Genetic Basis for Developmnet

The Genetic The Genetic Basis Basis

of Developmentof DevelopmentHow do cells with the same How do cells with the same

genes grow up to be so genes grow up to be so different?different?

Three Procceses of Three Procceses of DevelopmentDevelopment

The transformation from a zygote The transformation from a zygote into an organisminto an organism Results from three interrelated Results from three interrelated

processes: cell division, cell processes: cell division, cell differentiation, and morphogenesisdifferentiation, and morphogenesis

Figure 21.3a, b(a) Fertilized eggs of a frog

(b) Tadpole hatching from egg

Through a succession of mitotic cell divisionsThrough a succession of mitotic cell divisions The zygote gives rise to a large number of cellsThe zygote gives rise to a large number of cells

In cell differentiationIn cell differentiation Cells become specialized in structure and functionCells become specialized in structure and function

Morphogenesis encompasses the processesMorphogenesis encompasses the processes That give shape to the organism and its various That give shape to the organism and its various

partsparts

Some key stages of Some key stages of development in animals and development in animals and

plantsplantsAnimal development. Most animals go through some variation of the blastula and gastrula stages. The blastula is a sphere of cells surrounding a fluid-filled cavity. The gastrulaforms when a region of the blastula folds inward, creating a tube—a rudimentary gut. Oncethe animal is mature, differentiation occurs in only a limited way—for the replacement of damaged or lost cells.

Plant development. In plants with seeds, a complete embryo develops within the seed. Morphogenesis, which involves cell division and cell wall expansion rather than cell or tissue movement, occurs throughout the plant’s lifetime. Apical meristems (purple) continuously arise and develop into the various plant organs as the plant grows to an indeterminate size.

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 cells Zygote

(fertilized egg)

(a)

(b)

Differential gene Differential gene expressionexpression

Nearly all the cells of an organism have genomic equivalence, that is, they have the same genes

Differences between cells in a Differences between cells in a multicellular organismmulticellular organism differences in gene expressiondifferences in gene expression not from differences in the cells’ not from differences in the cells’

genomesgenomes

yields a variety of cell typesyields a variety of cell types each expressing a different each expressing a different combination of genescombination of genes multicellular eukaryotes multicellular eukaryotes

cells become specialized as a zygote cells become specialized as a zygote develops into a mature organismdevelops into a mature organism

Cell DifferentiationCell Differentiation

Different types of cells Different types of cells Make different proteins because different Make different proteins because different

combinations of genes are active in each typecombinations of genes are active in each type

Muscle cell Pancreas cells Blood cells

Cell DiferentiationCell Diferentiation

may retain all of their genetic may retain all of their genetic potentialpotential

Most retain a complete set of genesMost retain a complete set of genes May be totipotentMay be totipotent

Differentiated cellsDifferentiated cells

Totipotency in PlantsTotipotency in PlantsEXPERIMENT

Transversesection ofcarrot root

2-mgfragments

Fragments cultured in nutrient medium; stirring causessingle cells to shear off into liquid.

Single cellsfree insuspensionbegin todivide.

Embryonicplant developsfrom a culturedsingle cell.

Plantlet is cultured on agarmedium. Laterit is plantedin soil.

RESULTS A singleSomatic (nonreproductive) carrotcell developed into a mature carrotplant. The new plant was a genetic duplicate(clone) of the parent plant.

Adult plant

CONCLUSION At least some differentiated (somatic) cells in plants are toipotent, ableto reverse their differentiation and then give rise to all the cell types in a mature plant.

DNA packing in a eukaryotic chromosome

Wound around clusters of histone Wound around clusters of histone proteins, forming a string of proteins, forming a string of beadlike nucleosomesbeadlike nucleosomes

This beaded fiber is further wound This beaded fiber is further wound and folded and folded

DNA packing tends to block gene DNA packing tends to block gene expressionexpression

Presumably by preventing Presumably by preventing access of transcription access of transcription proteins to the DNAproteins to the DNA

DNA doublehelix(2-nmdiameter) Histones

Linker“Beads ona string”

Nucleosome(10-nm diameter)

Tight helical fiber(30-nm diameter)

Supercoil(300-nm diameter)

Metaphase chromosome

700nm

TE

M

TE

M

X chromosome inactivation in the cells of X chromosome inactivation in the cells of female mammalsfemale mammals

