Transcript 13.4 Gene Expression

Gene Regulation and Expression
Learning Objectives
 Describe gene regulation in prokaryotes.
 Explain how most eukaryotic genes are regulated.
 Relate gene regulation to development in multicellular
organisms.
Prokaryotic Gene Regulation
DNA-binding proteins in prokaryotes regulate genes
by controlling transcription.
One of the keys to gene
transcription in bacteria is
the organization of genes
into operons.
The Lac Operon
When lactose is not
present, the lac genes are
turned off by regulatory
proteins that bind to DNA
and block transcription.
Promoters and Operators
Located in front of the operon’s three genes are two regulatory
regions:
 A promoter
 An operator
The Lac Repressor Blocks Transcription
When the lac repressor binds to the O region, RNA polymerase
cannot reach the lac genes to begin transcription.
Repressor
protein
Lactose Turns On the Operon
When lactose is added to the medium, it diffuses into the cell
and attaches to the lac repressor.
RNA
polymerase
Lactose
Repressor protein with
changed shape
Eukaryotic Gene Regulation
A typical eukaryotic gene has a TATA box.
Transcription Factors
By binding DNA sequences in the regulatory regions of
eukaryotic genes, transcription factors control the expression
of those genes.
Cell Specialization
Complex gene regulation in eukaryotes is what makes
differentiation and specialization possible.
RNA Interference
Small RNA molecules that do not belong to any of the major
groups of RNA play a powerful role in regulating gene
expression.
They do so by interfering with mRNA.
RNA Interference
The small interfering RNA molecules fold into double-stranded
hairpin loops. The dicer enzyme cuts the double strands into
microRNA (miRNA).
RNA Interference
The two strands of the loops separate.
One of the miRNA pieces attaches to a cluster of proteins,
forming a silencing complex.
RNA Interference
Blocking gene expression by means of an miRNA silencing
complex is known as RNA interference.
The Promise of RNAi Technology
The discovery of RNAi has made
it possible for researchers to
switch genes on and off at will,
simply by inserting doublestranded RNA into cells.
It may provide new ways to treat
and perhaps even cure diseases.
Genetic Control of Development
Regulating gene expression
is important in shaping how a
multicellular organism develops.
Each of the specialized cell types
found in the adult originates from
the same fertilized egg cell.
Homeotic, Homeobox, and Hox Genes
 Homeotic genes
regulate organ
development.
 Homeobox genes
code for transcription
factors.
 Hox genes determine
the identities of each
body segment.
Environmental Influences
Environmental factors can
affect gene regulation.
Metamorphosis is an
example of how organisms
can regulate gene
expression in response to
change in their environment.