Download Chapter 15

Control over Genes
Chapter 15
Skin Cancer
Basal Cell Carcinoma
Squamous Cell Carcinoma
Malignant Melanoma
Changes in DNA
Trigger Cancer
Ultraviolet
radiation can
cause breaks
In-text figure
Page 238
Gene Control
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
Page 240
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
Page 241
Low Lactose
Repressor binds to
operator
Binding blocks promoter
Transcription is blocked
Figure 15.3b
Page 241
High Lactose
allolactose
lactose
mRNA
operator
promoter
operator
RNA
polymerase
gene 1
Figure 15.3c
Page 241
Lac-Operon Analogy
Like an elephant that likes to sit on a
railroad track blocking the train.
The only thing that the elephant likes
better than to sit on his spot on the track
is peanuts.
If peanuts are available, the elephant
leaves the track and eats the peanuts.
While he is gone the train can proceed
down the track until the elephant goes
back to sit on the track after eating all the
peanuts.
Controls in Eukaryotic Cells
Control of transcription
Transcript processing controls
Controls over translation
Controls following translation
Most Genes Are Turned Off
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
Page 245
Hormones
Signaling molecules
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
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