HUMAN BIOLOGY

414 Chapter 21

genetic engineering
Insertion of purposely
modified genes into an
organism.

genome All the DNA in
one (haploid) set of a
species’ chromosomes.

PCR The polymerase chain
reaction.

recombinant DNA A DNA
molecule that contains DNA
from more than one species.

F A collection of

recombinant plasmids

containing foreign DNA.

A A selected restriction enzyme

cuts wherever a specific base

sequence occurs in a molecule

of chromosomal DNA or cDNA.

B The same enzyme cuts the

same sequence in plasmid DNA.

C DNA or cDNA

fragments with

sticky ends.

E The

foreign DNA,

the plasmid

DNA, and

modification

enzymes

are mixed

together.

G Host cells

able to divide

rapidly take up

recombinant

plasmids.

D Plasmid DNA

with sticky ends.

molecules of “extra” DNA called plasmids, which contain a

few genes. The bacterium’s replication enzymes can copy

plasmid DNA. Bacteria also have restriction enzymes—

enzymes that can detect and cut apart specific short

sequences of bases in DNA. Today, plasmids and restric-

tion enzymes are basic parts of a tool kit for doing genetic

recombination in the laboratory.

Many restriction enzymes make staggered cuts that

leave single-stranded “tails” on the end of DNA fragments.

Depending on the molecule being cut, the fragments may

be long enough to be useful for studying the organization

of a genome. A genome is all the DNA in a haploid set of

a species’ chromosomes.

DNA fragments with staggered cuts have so-called

sticky ends. This means that a restriction fragment’s single-

stranded tail can base-pair with a complementary tail of

any other DNA fragment or molecule cut by the same

restriction enzyme. If you mix together some DNA frag-

ments cut by the same restriction enzyme, the sticky ends

of fragments that have complementary base sequences will

base-pair and form a recombinant DNA molecule. Another

enzyme seals the nicks.

Using the necessary enzymes, it’s possible to splice

foreign DNA into bacterial plasmids. The result is called a

F i g u r e 21.13 Animated! Recombinant DNA combines DNA of

different species. Steps A–F show the formation of recombinant

DNA—in this case, DNA fragments from a chromosome (cDNA) that

are spliced into bacterial plasmids. G The recombinant plasmids are

inserted into host cells that can rapidly amplify the spliced-in DNA.

(© Cengage Learning)

n Researchers can use advanced technology to change

genetic traits.

n Link to Cell division 18.2

Today researchers routinely cut

and splice DNA from different spe-

cies, then insert the modified mol-

ecules into cells such as bacteria

that can replicate genetic material

and divide. The cells copy the for-

eign DNA along with their own.

Copying produces large quantities

of recombinant DNA molecules.

This recombinant DNA technology

is used for genetic engineering,

in which genes are altered and

inserted back into the original organism or into a differ-

ent one.

enzymes and plasmids

from bacteria are basic tools

Recombinant DNA technology depends on the genetic

workings of bacteria. Many bacteria have small circular

21.7 tools for engineering Genes

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