Page 15 - An Identity Crisis

via hydrogen bonds to form the ladder’s rungs. T (RFLP) analysis in the first forensic use of DNA in
links only with A, and C links only G. Because of this a legal case. Restriction enzymes are proteins that
paring, each strand is complimentary to the other find all instances of short, specific DNA sequences,
and contains the information necessary to build the called cut sites, and cut the DNA molecule at
other strand by serving as a template. the sequence. These enzymes are chosen such
that they cut DNA at locations common to many
Genes are produced by DNA and determine the people. Mutations in DNA, however, may create
characteristics of each individual. Each person has new cut sites or change the sequence at that site,
two complete sets of DNA divided into 23 pairs of preventing a cut. The lengths of DNA between
chromosomes, one from their mother and the other the cut sites vary because of these mutations.
from their father. Each parent may give us the same The varying lengths of DNA fragments reflect the
version, or allele, of a gene (homozygous), or we uniqueness of the DNA sample, and these varying
may receive two different versions (heterozygous). length fragments are compared to fragment size
These genes are then used by our bodies to build a patterns generated from other DNA samples.
variety of proteins that are directly responsible Unfortunately, RFLP analysis requires a large
amount of intact DNA to obtain a meaningful
for the physical traits we possess. It is for this result. While some crime scenes have much DNA-
reason that people often exhibit a mixture of their containing evidence, most contain a small amount
parent’s physical traits. Within the entire human of DNA limiting the usefulness of RFLP analysis.
population, however, there is less than 0.1%
difference within our genetic makeups as 99.9% of The ability of forensic analysts to copy small
your DNA sequence is identical to all of the other amounts of DNA into larger quantities is critical
people in the world. The fraction of a percent that when faced with crime scenes with little biological
varies, however, is enough to produce all of the evidence – a cash register that has only been
genetic variation exhibited between individuals. touched by a suspect, for example. It is also
extremely important because it is likely that
Not all DNA base combinations produce genes that some, or all, of the biological material found at a
translate into proteins. Because these portions crime scene has been exposed to environment
of DNA do not affect the physical properties of degradation in the form of weather, time, or the
an individual, they are able to mutate with no sun’s UV rays. The polymerase chain reaction
ill effects. These mutations changes are then (PCR) is a laboratory technique that mimics the
transfered to that individual’s offspring. Therefore, biological process by which DNA replicates within a
this so-called “junk DNA” has a large degree of cell. Unlike RFLP, which cuts DNA apart to produce
variability. By exploiting this variability, forensic an identifying pattern, PCR makes many copies of a
analysts can purify DNA found at crime scenes, selected region of junk DNA that has variable length
generate a unique DNA profile for a sample, and within the population. The identifying pattern is
compare that profile to profiles from suspects or created when the collective lengths of eight to
other individuals. Ultimately, the forensic analyst sixteen of these regions, called short tandem
can determine a probability that two DNA samples repeats (STR), are compared.
came from the same individual.
The STRs used in DNA typing are located on
Forensic DNA analysis has had a short, but exciting, different chromosomes and follow the genetic
history. In 1984, Sir Alec Jefferys used a technique laws of segregation and independent assortment.
called Restriction Fragment Length Polymorphism These laws assert that we receive exactly one

THE MYSTERY OF LYLE AND LOUISE 15