Early embryo

X chromosomes

Allele fororange fur

Allele forblack fur

Cell divisionand random

X chromosomeinactivation

Two cell populationsin adult

Active X

Inactive X

Inactive X

Active X

Orangefur

Black fur

An Extreme Example of DNA An Extreme Example of DNA PackingPacking

Nuclear Transplantation

The nucleus of an unfertilized The nucleus of an unfertilized egg cell or zygote is replaced egg cell or zygote is replaced with the nucleus of a with the nucleus of a differentiated celldifferentiated cell

How to Clone a SheepHow to Clone a SheepBill Ritchie (Produced for the 1997 Royal Agricultural Show)

The EggThe Egg The unfertilized eggs are flushed out of a sheep which The unfertilized eggs are flushed out of a sheep which

has been induced to produce a larger than normal has been induced to produce a larger than normal number of eggs. number of eggs.

The CellThe Cell Previously a sample of tissue was from the Previously a sample of tissue was from the

udder of a six year old ewe was taken and udder of a six year old ewe was taken and cultured in a dish (Dolly 1). cultured in a dish (Dolly 1).

The cultured cells are starved to send them The cultured cells are starved to send them into a resting or quiescent stateinto a resting or quiescent state. .

The fusionThe fusion A cell is placed beside the egg and an electric current A cell is placed beside the egg and an electric current

used to fuse the coupletused to fuse the couplet. .

How to Clone a SheepHow to Clone a Sheep

CultureCulture The reconstructed embryo is put into culture The reconstructed embryo is put into culture

and grows for seven days. and grows for seven days.

DevelopmentDevelopment Embryos which grow successfully are taken Embryos which grow successfully are taken

and transferred to a sheep which is at the the and transferred to a sheep which is at the the same stage of the oestrus cycle as the egg. same stage of the oestrus cycle as the egg.

The sheep becomes pregnant and produces a The sheep becomes pregnant and produces a lamb after 21 weeks (Dolly). lamb after 21 weeks (Dolly).

““Copy Cat”Copy Cat”

Was the first cat ever clonedWas the first cat ever cloned

Figure 21.8

The Stem Cells of The Stem Cells of AnimalsAnimals

A stem cellA stem cell Is a relatively unspecialized cell Is a relatively unspecialized cell Can reproduce itself indefinitelyCan reproduce itself indefinitely Can differentiate into specialized cells of Can differentiate into specialized cells of

one or more types, given appropriate one or more types, given appropriate conditionsconditions

Totipotentcells

Liver cells Nerve cells

Early human embryoat blastocyst stage

(mammalian equiva-lent of blastula) From bone marrow

in this example

PluripotentcellsCultured

stem cells

Differentcultureconditions

Differenttypes ofdifferentiatedcells

Blood cells

Embryonic stem cells

Adult stem cells

Embryonic and Adult Stem Embryonic and Adult Stem CellsCells

Stem cells Stem cells can be can be isolated isolated From early From early

embryos embryos at the at the blastocyst blastocyst stagestage

Adult stem cells pluripotent

, able to give rise to multiple but not all cell types

Complex assemblies of proteins Complex assemblies of proteins control eukaryotic transcriptioncontrol eukaryotic transcription

A variety of regulatory proteins interact A variety of regulatory proteins interact with DNA and with each otherwith DNA and with each other

To turn the transcription of eukaryotic To turn the transcription of eukaryotic genes on or offgenes on or off

Transcriptional Regulation of Gene Transcriptional Regulation of Gene Expression During DevelopmentExpression During Development

Assist in initiating eukaryotic transcriptionAssist in initiating eukaryotic transcription

Enhancers Promoter

Gene

DNA Activatorproteins

Otherproteins

Transcriptionfactors

RNA polymerase

Bendingof DNA

Transcription

Transcription FactorsTranscription Factors

DNA

OFF OFF

Master control gene myoD Other muscle-specific genesNucleus

Embryonicprecursor cell

Determination and differentiation of Determination and differentiation of muscle cellsmuscle cells

DNA

OFF OFF

OFFmRNA

MyoD protein(transcriptionfactor)

Master control gene myoD

Determination. Signals from othercells lead to activation of a masterregulatory gene called myoD, andthe cell makes MyoD protein, atranscription factor. The cell, nowcalled a myoblast, is irreversiblycommitted to becoming a skeletalmuscle cell.

