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| DNA
Matching |
| 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 . |
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Read on by selecting one of the following topics:
--> What
is DNA?
--> The
advantage of DNA
--> Junk
--> Polymerase
chain reaction
--> Analysing
DNA
--> The
accuracy
--> The
next generation
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| What
is DNA? |
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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. |
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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. |
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| The
Advantage of DNA |
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| 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. |
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| Junk |
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| 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.
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Deoxyribonucleicacid can also be observed under the microscope. Photo
courtesy of California
Association of Criminalists. |
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| Polymerase
Chain Reaction |
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| 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. |
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| Analysing
DNA |
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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.
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.
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| *DNA
fingerprinting has epitomised modern forensic science. Photo copright
Mike Zeller,
Iowa State
University. |
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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.
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| The
Accuracy |
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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 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.
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Loose pieces of hair found at the crime scene may also be questionable,
as hair may be collected on clothing and bags. |
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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.
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| The
Next Generation |
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| 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. |
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