNA-destroying
bacteria and fungi. Each stained article should be packaged separately in a
paper bag or in a well-ventilated box. If feasible, the entire stained article
should be packaged and submitted for examination. If this is not possible, the
dried blood is removed from a surface with the aid of a sterile cotton-tipped
swab lightly moistened with distilled water from a dropper bottle. A portion
of the unstained surface material near the recovered stain must likewise be
removed and placed in a separate package. This is known as a substrate control.
The forensic examiner might use the substrate swab as a control to confirm that
the results of the tests performed were brought about by the stain and not by
the material on which it was deposited. However, this practice is normally not
necessary when DNA determinations are carried out in the laboratory. One point
is critical, and that is the collected swabs must not be packaged in a wet-state.
containing biological evidence should be refrigerated or stored in a cool location
out of direct sunlight until delivery to the laboratory. However, one common
exception is blood mixed with soil. Microbes present in soil will rapidly degrade
DNA. Therefore, blood in soil must be stored in a clean glass or plastic container
and immediately frozen.
will attain its full forensic value only when an analyst can compare each of
its DNA types to known DNA samples collected from victims and suspects. For
this purpose, at least 7 cc of whole blood should be drawn from individuals
by a qualified medical person. The blood sample should be collected in a sterile
vacuum tube containing the anticoagulant EDTA (ethylenediamine tetraacetic acid).
In addition to serving as an anticoagulant, EDTA inhibits the activity of enzymes
that act to degrade DNA. Prior to delivery to the laboratory, the tubes must
be kept refrigerated (do not freeze) while awaiting transportation to the laboratory.
Besides blood, there are other options for obtaining control DNA specimens.
The least intrusive DNA control and one that can readily be used by non-medical
personnel is the buccal swab. Here, cotton swabs are placed in the subject's
mouth and the inside of the cheek is vigorously swabbed, resulting in the transfer
of buccal cells onto the swab.
If an individual
is not available to give a DNA control sample, there are some interesting alternatives
available to evidence collectors to include: a toothbrush, combs and hair brushes,
a razor, and earplugs.
We have previously
seen forensic scientists are capable of detecting extremely small quantities
of DNA from biological evidence. With increased sensitivity comes a greater
chance that accidental contamination can be detected in crime scene evidence.
Contamination can occur by introducing foreign DNA into a stain while collecting
it; or there can be a transfer of DNA when items of evidence are in contact
with each other.
an examination of DNA band patterns in the laboratory readily reveals the presence
of contamination. For example with an STR, one will expect to see a two band
pattern. If one observes more than two bands, it becomes apparent that one could
be dealing with a mixture of DNA from more than one source.
There are some
relatively simple steps that crime scene investigators can take in order to
minimize the possible occurrence of contamination of biological evidence:
1. Always wear
disposable latex gloves when collecting biological evidence;
2. Always collect a
substrate control for possible subsequent laboratory examination;
3. Pick up small items
of evidence such as cigarette butts and stamps with clean forceps. Disposable
forceps are to be used so that they can be discarded after a single evidence
4. Always package each
item of evidence in its own well-ventilated container.