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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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| 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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| 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.
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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.
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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. 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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