Chapter 21Chapter 21

The Genetic Basis of The Genetic Basis of DevelopmentDevelopment

Model OrganismsModel Organisms

Embryonic DevelopmentEmbryonic Development

Cell DivisionCell Division

Cell differentiationCell differentiation

MorphogenesisMorphogenesis– Body plan and body axesBody plan and body axes– Animals: Movements of cells and tissuesAnimals: Movements of cells and tissues– Plants: Apical meristemsPlants: Apical meristems

LE 21-4

Animal development

Zygote(fertilized egg)

Eight cells Blastula(cross section)

Gastrula(cross section)

Adult animal(sea star)

Cellmovement

Gut

Cell division

Morphogenesis

Observable cell differentiation

Seedleaves

Shootapicalmeristem

Rootapicalmeristem

PlantEmbryoinside seed

Two cellsZygote(fertilized egg)

Plant development

Differential Gene ExpressionDifferential Gene Expression

Genomic equivalence?Genomic equivalence?

Totipotency in plants – Fig, 21.5Totipotency in plants – Fig, 21.5– Cells dedifferentiatedCells dedifferentiated

Dedifferentiation in animal cells?Dedifferentiation in animal cells?– CloningCloning

LE 21-5Transversesection ofcarrot root

2-mgfragments

Fragments cul-tured in nutrientmedium; stir-ring causessingle cells toshear off intoliquid.

Single cellsfree insuspensionbegin todivide.

Embryonicplant developsfrom a culturedsingle cell.

Plantlet is cul-tured on agarmedium. Laterit is plantedin soil.

A single somatic (nonreproductive)carrot cell developed into amature carrot plant. The newplant was a genetic duplicate(clone) of the parent plant.

Adult plant

LE 21-6

Frog embryo Frog egg cell Frog tadpole

UV

Less differ-entiated cell

Donornucleustrans-planted

Enucleatedegg cell

Most developinto tadpoles

<2% developinto tadpoles

Donornucleustrans-planted

Fully differ-entiated(intestinal) cell

LE 21-7

Mammarycell donor

Egg celldonor

Egg cellfrom ovary Nucleus

removedCells fusedCultured

mammary cellsare semistarved,arresting the cellcycle and causingdedifferentiation

Nucleus frommammary cell

Early embryo

Grown in culture

Implanted in uterusof a third sheep

Surrogatemother

Embryonicdevelopment

Lamb (“Dolly”) genetically identicalto mammary cell donor

Stem Cells in AnimalsStem Cells in Animals

Stem cellsStem cells

EmbryonicEmbryonic– TotipotentTotipotent

AdultAdult– Pluripotent stem cellsPluripotent stem cells

LE 21-9

Embryonic stem cells Adult stem cells

Pluripotentcells

Totipotentcells

Culturedstem cells

Differentcultureconditions

Differenttypes ofdifferentiatedcells

Liver cells Nerve cells Blood cells

DifferentiationDifferentiation

Transcriptional regulation of gene Transcriptional regulation of gene expression during developmentexpression during development– Determination and tissue-specific proteinsDetermination and tissue-specific proteins– Master regulatory genesMaster regulatory genes

Cytoplasmic determinants and cell-cell Cytoplasmic determinants and cell-cell signalssignals– Cytoplasmic determinantsCytoplasmic determinants– InductionInduction

LE 21-10_3

Nucleus

Embryonicprecursor cell

DNA

OFF OFF

Master control gene myoD Other muscle-specific genes

mRNA OFF

Determination

Myoblast(determined)

MyoD protein(transcriptionfactor)

Differentiation

Muscle cell(fully differentiated)

mRNA

MyoD

mRNA mRNA mRNA

Anothertranscriptionfactor

Myosin, othermuscle proteins,and cell-cycleblocking proteins

LE 21-11a

Sperm

Molecules of acytoplasmicdeterminant Fertilization

Nucleus

Molecules of anothercytoplasmic determinant

Unfertilized egg cell

Zygote(fertilized egg)

Mitotic cell division

Two-celledembryo

Cytoplasmic determinants in the egg

LE 21-11b

Early embryo(32 cells)

Signaltransductionpathway

Signalreceptor

Signalmolecule(inducer)

NUCLEUS

Induction by nearby cells

Pattern FormationPattern Formation

Pattern formation – the body planPattern formation – the body plan

Positional informationPositional information– Cytoplasmic determinantsCytoplasmic determinants– InductionInduction

LE 21-12a

Egg celldeveloping withinovarian follicle

Follicle cell

Nucleus

Egg cell

Fertilization

Nursecell

Fertilized egg

Embryo

Nucleus

Laying of egg

Egg shell

Multinucleatesingle cell

Early blastoderm

Plasmamembraneformation

Late blastoderm

Yolk

Bodysegments

Cells ofembryo

Segmentedembryo

0.1 mm

LE 21-12b

Hatching

Larval stages (3)

Pupa

Metamorphosis

Adult flyHead Thorax Abdomen

0.5 mm

BODYAXES

Dorsal

Ventral

PosteriorAnterior

LE 21-13

Eye

AntennaLeg

Wild type Mutant

Cell Signaling: Body PlanCell Signaling: Body Plan

Axis establishmentAxis establishment– Maternal effect genes (aka, egg-polarity Maternal effect genes (aka, egg-polarity

genes)genes)– MorphogensMorphogens

Segmentation pattern – genesSegmentation pattern – genes

Identity of body partsIdentity of body parts– Homeotic genes (Hox genes)Homeotic genes (Hox genes)

ApoptosisApoptosis

LE 21-14

Head

Tail

Tail

Tail

Wild-type larva

Mutant larva (bicoid)

Drosophila larvae with wild-type and bicoid mutant phenotypes

Developingegg cell

Bicoid mRNAin matureunfertilizedegg

Nurse cells Egg cell

bicoid mRNA

Bicoid proteinin earlyembryo

FertilizationTranslation of bicoid mRNA 100

Gradients of bicoid mRNA and Bicoid protein inNormal egg and early embryo

Anterior end

m

Table 21-UN425

LE 21-18Ced-9 protein (active)inhibits Ced-4 activity

Mitochondrion

Ced-4 Ced-3

Inactive proteins

Deathsignalreceptor

No death signal

Deathsignal

Ced-9(inactive)

Cellformsblebs

ActiveCed-4

ActiveCed-3

Otherproteases

NucleasesActivationcascade

Death signal

Morphological DiversityMorphological Diversity

Widespread conservation of Widespread conservation of developmental genes among animalsdevelopmental genes among animals– Homeobox Homeobox homeodomain homeodomain– Not only present in Hox genesNot only present in Hox genes

LE 21-23a

Adultfruit fly

Fruit fly embryo(10 hours)

Flychromosome

LE 21-23b

Mousechromosomes

Mouse embryo(12 days)

Adult mouse