Ch. 21: The Genetic Basis of Development

The development of an organism from a zygote to an embryo to an

adult is a delicate process that involves much genetic control!

Chapter 21: The Genetic Basis of Development

1. How do we study development in the genetics-based lab?

-Model organisms-fruit fly, nematode worm, mouse, etc.

Figure 21.2 Model Organisms for Genetic Studies of DevelopmentDROSOPHILA MELANOGASTER

(FRUIT FLY)

CAENORHABDITIS ELEGANS(NEMATODE)

0.25 mm

Drosophila- small, easy & cheap to culture- 2 week generation time- 4 chromosomes- LARGE literature of info

C elegans- easy to culture- transparent body with few cell types- zygote to mature adult in 3 days

Chapter 21: The Genetic Basis of Development

-The complete cell lineage of the C. elegans nematode is known…

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

MUS MUSCULUS(MOUSE)

DANIO RERIO(ZEBRAFISH)

ARABIDOPSIS THAMANA(COMMON WALL CRESS)

Mouse - vertebrate - LARGE literature - transgenics & knock-outs

Chapter 21: The Genetic Basis of Development2. How does a zygote transform into an organism?

1) Cell division

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

(b) Tadpole hatching from egg

2) Cell differentiation3) Morphogenesis—”creation of form/shape”

Figure 21.4a, b

Animal 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. Once the 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)

Chapter 21: The Genetic Basis of Development1. How do we study development in the genetics-based lab? 2. How does a zygote transform into an organism?3. How do cells become differentiated?

-All cells have the same DNA, so differential gene expression must be the explanation!

• -Once a cell is differentiated, it’s difficult to “de-differentiate.”

Figure 21.6

EXPERIMENT Researchers enucleated frog egg cells by exposing them to ultraviolet light, which destroyed the nucleus. Nuclei from cells of embryos up to the tadpole stage were transplanted into the enucleated egg cells.

Frog embryo Frog egg cell Frog tadpole

Less differ-entiated cell

Donornucleustrans-planted

Enucleatedegg cell

Fully differ-entiated(intestinal) cell

Donornucleustrans-planted

Most developinto tadpoles

<2% developinto tadpoles

However, plants behave differently…

EXPERIMENTTransversesection 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.

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 totipotent, ableto reverse their differentiation and then give rise to all the cell types in a mature plant.

Figure 21.5

Nucleusremoved

Mammarycell donor

Egg celldonor

Egg cellfrom ovary

Cultured mammary cells are semistarved, arresting the cellcycle and causingdedifferentiation

Nucleus frommammary cell

Grown in culture

Early embryoImplanted in uterusof a third sheep

SurrogatemotherEmbryonic

development

Lamb (“Dolly”)genetically identical to mammary cell donor

4

5

6

1 2

3 Cells fused

APPLICATION This method is used to produce cloned animals whose nuclear genes are identical to the donor animal supplying the nucleus.

TECHNIQUE Shown here is the procedure used to produce Dolly, the first reported case of a mammal cloned using the nucleus of a differentiated cell.

RESULTS The cloned animal is identical in appearance and genetic makeup to the donor animal supplying the nucleus, but differs from the egg cell donor and surrogate mother.

Nucleusremoved

Figure 21.7

Chapter 21: The Genetic Basis of Development4. What is a stem cell?

-a relatively unspecialized cell

Figure 21.9

Early human embryoat blastocyst stage

(mammalian equiva-lent of blastula)

From bone marrowin this example

Totipotentcells

Pluripotentcells

Culturedstem cells

Differentcultureconditions

Differenttypes ofdifferentiatedcells

Liver cells Nerve cells Blood cells

Embryonic stem cells Adult stem cells

-can differentiate into cells of different types under specific conditions-Embryonic = totipotent-Adult = pluripotent (can produce some, but not all, cell types)

Chapter 21: The Genetic Basis of Development5. What type of genetic signal leads to cell differentiation?

-Step 1: Cell receives signals from other cells

DNAOFF OFF

OFFmRNA

mRNA mRNA mRNA mRNA

Anothertranscriptionfactor

MyoDMuscle cell(fully differentiated)

MyoD protein(transcriptionfactor)

Myoblast (determined)

Embryonicprecursor cell

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

-Step 2: A regulatory gene is turned “on”, and a protein is made thatactivates other genes. (“point of no return”)-Step 3: Activated genes make proteins that determine cell type/structure/behavior.

