Biology 12

276 MHR • Unit 3 Molecular Genetics

more likely to bind to the operator and block

transcription. The tryptophan molecule thus

acts as a co-repressorto increase the affinity of

the repressor for the operator.

A prokaryote is a single-celled organism with a

relatively small genome and a short life span. Almost

all of its hereditary material is likely to be expressed

at some point during its life cycle. Therefore, gene

regulation in prokaryotes relies primarily on

mechanisms that can selectively turn off genes that

are not required under particular circumstances. In

contrast, eukaryotes may be multi-cellular organisms

with many specialized tissues. Most of the hereditary

material contained in any given eukaryotic cell is

therefore not expressed during the life of that cell.

As a result, eukaryotic cells are more dependent on

mechanisms that keep gene expression turned off

most of the time, and that turn on selected genes

only as they are needed.

Gene Expression in Eukaryotes

Figure 8.32 on the next page summarizes the

main steps in the protein synthesis pathway in

eukaryotes. These steps or control points (listed

below) offer more opportunities for control than

those available to prokaryotes.

pre-transcriptional control The cell controls the

extent to which DNA is exposed to transcription

enzymes, thus regulating the DNA’s availability

for transcription.

transcriptional control The cell controls whether

or not exposed DNA is transcribed into pre-mRNA.

post-transcriptional control The cell controls

the rate of processing of pre-mRNA into finished

mRNA.

translational control The cell manufactures

the mRNA, but then controls its transport to

ribosomes in the cytoplasm.

post-translational control The cell manufactures

the polypeptide product, but then modifies it

chemically or varies the rate at which it becomes

a functional protein. Another possibility is that

the cell may break the polypeptide product down

before it becomes a functional protein.

Like a prokaryotic cell, a eukaryotic cell can

save energy if regulation of gene expression occurs

sooner rather than later in the sequence of events

leading to protein synthesis. Nevertheless, a

eukaryotic cell is more likely to combine regulation

at the pre-transcriptional or transcriptional steps

with regulation at later steps. One reason for this

approach lies with the greater stability of mRNA

in eukaryotes. In prokaryotes, mRNA is typically

degraded within a few minutes after it is synthesized.

This is necessary because a gene that is expressed

will continue to be transcribed until it is turned off

by a regulatory mechanism. In contrast, mRNA in

eukaryotes tends to remain intact for hours or even

days, and it can be stored in the cell to be translated

at a later point. This means that a eukaryotic cell

can manufacture mRNA and then prevent it from

being translated until its protein product is required.

The following pages cover eukaryotic cell regulatory

mechanisms through the protein synthesis pathway.

Figure 8.31Co-repression in the E. coli trypoperon

tryp operon

repressor protein

binds to operator

P O

RNA polymerase

promoter and

operator sequences

tryp R gene

codes for

repressor

protein

tryp R

tryp D tryp B

tryp E tryp C tryp A

P O

tryp D tryp B

tryp E tryp C tryp A

repressor unbound

to operator

tryptophan co-repressor

binds to repressor

When tryptophan levels are low, the

repressor protein is present but does

not usually bind to the operator. RNA

polymerase is thus able to bind to the

promoter, so transcription takes place.

A

When tryptophan levels are high, tryptophan

binds to the repressor protein. As a result,

the repressor binds more readily to the

operator. When the repressor is bound to

the operator site, RNA polymerase cannot

bind and transcription will not proceed.

B