1、Gene Regulation and DevelopmentMaking of the Complex Living things In the development of multicell organisms,a single-cell zygote gives rise to many different cell types.Each type has different structure,under taking different chemical and biologic reactions and performs corresponding functions.All
2、these events are controlled by sets of genes in the cell and influenced by surrounding stimuli.Body system of multicell organisms Cells of similar types are organized into tissues Tissues with associated function into organs Organs of associated function into organ systems Organ systems coordinately
3、 to support the whole organism Process of embryonic development must give rise not only to the cells of different types but also to higher-level structures and coordinated functions.How life begin An organism arises from a fertilized egg as the result of three interrelated processes:cell division,ce
4、ll differentiation,and cell morphogenesis.During development,cells becomes polarized,starts divide unevenly,reprograms gene expression and become specialized in structure and function,a process called differentiation.The processes of morphogenesis is forming of shapes,give cells,tissues and organs t
5、heir proper shape to fit their corresponding functions.All these processes are part of development.Fertilization results in a zygote and triggers embryonic developmentPlasma membraneTailMiddlepieceNeckHeadNucleusAcrosomeMitochondrion(spiral shape)The spermapproachesthe egg1234The spermsacrosomal enz
6、ymesdigest the eggs jelly coatProteins on thesperm head bindto egg receptors5The spermnucleus enters the egg cytoplasmThe plasma membranesof sperm and egg fuse6A fertilizationenvelope formsSPERMNucleusAcrosomePlasmamembraneSpermheadAcrosomalenzymesJellycoat(Zona Pellucida)VitellinelayerPlasmamembran
7、eCytoplasmEGG CELLReceptor proteinmoleculesSpermnucleusEggnucleus7The nucleiof spermand egg fuseZygotenucleusCleavage startsFertilizationof ovumOviductSecondaryoocyteOvulationOvaryBlastocyst(implanted)EndometriumUterusENDOMETRIUMInner cell massCavityTrophoblastBlastocystAntibody against Oct4Trophobl
8、astEggSpermBlastocystEmbryoEmbryonic stem-cellcultureInner cellmassOnce sperm and egg cell fuse(zygote),cell start to divide and differentiate.The first differentiated structure:a blastocyst.The inner cell mass of the blastocyst develops into embryo.Embryonic stem cells(ES cells)areisolated from the
9、 inner cell massES Cell CultureDetermination The earliest changes that set a cell to a specialized stage without turning back at molecular level.Molecular changes that drives the process in the embryo is termed determination,the beginning of differentiation.During embryonic development,cells become
10、obviously different in structure and function as they differentiate.Differentiated Cells These cells produce the proteins that allow them to carry out their specialized roles in the organism.For example,only lens cells devote 80%of their capacity for protein synthesis to making just one type of prot
11、eins,crystallins.These form transparent fibers that allow the lens to transmit and focus light.Similarly,skeletal muscles cells have high concentrations of proteins specific to muscle tissues,such as muscle-specific version of the contractile protein myosin and the structural protein actin.They also
12、 have membrane receptor proteins that detect signals from nerve cells.PlacentaAllantoisYolk sacMothers bloodvesselsAmnioticcavityEmbryoChorionChorionicvilliFunctions of the chorion:1.Provide nutrients and O2 to the fetus2.Secrete hormones into the mother to help retain the fetus3.Repress the mothers
13、 immune response to prevent rejection of the fetus4.The placentas chorionic villi absorb food and oxygen from the mothers blood EctodermPattern FormationDifferentiation is carefully orchestrated:Proliferation Cell migration Interactions(Induction)Epithelial-mesenchymal transformations Epithelial fol
14、ding,movement,evagination,fusion Apoptosis ApoptosisLineage analysis of C.elegans highlights another outcome of cell signaling,programmed cell death or apoptosis.The timely suicide of cells occurs exactly 131 times in the course of C.eleganss normal development.At precisely the same points in develo
15、pment,signals trigger the activation of a cascade of“suicide”proteins in the cells destined to die.a.b.InhibitorCED-9Bcl-2CED-4Apaf1Caspase-8 or-9ApoptosisCED-3ApoptosisInhibitor:Activator:ApoptoticProtease:Caenorhabditis elegansMammalian CellOrganismInhibitionActivationIf it werent for apoptosis,yo
16、u may look a little different Apoptosis pathways in humans and other mammals are more complicated.Research on mammals have revealed a prominent role for mitochondria in apoptosis.Signals from apoptosis pathways or others somehow cause the outer mitochondrial membrane to leak,releasing proteins that
17、promote apoptosis.A cell must make a life-or-death“decision”by somehow integrating both the“death”and“life”(growth factor)signals that it receives.Apoptosis is essential to the development of animal morphogenesis(prevents webbing between fingers and toes).Model Organism(1)The Worm The nematode C.ele
