Transcript You Light Up My Life

PowerLecture:
Chapter 15
Controls over Genes
Impacts, Issues: Between You
and Eternity

Loss of gene controls can be disastrous

Some gene mutations, either inherited or spontaneously
mutated due to environmental factors, predispose
individuals to develop cancer

ERBB2, a type of membrane receptor, is encoded on
chromosome 17

This gene controls the cell cycle - overexpression or
mutation triggers cancerous transformations
Impacts, Issues: Between You
and Eternity
 BRCA1
and BRCA2 are tumor
suppressing proteins that fix damaged
DNA
 Breast
cancer cells often contain their
mutated forms
Changes in DNA
Trigger Cancer
 Ultraviolet
radiation can
cause breaks
 Can
promote
formation of
dimers
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
 Can
forms of normal genes
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
 Lethal
capacity for adhesion
unless eradicated
Apoptosis
 Programmed
 Signals
cell death
unleash molecular weapons of
self-destruction
 Cancer
cells do not commit suicide on cue
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
Control Mechanisms
 Promoters
 Enhancers
Chemical Modifications
 Methylation
of DNA
can inactivate
genes
 Acetylation of
histones allows
DNA unpacking
and transcription
Controls in Eukaryotic Cells
 Control
 Transcript
processing controls
 Controls
 Controls
of transcription
over translation
following translation
Controls in Eukaryotic Cells
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
Fig. 15-3, p.233
Chromosome Puff

Portion of the
chromosome in which the
DNA has loosened up to
allow transcription

Translation of transcripts
from puffed region
produces protein
components of saliva
X Chromosome Inactivation
 One
X inactivated in each cell of female
 Creates
a “mosaic” for X chromosomes
 Governed
by XIST gene
X Chromosome Inactivation
A
condensed X
chromosome (Barr
body) in the somatic
cell nucleus of a
human female
Fig. 15-4a, p.234
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
Phytochrome
 Signaling
molecule in plants
 Activated
by red wavelengths,
inactivated by far-red wavelengths
 Changes
in phytochrome activity
influence transcription of certain genes
petal
carpel
stamen
sepal
Fig. 15-6, p.235
B
A
1
C
2
3
petals
sepals
4
carpel
stamens
Fig. 15-6, p.235
Fig. 15-6, p.235
Fig. 15-6, p.235
Fig. 15-6, p.235
Fig. 15-6, p.235
Fig. 15-6, p.235
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
Knockout Experiments
Prevent a gene’s transcription or translation
 Differences between genetically engineered
knockout individuals and wild-type individuals
point to function of knocked out gene
 Knockout experiments shed light on genes that
function in Drosophila development

Knockout Experiments
Fig. 15-7c, p.237
A7 A5 A3 A1 T2 T2
A8
A4 A2 T3 T1
T2
T2
Body
Plan
A5
A4
A6
A7
A8
A3
A2
A1
T3
T1 Lb Mx Md
T2
A8
A7
A6
A4
A3 A2
A1 T3
T2
T1
A5 A4
A3 A2
A1 T3
T2
T1
A6
A7
A8
Fig. 15-8a, p.237
Body Plan
Fig. 15-8b, p.237
Body Plan
Fig. 15-8c, p.237
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
Fig.15-10, p. 241
High Lactose
allolactose
lactose
mRNA
operator
promoter
operator
RNA
polymerase
gene 1
Fig.15-10, p. 241
Low Lactose
 Repressor
 Binding
binds to operator
blocks promoter
 Transcription
is blocked
Fig.15-10, p. 241
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
 CAP
is little cAMP
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
 RNA
 The
complex forms
binds to promoter
polymerase can now bind
lactose-metabolizing genes are
transcribed rapidly
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
Polytene Chromosomes
 Occur
in salivary
glands of midge
larvae
 Consist of multiple
DNA molecules
 Can produce
multiple copies of
transcripts
Vertebrate Hormones
 Some

have widespread effects
Somatotropin (growth hormone)
 Others
signal only certain cells at
certain times

Prolactin stimulates milk
production
Fig. 15-11a, p.241
Fig. 15-11b, p.241