A Potent Weapon in Forensic Medicine.....DNA fingerprinting

Traditionally, one of the most accurate methods for
placing an individual at the scene of a crime has been
a fingerprint. With the advent of recombinant DNA
technology, a more powerful tool is now available:
DNA fingerprinting (also called DNA typing or DNA
profiling).
 

DNA fingerprinting is based on sequence polymorphisms,
slight sequence differences (usually single
base-pair changes) between individuals, 1 bp in
every 1,000 bp, on average. Each difference from the
prototype human genome sequence (the first one obtained)
occurs in some fraction of the human population;
every individual has some differences. Some of
the sequence changes affect recognition sites for restriction
enzymes, resulting in variation in the size of
DNA fragments produced by digestion with a particular
restriction enzyme. These variations are restriction
fragment length polymorphisms (RFLPs).
The detection of RFLPs relies on a specialized
hybridization procedure called Southern blotting. 

DNA fragments from digestion of genomic
DNA by restriction endonucleases are separated by
size electrophoretically, denatured by soaking the
agarose gel in alkali, and then blotted onto a nylon
membrane to reproduce the distribution of fragments
in the gel. The membrane is immersed in a solution
containing a radioactively labeled DNA probe. A probe
for a sequence that is repeated several times in the
human genome generally identifies a few of the thousands
of DNA fragments generated when the human
genome is digested with a restriction endonuclease.
Auto-radiography reveals the fragments to which the
probe hybridizes.
 

The genomic DNA sequences used in these tests
are generally regions containing repetitive DNA
(short sequences repeated thousands of times in tandem),
which are common in the genomes of higher
eukaryotes. The number of repeated
units in these DNA regions varies among individuals
(except between identical twins). With a suitable
probe, the pattern of bands produced by DNA fingerprinting
is distinctive for each individual. Combining
the use of several probes makes the test so selective
that it can positively identify a single individual in the
entire human population. However, the Southern blot
procedure requires relatively fresh DNA samples and
larger amounts of DNA than are generally present at
a crime scene. RFLP analysis sensitivity is augmented
by using PCR  to amplify vanishingly
small amounts of DNA. This allows investigators to
obtain DNA fingerprints from a single hair follicle, a
drop of blood, a small semen sample from a rape victim,
or samples that might be months or even many
years old.
 

 These methods are proving decisive in court cases
worldwide. The DNA from
a semen sample obtained from a rape and murder victim
was compared with DNA samples from the victim
and two suspects. Each sample was cleaved into fragments
and separated by gel electrophoresis. Radioactive
DNA probes were used to identify a small subset
of fragments that contained sequences complementary
to the probe. The sizes of the identified fragments
varied from one individual to the next, as seen here
in the different patterns for the three individuals (victim
and two suspects) tested. One suspect’s DNA exhibits
a banding pattern identical to that of a semen
sample taken from the victim. This test used a single
probe, but three or four different probes would be
used (in separate experiments) to achieve an unambiguous
positive identification

Such results have been used to both convict and
acquit suspects and, in other cases, to establish paternity
with an extraordinary degree of certainty. The
impact of these procedures on court cases will continue
to grow as societies agree on the standards and
as formal methods become widely established in forensic
laboratories. Even decades-old murder mysteries
can be solved: in 1996, DNA fingerprinting helped to
confirm the identification of the bones of the last Russian
czar and his family, who were assassinated in 1918.