1

Other muscle-specific genesNucleus

Myoblast (determined)



DNA

OFF OFF

OFFmRNA

mRNA mRNA mRNA mRNA

Anothertranscriptionfactor

MyoDMuscle cell(fully differentiated)

MyoD protein(transcriptionfactor)

Myoblast (determined)


Myosin, othermuscle proteins,and cell-cycleblocking proteins

Other muscle-specific genesMaster control gene myoDNucleus

Determination. Signals from othercells lead to activation of a masterregulatory gene called myoD, andthe cell makes MyoD protein, atranscription factor. The cell, nowcalled a myoblast, is irreversiblycommitted to becoming a skeletalmuscle cell.

1

Differentiation. MyoD protein stimulatesthe myoD gene further, and activatesgenes encoding other muscle-specifictranscription factors, which in turn activate genes for muscle proteins. MyoD also turns on genes that block the cell cycle, thus stopping cell division. The nondividing myoblasts fuse to become mature multinucleate muscle cells, alsocalled muscle fibers.

2


Cytoplasmic Determinants and Cell-Cell Signals in Cell Differentiation

Cytoplasmic determinants in the Cytoplasmic determinants in the cytoplasm of the unfertilized eggcytoplasm of the unfertilized egg Regulate the expression of genes in the zygote that Regulate the expression of genes in the zygote that

affect the developmental fate of embryonic cellsaffect the developmental fate of embryonic cells

Sperm

Molecules of another cyto-plasmic deter-minant

Unfertilized egg cell

Molecules of a a cytoplasmicdeterminant Fertilization

Zygote(fertilized egg)

Mitotic cell division

Two-celledembryo

Nucleus

Sperm

InductionInduction

Signal molecules from embryonic cells cause Signal molecules from embryonic cells cause transcriptional changes in nearby target cellstranscriptional changes in nearby target cells

Early embryo(32 cells)

NUCLEUS Signaltransductionpathway

Signalreceptor

Signalmolecule(inducer)

Induction by nearby cells. The cells at the bottom of the early embryo depicted here are releasing chemicals that signal nearby cells to change their gene expression.

(b)

Pattern FormationPattern Formation

Pattern formation in animals and plants Pattern formation in animals and plants results from similar genetic and cellular results from similar genetic and cellular mechanismsmechanisms

Pattern formationPattern formation Is the development of a spatial organization of Is the development of a spatial organization of

tissues and organstissues and organs Occurs continually in plantsOccurs continually in plants Is mostly limited to embryos and juveniles in Is mostly limited to embryos and juveniles in

animalsanimals

Cell PositioningCell Positioning

Positional informationPositional information Consists of molecular cues that control Consists of molecular cues that control

pattern formationpattern formation Tells a cell its location relative to the Tells a cell its location relative to the

body’s axes and to other cellsbody’s axes and to other cells

Cascades of gene expression and cell-to-cell Cascades of gene expression and cell-to-cell signaling direct the development of an signaling direct the development of an animalanimal

Early understanding of the relationship Early understanding of the relationship between gene expression and embryonic between gene expression and embryonic developmentdevelopment

Came from studies of mutants of the fruit Came from studies of mutants of the fruit fly fly Drosophila melanogasterDrosophila melanogaster

THE GENETIC CONTROL OF THE GENETIC CONTROL OF EMBRYONIC DEVELOPMENTEMBRYONIC DEVELOPMENT

Eye

Antenna

LegS

EM

50

Head of a normal fruit fly Head of a developmental mutant

Key Developmental Genes are Key Developmental Genes are Very AncientVery Ancient

Homeotic genes contain nucleotide sequences, Homeotic genes contain nucleotide sequences, called homeoboxescalled homeoboxes

That are very similar in many kinds of organismsThat are very similar in many kinds of organisms

Fly chromosome Mouse chromosomes

Fruit fly embryo (10 hours) Mouse embryo (12 days)

Adult fruit fly Adult mouse

Key developmental Key developmental events in the life events in the life cycle of cycle of DrosophilaDrosophila

Figure 21.12

Follicle cell Nucleus

Egg cell

FertilizationNursecell

Egg celldeveloping withinovarian follicle

Laying of egg

EggshellNucleus

Fertilized egg

Embryo

Multinucleatesingle cell

Early blastoderm

Plasmamembraneformation

Late blastoderm

Cells ofembryo

Yolk

Segmentedembryo

Bodysegments

0.1 mm

HatchingLarval stages (3)

