Transcript Power Point Notes

Control over Genes
Chapter 15
15.1 Control Mechanisms
Which genes are expressed in a
cell depends upon:
• 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
Control Mechanisms
• Negative control
– Regulatory proteins slow down
or curtail gene activity
• Positive control
– Regulatory proteins promote or
enhance gene activities
Chemical Modifications
• Methylation of
DNA can
inactivate genes
• Acetylation of
histones allows
DNA unpacking
and transcription
Figure 15.2
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15.2 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
operator
regulatory
gene
transcription,
translation
operator
gene 1
gene 2
gene 3
promoter
lactose operon
repressor protein
Figure 15.3a
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Low Lactose
• Repressor binds to operator
• Binding blocks promoter
• Transcription is blocked
Figure 15.3b
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High Lactose
allolactose
lactose
mRNA
operator
promoter
operator
RNA
polymerase
gene 1
Figure 15.3c
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CAP Exerts Positive Control
• CAP is an activator protein
• Adheres to promoter only when in
complex with cAMP
• Level of cAMP depends on level of
glucose
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
15.3 Controls in Eukaryotic Cells
• Control of transcription
• Transcript processing controls
• Controls over translation
• Controls following translation
14.4 Types of Control
Mechanisms
• Cells of a multicelled organism rarely
use more than 5-10 percent of their
genes at any given time
• The remaining genes are selectively
expressed
Homeotic Genes
• Occur in all eukaryotes
• Master genes that control development
of body parts
• Encode homeodomains (regulatory
proteins)
• Homeobox sequence can bind to
promoters and enhancers
X Chromosome
Inactivation
• One X inactivated in
each cell of female
• Creates a “mosaic”
for X chromosomes
• Governed by XIST
gene
Figure 15.6
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15.5 Signaling molecules
• Hormones
• Stimulate or inhibit activity in target cells
• Mechanism of action varies
– May bind to cell surface
– May enter cell and bind to regulatory
proteins
– May bind with enhancers in DNA
Polytene Chromosomes
• Occur in salivary
glands of midge
larvae
• Consist of multiple
DNA molecules
• Can produce
multiple copies of
transcripts
Figure 15.8
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Chromosome Puff
• Portion of the 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
Vertebrate Hormones
• Some have widespread effects
– Somatotropin (growth hormone)
• Others signal only certain cells at
certain times
– Prolactin stimulates milk
production
Phytochrome
• Signaling molecule in plants
• Activated by red wavelengths,
inactivated by far-red wavelengths
• Changes in phytochrome activity
influence transcription of certain genes
Controlling the Cell Cycle
• Cycle has built-in checkpoints
• Proteins monitor chromosome structure,
whether conditions favor division, etc.
• Proteins are products of checkpoint
genes
• Kinases
• Growth factors
Oncogenes
• Have potential to induce cancer
• Mutated forms of normal genes
• Can form following insertions of viral
DNA into DNA or after carcinogens
change the DNA
Cancer Characteristics
• Plasma membrane and cytoplasm
altered
• Cells grow and divide abnormally
• Weakened capacity for adhesion
• Lethal unless eradicated
Apoptosis
• Programmed cell death
• Signals unleash molecular weapons of
self-destruction
• Cancer cells do not commit suicide on
cue