dna and dna evidence 129Short segments of complementary DNA are used as oligonucleotide
primers to mark the starting point of target DNA during PCR.
Denature: During the laboratory process of amplification (PCR) and again
during the analysis step, the double helical strand of DNA must be
unwound into separate single strands. The separation of strands, or
denaturation, can be accomplished by the addition of certain chemi-
cals or by elevating the temperature to approximately 98°C. The latter
approach occurs during PCR. Denaturation is sometimes referred to
as melting. The opposite of denaturing is annealing, which describes
two complementary strands binding together.
DNA: Deoxyribonucleic acid is one of the body’s macromolecules and codes
for all proteins. Forensic scientists currently focus on three major
types of human DNA. All are identical at the molecular level, but each
varies in its protein-coding responsibilities and its location within the
cell. Y-DNA and X-DNA are the male and female sex chromo somes,
respectively. Autosomal DNA (autoDNA) refers to any of the remain-
ing twenty-two pairs of nonsex chromosomes. Sex and autosomal
chromosomes are all located within the nucleus of the cell and con-
tain the popular forensic targets called short tandem repeats (STRs)
and single nucleotide polymorphisms. Mitochondrial DNA (mtDNA),
on the other hand, is found outside the cell nucleus inside organelles
called mitochondria, which reside in the cellular cytoplasm. Forensic
scient ist s u su a l ly t a rget t he reg ions of t he mt DNA genome w it h k now n
hypervariable sequences or single nucleotide polymorphisms.
Electrophoresis: In the appropriate media, the negatively charged fragments
of DNA migrate in an electrical field according to their molecular
weight. Generally speaking, larger DNA fragments will move through
the medium slower than smaller fragments of DNA. In this way, the
analyst uses electrophoresis to separate DNA fragments of different
sizes to visualize the results of PCR analysis. The most informative
electrophoresis process takes place on a genetic analyzer.
Enzyme: Proteins that facilitate a biochemical reaction are called enzymes.
In forensic DNA science, the most commonly used enzyme is DNA
polymerase that, in the proper chemical environment, assists with
the synthesis of new strands of DNA from an existing template dur-
ing the polymerase chain reaction. Additionally, an enzyme called
proteinase K is used to break down cellular components and expose
DNA during the extraction process.
Extraction: The first of three laboratory processing steps in forensic DNA
science is called DNA extraction. It involves isolating the DNA by
preparing the gross sample, sometimes by grinding or macerating
it as in the case of bones, teeth, or tissues. Then proteinases are used
to disrupt the cellular barriers and allow the naked DNA to go into