Chapter 21: The Genetic Basis of Development6. What is induction?

-signal molecules (proteins or hormones) from embryonic cells cause trancriptional changes in nearby target cells.

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)

4

Anterior

EMBRYO

Posterior

ReceptorSignalprotein

Signal

Anteriordaughtercell of 3

Posteriordaughtercell of 3

Will go on toform muscle and gonads

Will go on toform adultintestine

12

43

3

Induction of the intestinal precursor cell at the four-cell stage.(a)

Figure 21.16b

Epidermis

Gonad Anchor cell

Signalprotein

Vulval precursor cells

Inner vulva

Outer vulva

Epidermis

ADULT

Induction of vulval cell types during larvaldevelopment.

(b)

Figure 47.25

EXPERIMENT

RESULTS

CONCLUSION

Spemann and Mangold transplanted a piece of the dorsal lip of a pigmented newt gastrula to the ventral side of the early gastrula of a nonpigmented newt.

During subsequent development, the recipient embryo formed a second notochord and neural tube in the region of the transplant, and eventually most of a second embryo. Examination of the interior of the double embryorevealed that the secondary structures were formed in part from host tissue.

The transplanted dorsal lip was able to induce cells in a different region of the recipient to form structures different from their normal fate. In effect, the dorsal lip “organized” the later development of an entire embryo.

Pigmented gastrula(donor embryo)

Dorsal lip ofblastopore

Nonpigmented gastrula(recipient embryo)

Primary embryo

Secondary (induced) embryoPrimarystructures:

Neural tube

Notochord

Secondarystructures:

Notochord (pigmented cells)

Neural tube (mostly nonpigmented cells)

Chapter 21: The Genetic Basis of Development

-cytoplasmic determinants in the unfertilized egg regulate geneexpression in the zygote that affects differentiation/development

Figure 21.11a

Unfertilized egg cell

Molecules of a a cytoplasmicdeterminant Fertilization

Zygote(fertilized egg)

Mitotic cell division

Two-celledembryo

Cytoplasmic determinants in the egg. The unfertilized egg cell has molecules in its cytoplasm, encoded by the mother’s genes, that influence development. Many of these cytoplasmic determinants, like the two shown here, are unevenly distributed in the egg. After fertilization and mitotic division, the cell nuclei of the embryo are exposed to different sets of cytoplasmic determinants and, as a result, express different genes.

(a)

Nucleus

Chapter 21: The Genetic Basis of Development

-Cytoplasmic determinants from mother’s egg initially establish the axes ofthe body of Drosophila.-bicoid gene

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

Figure 21.14a

Translation of bicoid mRNAFertilization

Nurse cells Egg cell

bicoid mRNA

Developing egg cell

Bicoid mRNA in mature unfertilized egg

100 µm

Bicoid protein inearly embryo

Anterior end

(b) Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo.

1

2

3

Figure 21.14b

Chapter 21: The Genetic Basis of Development-7. How does morphogenesis (pattern formation) occur in animals?

After the body’s axes are determined (by cytoplasmic determinants)…

-Segmentation genes produce proteins that direct formation of body segments.

Hierarchy 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

-Then, the development of specific features of the body segments is directedby HOMEOTIC GENES (Hox genes.)

Chapter 21: The Genetic Basis of Development8. What is the relationship among the genetic basis of development across organisms?

-Molecular analysis of the homeotic genes in Drosophila has shown that theyall include a sequence called a homeobox

-An identical (or very similar) DNA sequence has been discovered in the homeotic genes of vertebrates and invertebrates

Figure 21.23

Adultfruit fly

Fruit fly embryo(10 hours)

Flychromosome

Mouse chromosomes

Mouse embryo(12 days)

Adult mouse

Chapter 21: The Genetic Basis of Development9. What is apoptosis?

-programmed cell death (cell suicide)Ced-9protein (active)inhibits Ced-4activity

Mitochondrion

Deathsignalreceptor

Ced-4 Ced-3

Inactive proteins

(a) No death signalCed-9(inactive)

Cellformsblebs

Deathsignal

ActiveCed-4

ActiveCed-3

Activationcascade

Otherproteases

Nucleases

(b) Death signal

Figure 21.18 Molecular basis of apoptosis in C. elegans

Chapter 21: The Genetic Basis of Development8. What is apoptosis?

-programmed cell death (cell suicide)-necessary for development of hands/feet in vertebrates

Figure 21.19

Interdigital tissue

1 mm