Transcript Control of Gene Expression

How is Gene Expression Controlled?
• Transcriptional Control (whether gene is transcribed or not)
– Operon: series of genes that code for specific products, including
regulators that control whether these genes are transcribed
• Example: lac operon (bacteria) – genes for lactose metabolism only
activated if lactose is present (when lactose not present, a
repressor blocks transcription; if present, lactose blocks
repressor, and transcription occurs)
– Regulator genes control the expression of suites of genes; many
control development and/or body patterns (Hox genes in animals)
• Post-transcriptional Control: editing of exons
• Translational Control
– Involves whether or not m-RNA is used or stored in cytoplasm
• Ex., egg cells often with large amounts of m-RNA “ready for use”
• Post-translational Control
– Polypeptides may be inactive; may need to join another polypeptide
or may become activated by a co-factor
Fig. 18.6
Signal
NUCLEUS
Chromatin
Chromatin modification
DNA
Gene available
for transcription
Gene
Transcription
RNA
Exon
Primary transcript
Intron
RNA processing
Tail
Cap
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
mRNA in cytoplasm
Degradation
of mRNA
Translation
Polypeptide
Protein processing
Active protein
Degradation
of protein
Transport to cellular
destination
Cellular function
Precursor
Fig. 18.2
Feedback
inhibition
trpE gene
Enzyme 1
trpD gene
Regulation
of gene
expression
Enzyme 2
trpC gene
trpB gene
Enzyme 3
trpA gene
Tryptophan
(a) Regulation of enzyme
activity
(b) Regulation of enzyme
production
Figure 16.21a
trp operon
Promoter
Promoter
Genes of operon
DNA
trpR
Regulatory
gene
mRNA
5
Protein
trpE
3
Operator
Start codon
mRNA 5
RNA
polymerase
Inactive
repressor
E
trpD
trpC
trpB
trpA
B
A
Stop codon
D
C
Polypeptide subunits that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on
DNA
No RNA made
mRNA
Active
repressor
Protein
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off
Fig. 18.3
Regulatory
gene
Promoter
Operator
Fig. 18.4
lacZ
lacI
DNA
No
RNA
made
3
mRNA
RNA
polymerase
5
Active
repressor
Protein
(a) Lactose absent, repressor active, operon off
lac operon
DNA
lacZ
lacY
-Galactosidase
Permease
lacI
3
mRNA
5
RNA
polymerase
mRNA 5
Protein
Allolactose
(inducer)
lacA
Inactive
repressor
(b) Lactose present, repressor inactive, operon on
Transacetylase
Fig. 18.18
Eye
Leg
Antenna
Wild type
Mutant
Head
Thorax
Abdomen
Fig. 18.17
0.5 mm
Dorsal
BODY Anterior
AXES
Left
Right
Posterior
Ventral
(a) Adult
Follicle cell
1 Egg cell
developing within
ovarian follicle
Nucleus
Egg
cell
Nurse cell
Egg
shell
2 Unfertilized egg
Depleted
nurse cells
Fertilization
Laying of egg
3 Fertilized egg
Embryonic
development
4 Segmented
embryo
0.1 mm
Body
segments
Hatching
5 Larval stage
(b) Development from egg to larva
Figure
21.17
Adult
fruit fly
Fruit fly embryo
(10 hours)
Fly
chromosome
Mouse
chromosomes
Mouse embryo
(12 days)
Adult mouse
Exons (regions of genes coding for protein
or giving rise to rRNA or tRNA) (1.5%)
Fig. 21.7
Repetitive
DNA that
includes
transposable
elements
and related
sequences
(44%)
L1
sequences
(17%)
Introns and
regulatory
sequences
(24%)
Unique
noncoding
DNA (15%)
Repetitive
DNA
unrelated to
transposable
elements
(15%)
Alu elements
(10%)
Simple sequence Large-segment
DNA (3%)
duplications (5–6%)
How do Cells Become Specialized?
• Cell Differentiation: a process where a generalized
cell changes in form and function to a
specialized cell (ex. neurons, RBCs)
– Often triggered chemically by neighbor cells (induction)
• Cell Fate: specialized function that cell acquires
• Cell Potency: range of cell types that cell could
acquire if exposed to different inductive environments; potency always includes fate
– Totipotent cells: unlimited potency
– Pluripotent cells: high, but not unlimited potency
• Cell Determination: when potency becomes
restricted to fate; timing can vary
• Heterotopic transplantation: method for testing
potency and timing of cell determination
Fig. 18.14
(a) Fertilized eggs of a frog
(b) Newly hatched tadpole
Fig. 18.15
Unfertilized egg cell
Sperm
Fertilization
Nucleus
Two different
cytoplasmic
determinants
Zygote
Mitotic
cell division
Two-celled
embryo
(a) Cytoplasmic determinants in the egg
Early embryo
(32 cells)
Signal
transduction
pathway
Signal
receptor
Signal
molecule
(inducer)
(b) Induction by nearby cells
NUCLEUS
Fig. 18-16-3
Nucleus
Master regulatory gene myoD
Embryonic
precursor cell
Other muscle-specific genes
DNA
Myoblast
(determined)
OFF
OFF
mRNA
OFF
MyoD protein
(transcription
factor)
mRNA
MyoD
Part of a muscle fiber
(fully differentiated cell)
mRNA
Another
transcription
factor
mRNA
mRNA
Myosin, other
muscle proteins,
and cell cycle–
blocking proteins