1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 21 The Genetic Basis of Development

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: From Single Cell to Multicellular Organism Genetic analysis and DNA technology have revolutionized the study of development Researchers use mutations to deduce developmental pathways They apply concepts and tools of molecular genetics to the study of developmental biology

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 21.1: Embryonic development involves cell division, cell differentiation, and morphogenesis In embryonic development of most organisms, a single-celled zygote gives rise to cells of many different types, each with a different structure and corresponding function Development involves three processes: cell division, cell differentiation, and morphogenesis (“creation of form”)

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Through a succession of mitotic cell divisions, the zygote gives rise to a large number of cells In cell differentiation, cells become specialized in structure and function Morphogenesis encompasses the processes that give shape to the organism and its various parts

5. LE 21-4 Animal development Zygote (fertilized egg) Eight cellsBlastula (cross section) Gastrula (cross section) Adult animal (sea star) Cell movement Gut Cell division Morphogenesis Observable cell differentiation Seed leaves Shoot apical meristem Root apical meristem Plant Embryo inside seed Two cells Zygote (fertilized egg) Plant development

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 21.2: Different cell types result from differential gene expression in cells with the same DNA Differences between cells in a multicellular organism come almost entirely from gene expression, not differences in the cells’ genomes These differences arise during development, as regulatory mechanisms turn genes off and on

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evidence for Genomic Equivalence Many experiments support the conclusion that nearly all cells of an organism have genomic equivalence (the same genes) A key question that emerges is whether genes are irreversibly inactivated during differentiation

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Totipotency in Plants One experimental approach for testing genomic equivalence is to see whether a differentiated cell can generate a whole organism A totipotent cell is one that can generate a complete new organism Cloning is using one or more somatic cells from a multicellular organism to make a genetically identical individual

9. LE 21-5 Transverse section of carrot root 2-mg fragments Fragments cul- tured in nutrient medium; stir- ring causes single cells to shear off into liquid. Single cells free in suspension begin to divide. Embryonic plant develops from a cultured single cell. Plantlet is cul- tured on agar medium. Later it is planted in soil. A single somatic (nonreproductive) carrot cell developed into a mature carrot plant. The new plant was a genetic duplicate (clone) of the parent plant. Adult plant

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nuclear Transplantation in Animals In nuclear transplantation, the nucleus of an unfertilized egg cell or zygote is replaced with the nucleus of a differentiated cell Experiments with frog embryos have shown that a transplanted nucleus can often support normal development of the egg

11. LE 21-6 Frog embryoFrog egg cellFrog tadpole UV Less differ- entiated cell Donor nucleus trans- planted Enucleated egg cell Most develop into tadpoles <2% develop into tadpoles Donor nucleus trans- planted Fully differ- entiated (intestinal) cell

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reproductive Cloning of Mammals In 1997, Scottish researchers announced the birth of Dolly, a lamb cloned from an adult sheep by nuclear transplantation from a differentiated mammary cell Dolly’s premature death in 2003, as well as her arthritis, led to speculation that her cells were “older” than those of a normal sheep, possibly reflecting incomplete reprogramming of the original transplanted nucleus

13. LE 21-7 Mammary cell donor Egg cell donor Egg cell from ovary Nucleus removed Cells fused Cultured mammary cells are semistarved, arresting the cell cycle and causing dedifferentiation Nucleus from mammary cell Early embryo Grown in culture Implanted in uterus of a third sheep Surrogate mother Embryonic development Lamb (“Dolly”) genetically identical to mammary cell donor

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Since 1997, cloning has been demonstrated in many mammals, including mice, cats, cows, horses, and pigs “Copy Cat” was the first cat cloned

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Problems Associated with Animal Cloning In most nuclear transplantation studies, only a small percentage of cloned embryos have developed normally to birth Many epigenetic changes, such as acetylation of histones or methylation of DNA, must be reversed in the nucleus from a donor animal in order for genes to be expressed or repressed appropriately for early stages of development

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stem Cells of Animals A stem cell is a relatively unspecialized cell that can reproduce itself indefinitely and differentiate into specialized cells of one or more types Stem cells isolated from early embryos at the blastocyst stage are called embryonic stem cells The adult body also has stem cells, which replace nonreproducing specialized cells Embryonic stem cells are totipotent, able to differentiate into all cell types Adult stem cells are pluripotent, able to give rise to multiple but not all cell types

