C h a p t e r 5
Genetic Control of Cell Function and Inheritance
103
genes. However, several genetic disorders are not due
to missing genes, but to faulty gene activity. With this
in mind, some scientists are approaching the problem
by using
RNA interference
(RNAi) to stop genes from
making unwanted disease proteins. RNA interference is
a naturally occurring process in which small pieces of
double-stranded RNA (small interfering RNA [siRNA])
suppress gene expression. Scientists believe that RNAi
may have originated as a defense against viral infections
and potentially harmful genomic invaders. In viral infec-
tions, RNAi would serve to control the infection by pre-
venting the synthesis of viral proteins.
With the continued refinement of techniques to
silence genes, RNAi has already had a major impact on
molecular biology. For example, it has given scientists
the ability to practice reverse genomics, in which a gene’s
function can be inferred through silencing its expres-
sion. Increasingly, pharmacologic companies are using
RNAi to identify disease-related drug targets. There
also is considerable interest in harnessing RNAi for
therapeutic purposes, including the treatment of human
immunodeficiency virus (HIV) infection and hepatitis C.
Before this can occur, however, the therapeutic methods
must be shown to be safe and effective, and obstacles to
delivering RNAi into targeted cells must be overcome. It
is difficult for RNA to cross the cell membrane, and it is
quickly broken down by enzymes in the blood.
according to size by electrophoresis. The single-stranded
DNA is then transferred to nitrocellulose paper, baked
to attach the DNA to the paper, and treated with a series
of radioactive probes. After the radioactive probes have
been allowed to bond with the denatured DNA, radiog-
raphy is used to reveal the labeled DNA fragments.
When used in forensic pathology, this procedure is
applied to specimens from the suspect and the forensic
specimen. Banding patterns are then analyzed to see if they
match. With conventional methods of analysis of blood
and serum enzymes, a 1 in 100 to 1000 chance exists that
the two specimens match because of chance. With DNA
fingerprinting, these odds are 1 in 100,000 to 1 million.
When necessary, the polymerase chain reaction
(PCR) can be used to amplify specific segments of DNA
(see Chapter 14). It is particularly suited for amplifying
regions of DNA for clinical and forensic testing proce-
dures because only a small sample of DNA is required as
the starting material. Regions of DNA can be amplified
from a single hair or drop of blood or saliva.
GeneTherapy
Although quite different from inserting genetic mate-
rial into a unicellular organism such as bacteria, tech-
niques are available for inserting genes into the genome
of intact multicellular plants and animals. The adeno-
viruses are promising delivery vehicles for these genes.
These viruses are ideal vehicles because their DNA does
not become integrated into the host genome; however,
repeated inoculations are often needed because the
body’s immune system usually targets cells expressing
adenovirus proteins. Sterically stable liposomes also
show promise as DNA delivery mechanisms. This type
of therapy is one of the more promising methods for
the treatment of genetic disorders such as cystic fibrosis,
certain cancers, and a number of infectious diseases.
Two main approaches are used in gene therapy:
transferred genes can replace defective genes or they
can selectively inhibit deleterious genes. Cloned DNA
sequences are usually the compounds used in gene ther-
apy. However, the introduction of the cloned gene into
the multicellular organism can influence only the few
cells that get the gene. An answer to this problem would
be the insertion of the gene into a sperm or ovum; after
fertilization, the gene would be replicated in all of the
differentiating cell types. Even so, techniques for cell
insertion are limited. Not only are moral and ethical
issues involved, but these techniques cannot direct the
inserted DNA to attach to a particular chromosome or
supplant an existing gene by knocking it out of its place.
To date, gene therapy has been used successfully to
treat children with severe combined immunodeficiency
disease (see Chapter 16) and in a suicide gene transfer
to facilitate treatment of graft-versus-host disease after
donor lymphocyte infusion.
RNA InterferenceTechnology
One method of gene therapy focuses on the previ-
ously described replacement of missing or defective
SUMMARY CONCEPTS
■■
The genome is the gene complement of an
organism. Genomic mapping is a method used to
assign genes to particular chromosomes or parts of
a chromosome.The most important methods used
are family linkage studies, gene dosage methods,
and hybridization studies. Linkage studies assign
a chromosome location to genes based on their
close association with other genes of known
location. Dosage studies involve measuring
enzyme activity to determine if both members of
a gene pair are present and functioning normally.
Hybridization studies involve the fusion of human
somatic cells with those of a different species to
study gene location on chromosomes.
■■
Recombinant DNA studies involve the
extraction of specific types of mRNA used in
the synthesis of complementary DNA strands.
The complementary DNA strands, labeled with a
radioisotope, bind with the genes for which they
are complementary and are used as gene probes.
■■
Genetic engineering has provided the methods
for manipulating nucleic acids and recombining
genes (recombinant DNA) into hybrid molecules
that can be inserted into unicellular organisms
(continued)
