5 Steps to a 5 AP Biology, 2014-2015 Edition

Molecular Genetics ❮ 125

which tRNA binds next, and that tRNA molecule sits in the A site of the ribosome. An

enzyme helps a peptide bond form between the amino acid on the A site tRNA and the

amino acid on the P site tRNA. After this happens, the amino acid from the P site moves

to the A site, setting the stage for the tRNA in the P site to leave the ribosome. Now a step

called translocation occurs. During this step, the ribosome moves along the mRNA in such

a way that the A site becomes the P site and the next tRNA comes into the new A site car-

rying the next amino acid. This process continues until the stop codon is reached, causing

the completed protein to leave the ribosome.

Gene Expression

Let’s cover some vocabulary before diving into this section:

Promoter region:a base sequence that signals the start site for gene transcription; this is

where RNA polymerase binds to begin the process.

Operator:a short sequence near the promoter that assists in transcription by interacting

with regulatory proteins (transcription factors).

Operon:a promoter/operator pair that services multiple genes; the lac operonis a well-

known example (Figure 11.7).

Repressor:protein that prevents the binding of RNA polymerase to the promoter site.

Enhancer:DNA region, also known as a “regulator,” that is located thousands of bases

away from the promoter; it influences transcription by interacting with specific transcrip-

tion factors.

Inducer:a molecule that binds to and inactivates a repressor (e.g., lactose for the lac operon).

The control of gene expression is vital to the proper and efficient functioning of an

organism. In bacteria, operons are a major method of gene expression control. The lactose

operon services a series of three genes involved in the process of lactose metabolism. This

contains the genes that help the bacteria digest lactose. It makes sense for bacteria to pro-

duce these genes only if lactose is present. Otherwise, why waste the energy on unneeded

enzymes? This is where operons come into play—in the absence of lactose, a repressor binds

to the promoter region and prevents transcription from occurring. When lactose is present,

there is a binding site on the repressor where lactose attaches, causing the repressor to let

go of the promoter region. RNA polymerase is then free to bind to that site and initiate

transcription of the genes. When the lactose is gone, the repressor again becomes free to

bind to the promoter, halting the process.

Figure 11.7 General layout of an operon.

Enhancer Operator Promoter Gene 1 Gene 2 Gene 3

Inducer RNA polymerase

Repressor

CT teacher:

“Be able to write

about operons.”

BIG IDEA 4.C.2
Environmental fac-
tors (e.g., lactose in
a bacterial culture)
can influence gene
expression.

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