Chapter 13 Genetic Engineering
For thousands of years, people have bred animals and plants that
have desired traits. This technique is called selective breeding.
Humans use selective breeding, which takes advantage of
naturally occurring genetic variation, to pass desired traits
on to the next generation of organisms.
• Hybridization is a tool used by selective breeders. In
hybridization, individuals with different traits are crossed.
The goal is to produce offspring that have the best traits of
both parents. These offspring, called hybrids, are often
hardier than the parents.
• Breeders use inbreeding to maintain a group of plants
or animals with desired traits. In inbreeding, individuals
with similar traits are crossed. A disadvantage of inbreeding
is the risk of bringing together two recessive alleles for a
genetic defect.
Selective breeding would be nearly impossible without many
variations in traits. Breeders can increase the genetic variation
in a population by inducing mutations. Mutations are inheritable
changes in DNA. Mutations do occur naturally. However, breeders
can boost the rate of mutation through use of radiation and chemicals.
Many mutations are harmful. However, with luck, breeders
can produce useful mutations.

13–2 Manipulating DNA
To increase variation, scientists can make changes directly to DNA.
Genetic engineering is the intentional changing of an organism’s
DNA. Scientist use their knowledge of the structure of DNA and
its chemical properties to study and change DNA molecules.
• Extracting DNA. Scientists can extract, or separate, DNA
from the other cell parts using a chemical procedure.
• Cutting DNA. Scientists can cut DNA into smaller pieces
using restriction enzymes.
• Separating DNA. Scientists use gel electrophoresis, a
method in which DNA fragments are put at one end of a
porous gel. When an electric current is applied to the gel,
DNAmolecules move toward the positive end of the
gel. This technique allows scientists to compare the gene
composition of different organisms or different individuals.
Summary

Scientists also use different techniques to read, change, and
copy the DNA sequence.
• Scientists can read the order of nucleotide bases in a DNA
fragment. They make a copy of a single strand of DNA with
colored nucleotides inserted at random places. Reading the
order of colored bands in a gel gives the nucleotide sequence
of the DNA fragment.
• Scientists can change DNA sequences. Short sequences
of DNA made in the laboratory can be joined to the DNA
molecule of an organism. DNA from one organism can
be attached to the DNA of another organism. These DNA
molecules are called recombinant DNA because combining
DNAfrom different sources makes them.
• Scientists often need many copies of a certain gene to study
it. A technique called polymerase chain reaction (PCR)
allows scientists to do that. PCR is a chain reaction in which
DNAcopies become templates to make more DNA copies.

13–3 Cell Transformation
DNAfragments do not work by themselves. They must be part of
the DNA molecule in an organism. During transformation, a cell
takes in DNA from outside the cell. This external DNA becomes
a component of the cell’s DNA. Bacterial, plant, and animal cells
can be transformed.
To add DNA fragments to bacteria, the fragment is placed
in a plasmid. A plasmid is a small, circular DNA molecule that
occurs naturally in some bacteria. These plasmids with added
DNAfragments are recombinant DNA. The plasmids are mixed
in a solution with other bacteria. Some of the bacteria take up the
plasmids. These bacteria are transformed.
Plant cells can be transformed in several ways.
• Some plant cells in culture can take up DNA on their own.
These plant cells have had their cell walls removed.
• Scientists can also insert a DNA fragment into a plasmid.
This plasmid is transformed into a bacterium that
infects plants.
• Scientists can also inject DNA directly into some plant cells.
If transformation is successful, the recombinant DNA is integrated
into one of the chromosomes of the cell.
Animal cells can be transformed in ways similar to plant cells.
An egg cell may be large enough to inject DNA directly into its
nucleus. Once inside, the repair enzymes may help insert the DNA
fragment into the chromosomes of the egg.

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13–4 Applications of Genetic Engineering
Scientists wondered if genes from one organism could work in a
different organism. Some scientists isolated the gene from fireflies
that allows them to glow. Then they inserted this gene into the
DNAof plants. The plants glowed in the dark. This showed that
both plants and animals use the same process to translate DNA
into proteins. The glowing plant is transgenic because it has a
gene from another species.
Genetic engineering has spurred the growth of biotechnology,
which is changing the way we interact with the living world.
Some examples of genetic engineering include:
• Human genes have been added to bacteria. These transgenic
bacteria are used to make human proteins such as insulin,
human growth hormone, and clotting factor.
• Scientists have made transgenic animals to study the role of
genes and to improve the food supply. Transgenic animals
may be used to supply us with human proteins that can be
collected in the animal’s milk.
• Transgenic plants that can make their own insecticide
have been formed. Others are resistant to weed killers.
Some have been engineered to contain vitamins needed
for human health.
A clone is a member of a population of genetically identical
cells that were produced from a single cell. Clones are useful
in making copies of transgenic organisms. It is easy to produce
cloned bacteria and plants. Animals are difficult to clone.
However, in the 1990s, scientists in Scotland successfully cloned
a sheep. Animal cloning has risks. Studies suggest that cloned
animals may have genetic defects and other health problems.
The use of cloning also raises serious ethical and moral issues.
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