18、gans normally lives in the soil but is easily grown in petri dishes.Only a millimeter long,it has a simple,transparent body with only a few cell types and grows from zygote to mature adult in three and a half days.Its genome has been sequenced.Because individuals are hermaphrodites,it is easy to det
19、ect recessive mutations.Self-fertilization of heterozygotes will produce some homozygous recessive offspring with mutant phenotypes.A further important feature is that every adult C.elegans have exactly 959 somatic cells.These arise from the zygote in virtually the same way for every individual.Stud
20、ying WormsC.elegans is a very useful model organism for investigating the roles of cell signaling and induction in development.The pathway from fertilized egg to adult nematode involves four larval stages(the larvae look much like smaller versions of the adult)during which the vulva develops.Six cel
21、ls present on the ventral surface of the second-stage larva gives the vulva.A single cell in the embryonic gonad,the anchor cell,initiates a cascade of signals that establishes the fate of the vulva precursor cells.Vulva development in the nematode illustrates several important developmental concept
22、s:In the developing embryo,sequential inductions drive the formation of organs.The effect of an inducer can depend on its concentration.Inducers produce their effects via signal-transduction pathways similar to those operating in adult cells.The induced cells response is often the activation(or inac
23、tivation)of genes which establishes the pattern of gene activity characteristic of a particular cell type.Genetics as a powerful approach for elucidating the mechanisms of development.A fate map traces the development of an embryo.Model Organism(2)The Fruit Fly The fruit fly Drosophila melanogaster
24、was first chosen as a model organism by geneticist T.H.Morgan and intensively studied by generations of geneticists.The fruit fly is small and easily grown in the laboratory.It has a generation time of only two weeks and produces many offspring.Embryos develop outside the mothers body.In addition,th
25、ere are vast amounts of information on its genes and other aspects of its biology.However,because first rounds of mitosis occurs without cytokinesis,parts of its development are superficially quite different from what is seen in other organisms.Sabotaging the Fruit FliesIn the 1940s,Edward B.Lewis d
26、emonstrated that the study of mutants could be used to investigate Drosophila development.He studied bizarre developmental mutations and located the mutations on the flys genetic map.This research provided the first concrete evidence that genes somehow direct the developmental process.In the late 19
27、70s,Christiane Nsslein-Volhard and Eric Weischaus pushed the understanding of early pattern formation to the molecular level.Their goal was to identify all the genes that affect segmentation in Drosophila.Mutations that affect segmentation are likely to be embryonic lethals,leading to death at the e
28、mbryonic or larval stage.Because of maternal effects on axis formation in the egg,they also needed to study maternal genes.After a year of hard work,they identified 1,200(out of a fruit flies 13,000)genes essential for embryonic developmentAbout 120 of these were essential for pattern formation lead
29、ing to normal segmentation.They were able to map these genes.The results of detailed anatomical observations of development in several species and experimental manipulations of embryonic tissues laid the groundwork for understanding the mechanisms of development.Life cycle of the fruit flyHomeotic-S
30、elector HOX GenesAmong the most fascinating kinds of abnormalities in animals are those in which one normal body part is replaced by another.William Bateson(1894)catalogued several oddities of this nature coining the term homeotic to describe them.Calvin Bridges(1915)isolated a spontaneous mutant of
31、 Drosophila in which part of the haltere was transformed into wing tissue.The mutant was called bithorax(duplication of a thoracic segment).In the following decades other mutations affecting segment identity were identified e.g.certain Antennapedia mutations causing transformation of antennae into l
32、egs.BackgroundIn Drosophila,genes controlling segment identity were found to be transcription factors and coined to be homeotic-selector genes,meaning that the fate of a given segment was selected by the expression of specific homeotic genes.Homeobox All homeotic genes of Drosophila include a 180-nu
33、cleotide sequence called the homeobox,which specifies a 60-amino-acid homeodomain.An identical or very similar sequence of nucleotides(often called Hox genes)are found in many other animals,including humans.How Homeodomains Work Proteins with homeodomains probably regulate development by coordinatin
