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DNA
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A. Terminology
Chromosomes- strands of genetic material
 Genes- Fundamental unit of heredity
 Locus (loci)-Exact location on the DNA
molecule of a gene or area of interest
 Homozygous-Two identical gene pairs
 Heterozygous-Two different forms of the gene
 Recombinant DNA-Opening up the base pairs
of the helix and recombining it with another
strand
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DNA fingerprints
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Genetic code-Sequence of letters on a DNA
strand
Restriction enzymes-Chemicals that cut DNA
into fragments that can later be incorporated into
another DNA strand; about 150 are
commercially available
Probe- A single strand of nucleic acid, much like
RNA, that has been made in a way that its base
sequence lines up to hybridize areas in an allele;
usually labeled with radioactive material
Human gene- A project designed to
determine the order of bases on all 23
pairs of human chromosomes
 B. History
 1. James Watson and Francis Crick- 1953
discovered the configuration of the DNA
molecule
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Alec Jeffreys- Isolated DNA markers& called
them DNA fingerprints
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Kary Mullis- 1985 developed PCR testing
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C. Structure
Polymer- A very large molecule made by
linking together a series of repeating units
 Nucleotides- A unit composed of a sugar
molecule, a phosphorous group and a
nitrogen-containing base
 Bases-Adenine, cytosine, guanine, & thymine
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Double Helix- Two coiled DNA strands
 Base Pairing- A to T and G to C
 Proteins- Made by linking togerther a
combination amino acids from a
combination of up to 20 unknown
 Amino acid codes- building blocks of
protein are coded by a sequence of three
bases.
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D. Replication
1.
2.
process- unwinding the DNA strands in the
double helix; exposing the strand to a
collection of free nucleotides; letter by letter
the double helix is recreated in the proper
order
Polymerases- enzymes that assemble a
new DNA strand in the proper base
sequence determined by the original or
parent DNA strand
E. DNA Typing
 1. RFLP- restriction fragment length
polymorphisms; requires high molcular
weight DNA
 Visual evaluation- soak and free up the
DNA
 Assay- electrophoresis
 Digestion- by the restriction enzyme
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Test gel
 Prepare known samples
 Electrophoresis of all samples
 Southern blotting- transfer the fragments
to a nylon membrane
 Hybridization- nylon treated with
radioactive probes containing a base
sequence complementary to the RFLPs
being identified
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Autoradiography- nylon sheet is placed
against X-ray film and exposed for several
days
 Additional probes- adding more probes
increases probability of having a match
 2. PCR- polymerase chain reaction; need
only low molecular weight DNA
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DNA is denatured by heating it to 95 degrees
Celsius, splitting the bonds between the two
halves
Heat is reduced and DNA primer are added
which hybridizes to site-specific arrangements of
complementary bases
A DNA polymerase is added that replicate the
DNA; the primer attracts them to the open bases
and they pair up until the clone is complete
STEP c is repeated over and over
 3. match
 Inclusion- DNA bands match and the
suspect is still a suspect
 Exclusion- DNA bands don’t match and
the person is no longer a suspect
 F. Present and the Future
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CODIS- Computer Based DNA Information
System; a data base of DNA profiles of
individuals convicted of sex crimes and
other violent crimes
 TWGDAM- The Working Group for DNA
Analytical Methods wrote the standards for
DNA analysis that are part of a national
crime laboratory accreditation program
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