More often
heard about on television dramas than on the news, DNA
is the key to solving crimes the scientific way. Although
it has only been relatively recent (compared the course
of forensic history) that DNA has started being used
in court, its future in crime stopping is unquestionable
and ever more common .
DNA
is short for the scientific words, 'Deoxyribonucleic
Acid'. DNA looks like a ladder that is curled
around continuously and features about 3 billion
rungs that attached to the ladder. The rungs are
made up of simple natural base chemicals known
as guanine,
cytosine,
thymine
and adenine.
The order in which these chemicals are arranged
on a strand of DNA is unique to every individual
person, making DNA a very efficient tool for the
identification of a person as well.
*
A strand of DNA contains 4 different bases, with
each one represented by a different colour in
the image on the left. Guanine always forms a
rung with cytosine while cytosine only joins with
adenine. Photo is
courtesy of and copyright Wayne
Heim.
The fact that DNA exists
in every single cell in the body and only a minute amount
is needed for analysis, makes a sample easily obtainable.
DNA is also useful in identifying a victim, as we inherit
half of our DNA from each parent, and therefore, a part
match from the parent of a missing person can reveal
the relationship of an unidentified body.
Only a small
section of a DNA strand is responsible for our appearance,
while the remainder of the DNA is call 'junk' and appears
to have no specific purpose or function. However, it
is this 'junk' that can provide forensic scientists
with the most information in terms of identification.
It consists of small sequences of the base chemicals,
known as 'short tandem repeats' (STR's), which
continuously repeat end-to-end.
The number of times the STRs
repeat varies noticeably in each individual person
and, therefore, allows for identification. The repetition
of STR usually only needs to be counted up to thirteen
and it is at this point that we are able to make a
match in identity.
DNA is extracted from
a sample using a mixture of chloroform
and phenol,
which isolates the DNA strand from the other material
in the nucleus.
This method usually doesn't produce sufficient enough
DNA for analysis, so the strand is then artificially
increased using a method known as polymerase
chain reaction (PCR). This process involves an
enzyme
from the human body called a 'polymerase', which is
added to the already extracted DNA. As a catalyst,
the polymerase enzyme efficiently replicates the strand,
producing sufficient DNA for analysis.
The long
strand of DNA must then be separated into shorter pieces
(of different size) by using a restriction enzyme, which
cuts up the DNA each time a specific nucleotide pattern
occurs. These pieces of DNA must then be sorted according
to size, using the process of electrophoresis.
*DNA
fingerprinting has epitomised modern forensic science.
Photo copyright Mike Zeller,
Iowa
State University.
The fragments of DNA are poured into a narrow tube
of gel and a positive charge is applied to the bottom
of the gel while a negative charge is applied at the
top. Because DNA has a faintly negative charge, it
is attracted towards positives electrons in the same
way as north and south poles in magnets are attracted
to each other, and the DNA begins to move toward the
bottom (positive charge). However, smaller DNA travels
faster and sinks further down, while larger pieces
move relatively slower. This eventually creates 'bands'
on the gel, which are used for comparison with other
samples.
DNA matching has really revolutionised the solving
of crimes in forensic science. Samples that have been
taken from a suspect and crime scene can now be compared
using DNA databases, which can easily match samples
to prove a suspect guilty. However, DNA matching has
been subject to a lot of criticism, especially when
used as evidence in a court of law.
DNA databases have built a reputation as being a
fast and efficient way of solving crimes. The identity
of an individual is encoded in a database using a
compilation of numbers no longer than four telephone
numbers, making it extremely simple to match the numbers
from the criminal with the numbers from the crime
scene.
DNA matching is however, constantly under question
when used as evidence in court, as contamination of
a sample is possible, even though strict precautions
are put in place to prevent contamination. For example
- a stain containing DNA was found at a crime scene
in a country that has a population of 10 million people,
and the crime scene DNA sample is
*
Loose pieces of hair found at the crime scene may also
be questionable, as hair may be collected on clothing
and bags.
accurate enough to match 1%
of the population. A suspect is arrested and that
person's DNA sample matches perfectly with the one
found at the crime scene. The prosecutor
argues that because only 1% of the population shares
the same DNA profile, there is only a 1 in 100 chance
that the person is innocent. The defence however,
then argues that if 1% of the population share the
same DNA, then there could be 99 999 (1% of 10 million
minus 1) other individuals who could have possibly
been at the scene of the crime. Presuming innocence,
the odds of the suspect being guilty are actually
1 in 100 000. This example shows the hazards of relying
too much on DNA as evidence.
If there is enough evidence to support
the DNA sample, then this amplifies the suspicion
of guilt and makes a very persuasive case. However,
if the there is little or no evidence to support the
DNA sample, then the sample is practically useless.
The future of DNA matching
has a very promising outlook, with the completion of
the mapping of the human
genome in 2001. Scientists are now able to identify
the genes responsible for inherited traits and using
this, can reveal the suspect's hair colour. Scientists
predict that future DNA study will be able to reveal
what height and race a suspect is and possibly the building
of a suspect's face from just a single drop of blood.
This is still a long way off, but as research grows
and technology improves, advancements in this area are
constant.
