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
lactose
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.
allolactosetranslation into polypeptidechains for the three enzymes
mRNA
RNA polymerase
operator operator gene 1promoter
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
DNApre-mRNAtranscript
mRNA
NUCLEUS
mRNA
CTYOPLASMtranslational
controlproteinproduct
proteinproductcontrol
inactivatedprotein
inactivatedmRNA
mRNAdegradation
control
mRNAtransportcontrol
transportprocessing
control
transcriptioncontrol
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 organsand 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