Genetic Basis for Developmnet
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Transcript of 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)
Embryonicprecursor cell
Determination and differentiation of Determination and differentiation of muscle cellsmuscle cells
DNA
OFF 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.
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Determination and differentiation of Determination and differentiation of muscle cellsmuscle cells
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