Biology (Holt)

Operator

1

2 3

Promoter

Genes Involved in lactose use

RNA polymerase

Lactose absent—the lac operon is off.

Lactose present—the lac operon is on.

Repressor protein

Operator

1

2 3

Promoter

Lactose bound to repressor protein Transcription proceeds

Lactose

Protein Synthesis in Prokaryotes

Although prokaryotic organisms, such as bacteria, might seem simple

because of their small size, prokaryotic cells typically have about

2,000 genes. The human genome, which is the largest genome

sequenced to date, has about 30,000 genes. Not all of the genes, how-

ever, are transcribed and translated all of the time; this would waste

the cell’s energy and materials. Both prokaryotic and eukaryotic cells

are able to regulate which genes are expressed and which are not,

depending on the cell’s needs.

An example of gene regulation that is well understood in prokary-

otes is found in the bacterium Escherichia coli. When you eat or drink

a dairy product, the disaccharide lactose (“milk sugar”) reaches the

intestinal tract and becomes available to the E. coli living there. The

bacteria can absorb the lactose and break it down for energy or for

making other compounds. In E. coli, recognizing, consuming, and

breaking down lactose into its two components, glucose and galac-

tose, requires three different enzymes, each of which is coded for by

a different gene.

As shown in Figure 6,the three lactose-metabolizing genes are

located next to each other and are controlled by the same promoter

site. There is an on-off switch that “turns on” (transcribes and then

translates) the three genes when lactose is available and “turns off”

the genes when lactose is not available.

Gene Regulation

and Structure

SECTION 2Gene Regulation and Structure 215

Section 2

Objectives

Describehow the lac

operon is turned on

or off.

Summarizethe role of

transcription factors in

regulating eukaryotic gene

expression.

Describehow

eukaryotic genes are

organized.

Evaluatethree ways

that point mutations

can alter genetic

material.

Key Terms

operator

operon

lacoperon

repressor

intron

exon

point mutation

The lacoperon allows a bacterium to build the proteins needed for lactose

metabolism only when lactose is present.

Figure 6 Turning prokaryotic genes on and off

4B 6B

4A 6A

6A 6B

4B 6A 6C