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THE ATTIC
: DNA Profiling - Background |
Date
Added
OCT 2002 |
Perhaps the most potentially important discovery in the recent scientific
history of forensics has been the development of DNA profiling. There already
exist biological tests which, although advanced to a high degree, have distinct
disadvantages by comparison with DNA profiling for the same requirements.
Blood grouping, for example, can exclude a putative father in a paternity
dispute, but it cannot positively prove paternity. Immunological techniques are
widely used in forensic sciences - notably to identify the components of
blood and the specific origin of tissue samples. With both these techniques
problems can arise if the samples are old, impure, or insufficient in quantity
for accurate analysis. These difficulties can be overcome with the use of DNA as
the basic sample - it is so stable that DNA has been extracted from
mummies millennia old. It is the basic unit of the human body, each
individual's DNA is unique, and only minute samples are required for analysis.
It was in 1984 that Professor Alec Jeffrey's, FRS, a Research Fellow at the
Lister Institute, Leicester University, achieved the breakthrough that made DNA
a practical tool to identify positively any individual from the minutest body
trace. Professor Jeffrey's discovered that with the DNA molecule a particular
sequence of information exists which varies greatly in unrelated individuals.
These sequences can be 'visualised' in the laboratory as prints from x-ray film
which are as unique to an individual as a conventional
fingerprint - hence the colloquial term 'DNA fingerprinting'. A UK patient
was granted 1987, and through a commercial agreement with the Lister Institute,
a branch of ICI - Cellmark Diagnostics - hold exclusive rights to the
technology. |
Technique |
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Every human cell contains within
it all the information required to create a whole human body - in other words a
'blueprint for life'. This information is carried in the chromosomes.
Chromosomes are found in the nuclei of all human cells; each nucleus contains
coded information in the form of DNA (deoxyribonucleic acid) arranged into
groups called genes. These in turn are arranged on entwined strands of DNA which
are chromosomes. The four building blocks, called 'bases', which make up DNA are
Adenine, Cytosine, Guanine, and Thymine - known by their initial letters ACGT.
The ribbon of DNA has been compared to a zip fastener with the bases ACGT
forming the teeth. The difference is that in order to join the two strands of
DNA, A must pair with T, and G must pair with C.
There are an enormous number of these bases in a chromosome, though as far as
can be determined only about forty five percent are required for cell operation
- the function of the remaining fifty five percent is not yet known.
The forty five percent functional units are distributed throughout the length of
the chromosome, interspersed with the unexplained fifty five percent. Along the
length at random intervals bases in certain combinations of six occur and are
called 'restriction sites'; the sites are palindromic, that is, the order of
bases on the top strand is the exact reverse of the order of bases on the bottom
strand:
Natural defence systems known
as 'restriction enzymes' can be found in certain bacteria and they are capable
of recognising and cutting DNA at its palindromic sequences. The fragments are
then separated from each other by gel electrophoresis. The technique is
basically this : the DNA sample is put in a slit at one end of a 10 x 14 x 0.5cm
block of gel; a high voltage electrical current is applied across the gel
causing the DNA fragments to move through it, the distance of each fragment
moves being determined by its size. The separated fragment can be lifted from
the gel on to a nylon membrane, rather as blotting paper will lift ink (in fact,
the process is known as 'Southern Blotting' after its inventor, Professor Ed
Southern). It should be added that at this stage the characteristic 'bands' are
invisible to the eye.
Besides the palindromic restrictions sites the chromosome chain also has
sections along its length where base sequences are repeated again and again; it
is this feature which allows the DNA fragments to match and rejoin. The membrane
is incubated overnight with what is called a radioactive 'probe' - a solution
consisting of fragments of DNA containing repeating sequences which have been 'labelled'
with radioactivity so that they can be detected by matching repeat sequences on
the membrane which will unite, or 'hybridise', with them. For example, if the
membrane contains a strand with the sequence AGCCTGCTCTTA and the probe a
complementary sequence TCGGACGAGAAT they will bind to form a double band. |
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The next stage is to wash the membrane clean of
any unbound probe material and place it in contact with x-ray film. The
radioactivity of the sections where the probe has bound to the DNA will produce
dark bands on the film. The x-ray film is printed out revealing a pattern of
bands not unlike supermarket bar codes, and with the exception of genetically
identical twins every individual has a unique pattern. |
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Keeping it in the Family |
Although the news concerning this remarkable
discovery tended to centre around the more spectacular cases of trapping killers
and rapists, by far the greatest number of tests is conducted routinely to
establish the identity of a father in a paternity dispute, and to prove or
disprove family links in applications for immigration.
A child inherits half its chromosomes (and hence DNA) from each of its parents.
In cases of disputed paternity DNA fingerprints are obtained for the mother, the
child and the two (or more) putative fathers. Each band on the child's chart
must match one on either its mother's or its father's chart; so lining up all
four charts it is possible to identify the father absolutely.
This could have serious repercussions for fathers attempting to avoid supporting
their illegitimate children by denying paternity. In June 1991 for the first
time, an English Judge ordered a man whose
blood test was inconclusive (it determined that he could not be excluded) to
submit to DNA Profiling. |
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Criminal Matters |
The application of DNA profiling to criminal
investigations has revolutionized the whole concept of 'positive
identification'. For example, a
blood or semen stain found at the
scene of the crime - even if it is minute in size and of considerable age -
can be analysed and a DNA profile obtained.
In due course this can be compared with the DNA profile derived from a suspect
or suspects. The importance of the technique lies in its certainty - either the
profiles match and the suspect is guilt, or they do not and he can be eliminated
from police inquiry.*
(* An anomaly of the British legal system at present under
investigations by the Royal Commission on Criminal Justice is that a suspect can
legally refuse to give a sample of his/her blood if he feels disinclined. It has
been pointed out that alleged
drink drivers are obligated to give samples, but alleged murderers and
rapists are not. It is expected that the Commission will ask for police to be
given the power to ask a court to demand a sample, with heavy penalties for
defiance of the order.) |
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Recent Advances |
The technique of DNA profiling has advanced so rapidly since its discovery that
profiles can be obtained from any sample of human tissue - in June 1989 a rapist
was convicted in St Albans Crown Court on the evidence of a DNA profile derived
from one of his hairs.
This is not to say that the brief history of genetic profiling has been a one
hundred percent success. It has, like all new techniques, been subject to a
number of shortcomings - not least the length of time that it takes to derive a
conventional profile (up to 6 weeks). However, in November 1991, Dr Alec
Jeffreys announced new breakthrough that is hailed as taking a 'quantum leap' in
the process of DNA fingerprinting, which at a single stroke solves most of the
existing problems. They system is called Digital DNA Profiling, and pilot
studies have shown that it is up to 100 times more sensitive than the previous
method, and has reduced processing time to two days. In addition, it will
enhance the samples degraded from by body
putrefaction and easily distinguish between mixed samples - as when semen
and the victims own DNA are present in the vagina in a rape case. However, the
most remarkable new feature is that the profile emerges as a sequence of
numbers, a digital code of between fifty and seventy figures, thus eliminating
objections to the previous sometimes ambiguous bar code profile. It also means
that the digitalised sequence of numbers are easier to store on a computer. This
has met with an enthusiastic response from police forces already building
profile indexes. It also raises again the whole contentious debate on a
Universal Register. |
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