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NEL Molecular Genetics 683

Each PCR cycle doubles the number of DNA molecules. After just 10 cycles there are

210 (over two million) copies of the DNA template. Since scientists can use PCR to syn-

thesize many identical copies from a very small sample of DNA, this technology has led

to many advances in medicine, evolutionary biology, genetic engineering, and forensic

science. Mullis was awarded the Nobel Prize in Chemistry in 1993 for his invention.

Section20.3

vectora vehicle by which foreign

DNA may be introduced into a cell

transgenica cell or an organism

that is transformed by DNA from

another species

plasmida small double-stranded

circular DNA molecule found in

some bacteria

Figure 8

Chromosomal and plasmid

DNA coexist in many bacteria.

plasmid DNA

(several thousand

base pairs each)

circular chromosomal

DNA (4 million base

pairs)

bacterial

cell

Purpose Design Analysis

Problem Materials Evaluation

Hypothesis Procedure Synthesis

Prediction Evidence

To perform this investigation, turn to page 696.

Restriction Enzyme Digestion of

Bacteriophage DNA

Using restriction enzymes and electrophoresis, molecular
biologists are able to excise and isolate target sequences from
DNA. How would the banding patterns compare if the same
fragment of DNA were digested with different restriction
enzymes? In this investigation, you will conduct electrophoresis of

bacteriophage DNA that has been digested with restriction

enzymes.

INVESTIGATION 20.2Introduction Report Checklist

Transformation

So far, you have seen that mapping and sequencing can be used to identify the relative

position and nucleotide sequence of genes in a DNA molecule. Using various enzymes,

scientists can isolate DNA fragments containing a gene or genes. Multiple copies of the

fragment can be prepared using PCR. The DNA fragment may also be joined (annealed)

to other DNA fragments.

In genetics, transformation is any process by which foreign DNA is incorporated into

the genome of a cell. A vectoris the delivery system used to move the foreign DNA into

a cell. The specific vector used for transformation is chosen based on the size and sequence

of the foreign DNA fragment, the characteristics of the cells to be transformed, and the

goal of the transformation. The goal of most genetic transformation is to express the

gene product(s), and so change the traits of the organism that receives the foreign DNA.

An organism with foreign DNA in its genome is said to be transgenic.

Transformation of Bacteria

Bacteria are the most common organisms that are transformed by molecular biologists.

Transgenic bacteria may be used to study gene expression or gene function, to create

and maintain a stock of a particular DNA fragment, or to synthesize a useful gene

product. For example, transgenic bacteria have been engineered to produce human

growth hormone, used in the treatment of pituitary dwarfism.

The first stage of transformation for any organism is to identify and isolate the DNA

fragment that is to be transferred. The DNA fragment is then introduced into the vector.

Plasmids are small, circular, double-stranded DNA molecules that occur naturally in the

cytoplasm of many bacteria (Figure 8). Plasmids are commonly used as vectors for bac-

terial transformation. A plasmid contains genes, and it is replicated and expressed inde-

pendently of the large bacterial chromosome. There can be many copies of a plasmid in

a single bacteria cell and, under certain conditions, plasmids can pass through the cell

membrane.

Figure 9, on the next page, is a diagram of the basic steps in producing transgenic

bacteria. First, both the plasmid vector and the DNA containing the desired sequence are

cut by the same restriction enzyme(s). In this example, both DNA molecules are cut by

EcoRI, generating sticky ends. The cut plasmid and DNA fragment are then mixed

together and incubated with DNA ligase. This produces recombinant plasmids that