HUMAN BIOLOGY

418 Chapter 21

DNA fingerprint A DNA
profile used in criminal
investigations.

DNA profile A set of
repeated DNA fragments
unique to each person.

gene therapy Inserting
one or more normal genes
into body cells to correct a
genetic defect or enhance
the activity of specific
genes.

Normal gene

Inject cells

into patient.

Retrovirus

Clone normal gene

into viral vector.

Infect patient’s white

blood cells with virus.

In some cells,

viral DNA inserts

into chromosome.

Viral nucleic acid

affected person’s bone marrow fail to make the immune sys-

tem’s infection-fighting lymphocytes (Section 9.1). Affected

children (sometimes called “bubble babies”) must live in

germ-free isolation tents. Some years ago, a virus was used

to insert copies of a normal allele into stem cells taken from

the bone marrow of eleven children with the disorder.

Then the genetically modified stem cells were infused back

into the marrow, which began to produce lymphocytes.

Seven of the treated children, including Rhys Evans, the

boy shown in Figure 21.20, eventually developed working

immune systems. Later on, however, two of the children

developed leukemia as a side effect of their treatment.

Cystic fibrosis has been another target. The idea is to

introduce normal copies of the defective CFTR gene into

cells of the respiratory system, using a viral vector in

a nasal spray. In the trials thus far, only about 5 percent of

affected cells have taken up the normal gene. The retrovirus

applying Biotechnology to human Concerns

F i g u r e 21.19 This diagram shows the basics of one gene therapy

method. Here, white blood cells are removed from a patient with a

genetic disorder. Next, a retrovirus is used as a vector to insert a

normal gene into the DNA. After the normal gene begins to produce

its protein, the cells are placed back into the patient. If enough of

the normal protein is present in the patient, the disorder’s symptoms

diminish. Because the genetically altered blood cells live but a few

months, however, the procedure must be repeated regularly.

n Practical applications of biotechnology are coming along

almost as fast as the leaps in our understanding of human

genetics. they include genetic fixes for diseases and using

DNA as a personal identifier.

n Link to inheritance of genes on autosomes 20.5

Researchers are exploring gene therapy

There are about 15,500 known human genetic disorders.

Some, such as cystic fibrosis and hemophilia, develop when

one single gene mutates, producing a malfunctioning pro-

tein. Many cancers also arise following changes in single

genes. Gene therapy aims to replace such mutated genes

with normal ones that will encode functional proteins, or to

insert genes that restore normal controls over gene activity.

Scien tists around the world are exploring these possibilities.

Genes can be inserted two ways

The size of a gene (how many base pairs it has) helps

determine how it might be inserted into a host cell. Smaller

genes can be carried into animal cells by a vector such as a

virus. Larger genes must enter a host cell some other way.

In transformation, cells grown in the laboratory are

exposed to DNA that contains a gene of interest, and some

of the foreign DNA may become integrated into the host

cell’s genome. Exposing the host cells to a weak electric

current seems to help. Even so, sometimes only one cell in

ten million takes up a new gene.

In transfection, a gene is inserted into a virus. The first

step is to remove from the virus its genetic instructions that

would allow it to replicate and cause

disease. In their place goes the gene

to be transferred. Next, the virus is

allowed to infect target cells (Fig-

ure  21.19). Once the virus is inside

the host cell, the desired foreign DNA

usually becomes integrated into the

host cell’s DNA.

Transfection can be used only

with genes that are expressed in the

tissues into which the new DNA will

be inserted. In addition, many intro-

duced genes turn off within a few

days or weeks. Several patients have died from complications

of the procedure, which raises serious safety questions.

Gene therapy results have been mixed

The first federally approved gene therapy test on humans

began in the early 1990s. It aimed to replace a defective

gene that causes one type of severe combined immune

deficiency, called SCID-X1. In this disorder, stem cells in the

21.10

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