Transcript You Light Up My Life

Controls Over Genes
Chapter 14
Gene Control
Which genes are being expressed in a
cell depends upon:
• The type of cell
• Internal chemical conditions
• External signals
• Built-in control systems
Mechanisms of Gene Control
Controls related to transcription
Transcript-processing controls
Controls over translation
Post-translation controls
Regulatory Proteins
Can exert control over gene
expression through interactions with:
– DNA
– RNA
– New polypeptide chains
– Final proteins
Activator and Repressor
Proteins
• Positive Control System
– Activator protein enhances
some activity
• Negative Control System
– Repressor protein inhibits
some activity
Some Controls Affect
Noncoding DNA
• Promoter signals beginning
of gene
• Enhancer is a binding site for
activator protein
Chemical Modifications
• Methylation of
DNA can
inactivate genes
• Acetylation of
histones allows
DNA unpacking
and transcription
Gene Control in Prokaryotes
• No nucleus separates DNA from
ribosomes in cytoplasm
• When nutrient supply is high,
transcription is fast
• Translation occurs even before
mRNA transcripts are finished
The Lactose Operon
Negative Control –
Low Lactose
b In the absence of lactose, the repressor
binds to two operators in DNA. It makes
the DNA loop out in a way that blocks
operon gene transcription; it stops RNA
polymerase from binding to its promoter.
Negative Control – High Lactose
allolactose
translation into polypeptide
chains for the three enzymes
lactose
mRNA
RNA polymerase
operator
promoter
operator
gene 1
c When lactose is present, some is converted to a form that binds to the repressor
and alters its shape. The altered repressor can’t bind to operators, so RNA
polymerase is free to transcribe the operon genes.
Positive Control of
Lactose Operon
• CAP is an activator protein
• CAP affects promoter
• CAP will adhere to promoter only
when in complex with cAMP; when
glucose levels are high
Positive Control –
High Glucose
• There is little cAMP
• CAP cannot be activated
• The promoter is not good at binding
RNA polymerase
• The lactose-metabolizing genes are
not transcribed very much
Positive Control –
Low Glucose
• cAMP accumulates
• CAP-cAMP complex forms
• Complex binds to promoter
• RNA polymerase can now bind
• The lactose-metabolizing genes are
transcribed rapidly
Cell Differentiation
• All cells in a multicellular organism
inherited the same genes
• Some of those genes are used in all cell
types
• During development, cells activate and
suppress other genes in selective ways
Controls of Eukaryotic
Gene Expression
NUCLEUS
DNA
pre-mRNA
transcript
transcription
control
CTYOPLASM
translational
control
mRNA
transport
processing
control
mRNA
mRNA
transport
control
mRNA
degradation
control
inactivated
mRNA
protein
product
protein
product
control
inactivated
protein
Most Genes Are Turned Off
• Cells of multicelled organisms rarely
use more than 5-10 percent of their
genes at any given time
• The remaining genes are selectively
expressed
Polytene Chromosomes
• Occur in salivary glands of midge larvae
• Consist of multiple DNA molecules
• Can produce multiple copies of
transcripts
Chromosome Puff
• Portion of the polytene chromosome in
which the DNA has loosened up to
allow transcription
• Appears in response to ecdysone
• Translation of transcripts from puffed
region produces protein components
of saliva
Controls Over
Gene Expression
• Work at certain stage before, during,
and after transcription and translation
• Most controls over gene expression
occur at translation
Selective Gene Expression
• All differentiated cells in a complex,
multicelled body use most of their genes
in much the same way
• Each type also uses a fraction of those
genes in a unique, selective way
Homeotic Genes
• A class of master genes in most
eukaryotic organisms
• Transcribed in specific locations in the
developing embryo – products form in
local tissue regions
• Guide formation of organs
and limbs
X Chromosome Inactivation
• Mammalian females have two X
chromosomes per cell
• One X is inactivated in each cell
• Inactivation is random
• Female is a “mosaic”
Barr Body
• Condensed X chromosome
• Visible in micrographs
• May be either the maternal or
the paternal X chromosome
Clues to Gene Controls
• Research using
Drosophila
melanogaster has led to
remarkable discoveries
about how embryos
develop