section 23.7
Diagnostic and Clinical Applications of DNA
TABLE 23-2
A Partial List o f Diseases for Which Single Genes Have Been Isolated and Characterized*
Familial amyloid polyneuropathies
Niemann-Pick disease
X-linked Charcot-Marie-Tooth disease
Familial hypercholesterolemia
Familial colon cancer
Christmas factor deficiency
Hereditary retinal degenerative diseases
Retinoblastoma, Wilson’s disease
Ototoxic deafness
Type IV collagen deficiencies
Hydatidiform moles and choriocarcinomas
Compulsive disorders
Tay-Sachs disease
Myotonic muscular dystrophy
Alzheimer’s disease
Familial dysalbuminemic hyperthuroxinemia
Gaucher’s disease
Long QT syndrome
Charcot-Marie-Tooth 1A disease
Cerebral autosomal dominant arteriopathy
Williams syndrome
Cancer-associated mar binding protein
Von Hippel-Lindau disease
Breast or ovarian cancer
Breast cancer (BRCA1)
Idiopathic dilated cardiomyopathy
Prostate cancer
Hereditary neuropathy with liability
*Each of these mutant alleles contributes to a greater or lesser degree to the pathology of the disease or inherited disorder. As of December
1997, patents had been issued to universities or companies for the commercial use of these genes.
Forensic DNA Analysis
are also widely used in forensic pathology,
criminology, and cases of contested paternity. A par-
ticular set of DNA probes is specific for hypervari-
able sequences in the human genome that are inher-
ited in a Mendelian pattern identical to the inheritance
of genes.
Hypervariable sequences
are highly polymor-
phic minisatellite loci that are unique to each individ-
ual just as each individual has a unique set of genes.
These hypervariable sequences can be detected by a
special set of DNA probes called
Jeffreys probe
ter their discoverer. A Southern blot analysis of frag-
ments of DNA from an individual using the Jeffreys
probe constitutes a unique “genetic fingerprint.” Only
monozygous twins have the same pattern of hyper-
variable sequences and identical genetic fingerprints
(Figure 23-15).
DNA obtained from a blood stain, a human hair cell, or
even a few sperm provides enough material to match with
the DNA obtained from a suspect. DNA testing of suspects
in rape, murder, and other crimes in which a sample of
DNA was obtained at the scene of the crime is now a
routine procedure.
DNA sequences can be amplified by the
chain reaction (PCR)
technique. Only an infinitesimal
amount of DNA is needed, e.g., a sample of DNA from a
single hair follicle or sperm. The DNA to be amplified is
mixed with three other components in an automated PCR
1. A heat-stable DNA polymerase that is isolated from a
thermophilic bacterium,
2. An excess of two short primer DNAs that are
complementary to opposite strands of the DNA
fragment that is to be amplified, and
3. An excess of deoxyribonucleotide triphosphates.
PCR consists of repeated cycling of three reactions:
dénaturation of the DNA by heating, reannealing of the
primers with the target DNA by cooling, and synthesis of
new DNA strands. The sequence of reactions is automati-
cally repeated at defined intervals to yield an exponential
increase in the amount of DNA. Twenty cycles of PCR
amplify DNA by about a factor of 10
and 30 cycles by
1 0
PCR amplification of DNA is one of the most widely
used techniques in medical research and diagnostics. PCR
is used in forensic pathology (to identify human remains),
in rapid identification of infectious microorganisms, in di-
agnosis of inherited diseases, and in archaeology and an-
thropology where small DNA samples can be recovered.
Sequencing DNA
Until the development of automated DNA sequencing ma-
chines in the 1990s, two techniques were used to sequence
the bases in a segment of DNA. Each of the techniques in-
volves the isolation of a restriction fragment containing
a few hundred or a few thousand base pairs. The DNA
is denatured and each strand is sequenced separately so
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