34、g the transcription of batteries of developmental genes.In Drosophila,different combinations of homeobox genes are active in different parts of the embryo and at different times,leading to pattern formation.Action of Signaling molecules Paracrine factors Inducing factors controlling forms of develop
35、ing organ Mitogen regulating cell proliferation Morphogen acting in a dose-dependent way to pattern the cell fates within a target fieldMorphogens Product of the mothers bicoid gene is essential for setting up the anterior end of the fly.The genes products are concentrated at the future anterior end
36、.This is a specific version of a general gradient hypothesis,in which gradients of morphogens establish an embryos axes and other features.The bicoid protein and other morphogens are transcription factors that regulate the activity of some of the embryos own genes.Gradients of these morphogens bring
37、 about regional differences in the expression of segmentation genes,the genes that direct the actual formation of segments after the embryos major axes are defined.The direction of homeotic transformations depends on whether the mutation causes loss of homeotic gene function where the gene normally
38、acts or gain of function where the gene normally does not act.Ultrabithorax(Ubx)acts in the haltere to promote haltere development and repress wing development.Loss of function mutations in Ubx transform the haltere into a wing.Dominant mutations that cause Ubx to gain function in the wing transform
39、 that structure into a haltere.In antenna-to-leg transformations of Antennapedia the mutants reflect a dominant gain of Antennapedia gene function in the antennae.Homeotic MutationsExamples of Homeotic MutationsNormal adult flyAntennapedia mutantBithorax mutantHomeotic-Selector Genes Homeotic gene p
40、roducts regulate development within parasegmental domains.At this stage each band of cells along the A-P axis express a unique combination of transcription factors which control subsequent cell development.Patterns of selector gene expression are determined in the early embryo and maintained into th
41、e adult fly.Continuous expression is required to specify structures along the A-P axis.The genes are ordered on the chromosome in the order of their expression with respect to the parasegments of the Drosophila embryo.Two of the best studied selector genes are Antennapaedia(Antp)which dominates deve
42、lopment of the fourth parasegment and Ultrabithorax(Ubx)which largely controls the sixth parasegment.Transcription of the Antp and Ubx genes begins in the third hour of embryogenesis in domains determined in part by various gap-gene products with boundaries set up by the segmentation genes.Homeotic
43、genes act in a cell autonomous manner a cell will behave according to homeotic gene expression of its own regardless of the pattern of expression of its neighbours Maintenance of homeotic gene expression is through self regulation and the Polycomb-like class of genes.Selector genes specify epidermal
44、 structures the musculature,neural tissue and gut along the A-P axis.Two homeotic selector gene clusters occur in two groups on the same chromosome five(the Antennapedia Complex ANT-C)are expressed in the head region while genes in the Bithorax Complex(BX-C)are mainly expressed in thorax or abdomina
45、l regions.Homeotic-Selector GenesEight Genes Regulate the Identity of Within the Adult and Embryolabial(lab)proboscipedia(pb)Deformed(Dfd)Sex combs reduced(Scr)Antennapedia(Antp)Ultrabithorax(Ubx)abdominal A(abd-A)Abdominal B(Abd-B)Homeotic genes encode nuclear proteins containing a DNA-binding moti
46、f called a homeodomain.The products are transcription factors that specify segment identity by activating multiple gene expression events.The genes are initially activated imprecisely by the concentration gradients of gap gene products.e.g.Ubx is switched on between certain concentrations of hunchba
47、ck to give a broad band of expression near the middle of the embryo.Later,fushi tarazu and even skipped sharpen the limits of Ubx expression which comes into register with the anterior boundaries of specific parasegments.The BX-C and ANT-C genes have extensive non-coding sequences(introns)that are c
48、ritical in regulating their individual expression.Homeotic-Selector Genes The embryo is shown in side view and in cross-section,displaying the relationship between the dorsoventral subdivision into future major tissue types and the anteroposterior pattern of future segments.A heavy line encloses the
49、 region that will form segmental structures.During gastrulation the cells along the ventral midline invaginate to form mesoderm,while the cells fated to form the gut invaginate near each end of the embryo.Thus,with respect to their role in gut formation,the opposite ends of the embryo,although far a
50、part in space,are close in function and in final fate.Fate map of a Drosophila embryo at the cellular blastoderm stage.The patterns of expression compared to the chromosomal locations of the genes of the HOM complex.The sequence of genes in each of the two subdivisions of the chromosomal complex cor
