Biology (Holt)

Enhancer

Activator RNA polymerase Transcription

factor

Enhancer

Promoter

Promoter

Transcription begins

Coding region of gene

Coding region of gene

DNA

Protein Synthesis in Eukaryotes

Eukaryotic cells contain much more DNA than prokaryotic cells do.
Like prokaryotic cells, eukaryotic cells must continually turn certain
genes on and off in response to signals from their environment.
Operons have not been found often in eukaryotic cells. Instead, genes
with related functions are often scattered on different chromosomes.
Because a nuclear envelope physically separates transcription
from translation in a eukaryotic cell, more opportunities exist for
regulating gene expression. For example, gene regulation can occur
before, during, and after transcription. Gene regulation can also
occur after mRNA leaves the nucleus or after translation, when the
protein is functional.

Controlling the Onset of Transcription

Most gene regulation in eukaryotes controls the onset of transcrip-
tion—when RNA polymerase binds to a gene. Like prokaryotes,
eukaryotic cells use regulatory proteins. But many more proteins are
involved in eukaryotes, and the interactions are more complex. These
regulatory proteins in eukaryotes are called transcription factors.
As shown in Figure 7,transcription factors help arrange RNA
polymerases in the correct position on the promoter. A gene can be
influenced by many different transcription factors.
An enhancer is a sequence of DNA that can be bound by a tran-
scription factor. Enhancers typically are located thousands of
nucleotide bases away from the promoter. A loop in the DNA may bring
the enhancer and its attached transcription factor (called an activator)
into contact with the transcription factors and RNA polymerase at
the promoter. As shown in Figure 7, transcription factors bound to
enhancers can activate transcription factors bound to promoters.

SECTION 2Gene Regulation and Structure 217

Organizing Information

Make a table to organize

information about the regu-

lation of protein synthesis.

Across the top write the

headings Prokaryotesand

Eukaryotes.Along the sides

write Protein(s) that regu-

late(s) the genes and

Details of regulation.Add

information to the table as

you read Section 2.

Transcription factors bind to the enhancer and to the RNA polymerase. The
binding activates transcription factors bound to the promoter.

Figure 7 Controlling transcription in eukaryotes