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

DNA

Signal

Gene

NUCLEUS

Chromatin modification

Chromatin

Gene availablefor transcription

Exon

Intron

Tail

RNA

Cap

RNA processing

Primary transcript

mRNA in nucleus

Transport to cytoplasm

mRNA in cytoplasm

Translation

CYTOPLASM

Degradationof mRNA

Protein processing

Polypeptide

Active protein

Cellular function

Transport to cellulardestination

Degradationof protein

Transcription

Fig. 18.2

Regulationof geneexpression

trpE gene

trpD gene

trpC gene

trpB gene

trpA gene

(b) Regulation of enzyme production

(a) Regulation of enzyme activity

Enzyme 1

Enzyme 2

Enzyme 3

Tryptophan

Precursor

Feedbackinhibition

Figure 16.21a

Fig. 18.3

Polypeptide subunits that make upenzymes for tryptophan synthesis

(b) Tryptophan present, repressor active, operon off

Tryptophan(corepressor)

(a) Tryptophan absent, repressor inactive, operon on

No RNA made

Activerepressor

mRNA

Protein

DNA

DNA

mRNA 5

Protein Inactiverepressor

RNApolymerase

Regulatorygene

Promoter Promoter

trp operon

Genes of operon

OperatorStop codonStart codon

mRNA

trpA

5

3

trpR trpE trpD trpC trpB

ABCDE

Fig. 18.4

(b) Lactose present, repressor inactive, operon on

(a) Lactose absent, repressor active, operon off

mRNA

Protein

DNA

DNA

mRNA 5

ProteinActiverepressor

RNApolymerase

Regulatorygene

Promoter

Operator

mRNA5

3

Inactiverepressor

Allolactose(inducer)

5

3

NoRNAmade

RNApolymerase

Permease Transacetylase

lac operon

-Galactosidase

lacYlacZ lacAlacI

lacI lacZ

Fig. 18.18

Antenna

MutantWild type

Eye

Leg

Fig. 18.17ThoraxHead Abdomen

0.5 mm

Dorsal

Ventral

Right

PosteriorLeft

AnteriorBODYAXES

Follicle cell

(a) Adult

Nucleus

Eggcell

Nurse cell

Egg celldeveloping withinovarian follicle

Unfertilized egg

Fertilized egg

Depletednurse cells

Eggshell

FertilizationLaying of egg

Bodysegments

Embryonicdevelopment

Hatching

0.1 mm

Segmentedembryo

Larval stage

(b) Development from egg to larva

1

2

3

4

5

Figure 21.17

Adultfruit fly

Fruit fly embryo(10 hours)

Flychromosome

Mousechromosomes

Mouse embryo(12 days)

Adult mouse

Fig. 21.7

Exons (regions of genes coding for proteinor giving rise to rRNA or tRNA) (1.5%)

RepetitiveDNA thatincludestransposableelementsand relatedsequences(44%)

Introns andregulatorysequences(24%)

UniquenoncodingDNA (15%)

RepetitiveDNAunrelated totransposableelements (15%)

L1sequences(17%)

Alu elements(10%)

Simple sequenceDNA (3%)

Large-segmentduplications (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 environ-ments; 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

(b) Induction by nearby cells(a) Cytoplasmic determinants in the egg

Two differentcytoplasmicdeterminants

Unfertilized egg cell

Sperm

Fertilization

Zygote

Mitoticcell division

Two-celledembryo

Signalmolecule(inducer)

Signaltransductionpathway

Early embryo(32 cells)

Nucleus

NUCLEUS

Signalreceptor

Fig. 18-16-3

Embryonicprecursor cell

Nucleus

OFF

DNA

Master regulatory gene myoD Other muscle-specific genes

OFF

OFFmRNA

MyoD protein(transcriptionfactor)

Myoblast(determined)

mRNA mRNA mRNA mRNA

Myosin, othermuscle proteins,and cell cycle–blocking proteinsPart of a muscle fiber

(fully differentiated cell)

MyoD Anothertranscriptionfactor