Pupa

Metamorphosis

Head Thorax Abdomen

0.5 mm

Adult fly

Dorsal

Anterior Posterior

Ventral

BODYAXES

BicoidBicoid Mutation Mutation

Head

Wild-type larva

Tail Tail

Mutant larva (bicoid)

Drosophila larvae with wild-type and bicoid mutant phenotypes. A mutation in the mother’s bicoid gene leads to tail structures at both ends (bottom larva). The numbers refer to the thoracic and abdominal segments that are present.

(a)

T1 T2T3

A1 A2 A3 A4 A5 A6 A7A8

A8A7 A6 A7

A8

Tail

Summary of Gene Activity During Summary of Gene Activity During DrosophilaDrosophila Development DevelopmentHierarchy of Gene Activity in Early Drosophila Development

Maternal effect genes (egg-polarity genes)

Gap genes

Pair-rule genes

Segment polarity genes

Homeotic genes of the embryo

Other genes of the embryo

Segmentation genesof the embryo

C. elegansC. elegans: The Role of Cell : The Role of Cell SignalingSignaling

The complete cell lineage The complete cell lineage Of each cell in the nematode roundworm Of each cell in the nematode roundworm

C. elegansC. elegans is known is known

Figure 21.15

Zygote

Nervoussystem,outerskin, mus-culature

Musculature,gonads

Outer skin,nervous system

Germ line(futuregametes)

Musculature

First cell division

Tim

e af

ter

fert

iliza

tion

(hou

rs)

0

10Hatching

Intestine

Intestine

Eggs Vulva

ANTERIOR POSTERIOR1.2 mm

Induction As early as the four-cell stage in As early as the four-cell stage in C. elegansC. elegans

Cell signaling helps direct daughter cells down the Cell signaling helps direct daughter cells down the appropriate pathways, a process called inductionappropriate pathways, a process called induction

Figure 21.16a

4

Anterior

EMBRYO

Posterior

ReceptorSignalprotein

Signal

Anteriordaughtercell of 3

Posteriordaughtercell of 3

Will go on toform muscle and gonads

Will go on toform adultintestine

1

2

43

3

(a)

InductionInduction also critical later in nematode developmentalso critical later in nematode development

As the embryo passes through three larval As the embryo passes through three larval stages prior to becoming an adultstages prior to becoming an adult

Figure 21.16b

Epidermis

Gonad Anchor cell

Signalprotein

Vulval precursor cells

Inner vulvaOuter vulva

Epidermis

ADULT

Programmed Cell Death (Apoptosis) In apoptosisIn apoptosis

Cell signaling is involved in programmed cell deathCell signaling is involved in programmed cell death

2 µmFigure 21.17

In vertebrates In vertebrates Apoptosis is essential for normal Apoptosis is essential for normal

morphogenesis of hands and feet in morphogenesis of hands and feet in humans and paws in other animalshumans and paws in other animals

Figure 21.19

Interdigital tissue

1 mm

Plant Development: Cell Signaling and Transcriptional Regulation

Thanks to DNA technology and clues Thanks to DNA technology and clues from animal researchfrom animal research Plant research is now progressing Plant research is now progressing

rapidlyrapidly

Mechanisms of Plant Development

In general, cell lineageIn general, cell lineage Is much less important for pattern Is much less important for pattern

formation in plants than in animalsformation in plants than in animals The embryonic development of most The embryonic development of most

plantsplants Occurs inside the seedOccurs inside the seed

Pattern Formation in Pattern Formation in FlowersFlowers

Floral meristemsFloral meristems Contain three cell types that affect Contain three cell types that affect

flower developmentflower development

Figure 21.20

Carpel

Petal

Stamen

Sepal

Anatomy of a flowerFloral meristem Tomato flower

Celllayers

L1L2L3

Organ Identity GenesOrgan Identity Genes

Organ identity genesOrgan identity genes Determine the type of structure that Determine the type of structure that

will grow from a meristemwill grow from a meristem Are analogous to homeotic genes in Are analogous to homeotic genes in

animalsanimals

Figure 21.22Wild type Mutant

Documents

Genetic Basis for Developmnet