Biology (Holt)

Steps in a Genetic Engineering Experiment
Genetic engineering experiments use different approaches, but
most share four basic steps, as illustrated in Figure 2.

Step Cutting DNA The DNA from the organism containing the

gene of interest (in our example, the insulin gene) is cut by

restriction enzymes. are bacterial

enzymes that recognize and bind to specific short sequences

of DNA, and then cut the DNA between specific nucleotides

within the sequences. The DNA from a vector also is cut. A

is an agent that is used to carry the gene of interest

into another cell. Commonly used vectors include viruses,

yeast, and plasmids. ,

shown in Figure 2, are circular

DNA molecules that can repli-

cate independently of the main

chromosomes of bacteria.

Step Making recombinant DNA

The DNA fragments from the

organism containing the gene of

interest are combined with the

DNA fragments from the vector.

An enzyme called DNA ligase is

added to help bond the ends of

DNA fragments together. In our

example, human DNA frag-

ments are combined with plas-

mid DNA fragments. The host

cells then take up the recombi-

nant DNA.

Step CloningIn a process called

, many copies of

the gene of interest are made

each time the host cell repro-

duces. Recall from your reading

that bacteria reproduce by

binary fission, producing identi-

cal offspring. When a bacterial

cell replicates its DNA, its plas-

mid DNA also replicates.

Step Screening Cells that have

received the particular gene of

interest are distinguished, or

separated, from the cells that did

not take up the vector with the

gene of interest. The cells can

transcribe and translate the

gene of interest to make the

protein coded for in the gene.

gene cloning

Plasmids

vector

Restriction enzymes

BIO

graphic

Genetic Engineering

Many genetic engineering experiments

use one or more of these basic steps.

1 DNA is cut.

2 Recombinant DNA is produced.

3 The gene is cloned when bacteria are allowed to reproduce.

4

Human

chromosome

carrying

insulin gene

Human

insulin

gene

Insert into

bacteria

Bacterial cells

with the insulin

gene are later

isolated.

Cut with

restriction enzyme

Plasmid DNA

Bacterium

TTAAAATT

AATT TTAA

Cells undergo selection and then are screened.

SECTION 1Genetic Engineering 229

Figure 2