Fragments
of human DNA are unique to the individual. These fragments are
called polymorphic because they vary in shape from person to
person. DNA profiling is the process of separating an individual's
unique, polymorphic, fragments from the common ones.
With
the use of PCR in forensics, law enforcement are revisiting
older, unsolved cases. PCR can start with the tiniest DNA sample
and rapidly create identical copies until investigators have
enough DNA to compare in tests.
"PCR
acts as a chemical photocopier.
This often
works even with degraded DNA," said Dr. Beverly Himick, a forensic
scientist at the Washington State Patrol Crime Lab.
PCR
breaks apart the double stranded DNA helix and exposes the 2
single strands to enzymes known as "polymerases" and
the 4 basic building blocks of DNA, nucleotides known as A,
C, T and G.
The
polymerases use the nucleotides to rebuild each single strand
back into a complete double helix. Where there was just 1 complete
strand of DNA, with PCR you get 2, then 4, 8, 16, 32, 64, 128.
Polymerase
chain reaction is a process catalyzed by an enzyme, polymerase
replicating DNA from templates or strands to create a larger
pool for extensive testing and characterization.
Now
a refined version of PCR, known as "short, tandem repeats" or
STR-PCR, is available for cases such as cold
felony cases. STR has used skin cells from a doorknob. No two
individuals have the same STR patterns.
STR,
Short Tandem Repeats are nucleotides along the backbone of a
chromosome used as markers.
STR
heats a piece of DNA until it separates into the 2 strands of
its characteristic double helix, then is incubated with man
made DNA matching the genetic code of each strand. This creates
2 new pieces of DNA from the original one. The process is repeated
until there are enough copies for capillary electrophoresis,
which produces a chart mapping a person's exact genetic makeup.
This is the information used to compare suspects to crime.
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