Download Lecture 4 – Gene Expression Control and Regulation

Regulation and Control of Gene
Expression
15.1 Gene Expression in Eukaryotic Cells
 Gene controls govern the kinds and amounts of
substances in a cell at any given interval
 Various control processes regulate all steps
between gene and gene product
Which Genes Get Tapped?
 Differentiation
• The process by which cells become specialized
• In multicelled organisms, most cells differentiate
when they start expressing a unique subset of
their genes
• Which genes are expressed depends on the type
of organism, its stage of development, and
environmental conditions
Control of Transcription
 Transcription factors
• Regulatory proteins that affect the rate of
transcription by binding to special nucleotide
sequences in DNA
• Activators speed up transcription when bound to
a promoter; or may bind to distant enhancers
• Repressors slow or stop transcription
Control of Transcription
 Chemical modifications and chromosome
duplications affect RNA polymerase’s access to
genes
• Interactions between DNA and histone proteins
(methylation) prevent transcription
• Polytene chromosomes (many copies) increase
transcription rates in some organisms
Controls of mRNA Transcripts
 mRNA processing
• DNA splicing controls products of translation
 mRNA transport controls delivery of transcripts
• Passage through nuclear pores
• Delivery within cytoplasm (mRNA localization)
Translational Controls
 Controls over molecules involved in translation
 Controls over mRNA stability
• Depends on base sequence, length of poly-A tail,
and which proteins are attached to it
 RNA interference
• Expression of a microRNA complementary to a
gene inhibits expression of the gene
Post-Translational Modification
 Post-translational modification can inhibit,
activate, or stabilize many molecules, including
enzymes that participate in transcription and
translocation
Points of Control over
Eukaryotic Gene Expression
DNA
NUCLEUS
A Transcription
Binding of transcription factors to special sequences in DNA slows or
speeds transcription. Chemical modifications and chromosome
duplications affect RNA polymerase’s physical access to genes.
new RNA
transcript
B mRNA Processing
New mRNA cannot leave the nucleus before being modified, so controls
over mRNA processing affect the timing of transcription. Controls over
alternative splicing influence the final form of the protein.
mRNA
C mRNA Transport
RNA cannot pass through a nuclear pore unless bound to certain
proteins. Transport protein binding affects where the transcript will be
delivered in the cell.
CYTOPLASM
mRNA
D Translation
An mRNA’s stability influences how long it is translated. Proteins that
attach to ribosomes or initiation factors can inhibit translation. Doublestranded RNA triggers degradation of complementary mRNA.
polypeptide
chain
active
protein
E Protein Processing
A new protein molecule may become activated or disabled by enzymemediated modifications, such as phosphorylation or cleavage. Controls
over these enzymes influence many other cell activities.
Stepped Art
Fig. 15-2, p. 230
15.1 Key Concepts: Overview of
Controls Over Gene Expression
 A variety of molecules and processes alter gene
expression in response to changing conditions
both inside and outside the cell
 Selective gene expression also results in cell
differentiation, by which different cell lineages
become specialized
15.2 A Few Outcomes
of Eukaryotic Gene Controls
 Selective gene expression can give rise to
visible traits
X Chromosome Inactivation
 X chromosome inactivation
• In cells of female mammals, either the maternal
or paternal X chromosome is randomly
condensed (Barr body) and is inactive
• Occurs in an early embryonic stage, so that all
descendents of that particular cell have the same
inactive X chromosome, resulting in “mosaic”
gene expression
X Chromosome Inactivation
Calico: Mosaic Gene Expression
in a Female Mammal
Prokaryotic Gene Control
 In prokaryotes, genes that are used together
often occur together on chromosomes
 Operon
• A promoter and one or more operators that
collectively control transcription of multiple genes
 Operators
• DNA regions that are binding sites for a repressor
The Lactose Operon
 E. coli digest lactose in guts of mammals using a
set of three enzymes controlled by two operators
and a single promoter (the lac operon)
• When lactose is not present, repressors bind to
the operators and inactivate the promoter;
transcription does not proceed
• When lactose is present, allolactose binds to the
repressors; repressors don’t bind to operators to
inactivate the promoter; transcription proceeds
The Lactose Operon Repressor
Lactose Intolerance
 Human infants and other mammals produce the
enzyme lactase, which digests the lactose in
milk – adults tend to lose the ability to produce
lactase, and become lactose intolerant