G E N E C O N T R O L I N P R O K A R Y O T E S
(Nelson Biology 12, pp. 255-256)

Introduction

Some intestinal bacteria (E. coli) utilize _____________ that you ingest in as an energy source. Since you don't drink milk 24 hours a day, it would be wasteful for the bacteria to produce ___________-degrading _____________ when lactose is not present.

Thus, bacteria need a way to control or regulate when the DNA that codes for lactose-degrading enzymes is active (being ____________________) and when it is not.

Lactose Operon Structure (Fig. 2, pg. 256)

An "operon" is a stretch of DNA with genes that are associated with one major function. The lactose operon (also called the ______ ____________) is a stretch of _________ that can be turned on or off to regulate production of lactose-degrading enzymes.

The operon itself includes 3 important regions:

___________________ sequence (binding site for ________ ___________________) to begin transcription.

___________________ (binding site for ___________________ protein).

_____________________________________ genes:

lacZ codes for _________________________________

lacY codes for _________________________________

lacA codes for _________________________________

Just upstream from the lactose operon is a ___________________ gene which codes for production of a ________________ protein (called the _______ protein) which turns the operon on or off (ie. allows or prevents DNA transcription).

How it Works: Lactose Absent

The _____________________ gene is constantly transcribed (ie. ____________ is produced) and translated producing a _____________ protein. The _________________ protein binds to the ___________________ region on the DNA immediately upstream from the __________________________ genes.

When it binds, the _____________________ protein blocks the access of _______ ______________________ enzyme to the __________________ sequence (RNA polymerase must bind here to begin transcription of the structural genes). Since the enzyme cannot bind to the ___________________, transcription of the ___________________ genes does not occur and the 3 enzymes for lactose degradation are not transcribed (hence, no translation either). So the enzymes aren't made and wasted when they aren't needed.

How it Works: Lactose Present

The _____________________ gene is still being transcribed and translated producing the ________________ protein.

Since _________________ is now present, some of the lactose binds to the __________________ protein causing a change in its __________________. The repressor is now unable to _____________ correctly to the __________________ region. Thus, ________ __________________ is able to bind to the _______________________ sequence.

When RNA polymerase binds to the ___________________ sequence on the DNA it transcribes the ___________________ genes. The mRNA is then translated on a _______________________ and the 3 lactose-degrading enzymes are produced.

Because the presence of lactose turned the operon on, lactose is called an ______________________ or __________________ and the enzymes were only made when they were needed.

Negative Versus Positive Control of DNA

This is an example of "Negative Control" of DNA because the binding of a _____________________ molecule (the ________________ protein) INHIBITS transcription (ie. has a "negative" effect).

In contrast, you will see that "Positive Control" occurs when binding of a _____________________ molecule STIMULATES transcription.