17. LE 21-9 Embryonic stem cellsAdult stem cells Pluripotent cells Totipotent cells Cultured stem cells Different culture conditions Different types of differentiated cells Liver cellsNerve cellsBlood cells

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transcriptional Regulation of Gene Expression During Development Cell determination precedes differentiation and involves expression of genes for tissue-specific proteins Tissue-specific proteins enable differentiated cells to carry out their specific tasks

19. LE 21-10_2 Nucleus Embryonic precursor cell DNA OFF Master control gene myoDOther muscle-specific genes mRNAOFF Determination Myoblast (determined) MyoD protein (transcription factor)

20. LE 21-10_3 Nucleus Embryonic precursor cell DNA OFF Master control gene myoDOther muscle-specific genes mRNAOFF Determination Myoblast (determined) MyoD protein (transcription factor) Differentiation Muscle cell (fully differentiated) mRNA MyoD mRNA Another transcription factor Myosin, other muscle proteins, and cell-cycle blocking proteins

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cytoplasmic Determinants and Cell-Cell Signals in Cell Differentiation Maternal substances that influence early development are called cytoplasmic determinants These substances regulate expression of genes that affect the cell’s developmental fate Animation: Cell Signaling Animation: Cell Signaling

22. LE 21-11a Sperm Molecules of a cytoplasmic determinant Fertilization Nucleus Molecules of another cytoplasmic determinant Unfertilized egg cell Zygote (fertilized egg) Mitotic cell division Two-celled embryo Cytoplasmic determinants in the egg

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The other important source of developmental information is the environment around the cell, especially signals from nearby embryonic cells In the process called induction, signal molecules from embryonic cells cause transcriptional changes in nearby target cells

24. LE 21-11b Early embryo (32 cells) Signal transduction pathway Signal receptor Signal molecule (inducer) NUCLEUS Induction by nearby cells

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 21.3: Pattern formation in animals and plants results from similar genetic and cellular mechanisms Pattern formation is the development of a spatial organization of tissues and organs It occurs continually in plants, but it is mostly limited to embryos and juveniles in animals Positional information, the molecular cues that control pattern formation, tells a cell its location relative to the body axes and to neighboring cells

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Drosophila Development: A Cascade of Gene Activations Pattern formation has been extensively studied in the fruit fly Drosophila melanogaster Combining anatomical, genetic, and biochemical approaches, researchers have discovered developmental principles common to many other species, including humans

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Life Cycle of Drosophila After fertilization, positional information specifies the body segments in Drosophila Positional information triggers the formation of each segment’s characteristic structures Sequential gene expression produces regional differences in the formation of the segments

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Analysis of Early Development: Scientific Inquiry Study of developmental mutants laid the groundwork for understanding the mechanisms of development Mutations that affect segmentation are likely to be embryonic lethals, leading to death at the embryonic or larval stage

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Axis Establishment Maternal effect genes encode for cytoplasmic determinants that initially establish the axes of the body of Drosophila These maternal effect genes are also called egg- polarity genes because they control orientation of the egg and consequently the fly

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings One maternal effect gene, the bicoid gene, affects the front half of the body An embryo whose mother has a mutant bicoid gene lacks the front half of its body and has duplicate posterior structures at both ends This phenotype suggests that the product of the mother’s bicoid gene is concentrated at the future anterior end This hypothesis is an example of the gradient hypothesis, in which gradients of substances called morphogens establish an embryo’s axes and other features

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The bicoid research is important for three reasons: 1. It identified a specific protein required for some early steps in pattern formation 2. It increased understanding of the mother’s role in embryo development 3. It demonstrated a key developmental principle that a gradient of molecules can determine polarity and position in the embryo

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Segmentation Pattern Segmentation genes produce proteins that direct formation of segments after the embryo’s major body axes are formed Positional information is provided by sequential activation of three sets of segmentation genes: gap genes, pair-rule genes, and segment-polarity genes

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Identity of Body Parts The anatomical identity of Drosophila segments is set by master regulatory genes called homeotic genes Mutations to homeotic genes produce flies with strange traits, such as legs growing from the head in place of antennae

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings C. elegans: The Role of Cell Signaling The nematode C. elegans is a very useful model organism for investigating the roles of cell signaling, induction, and programmed cell death in development Researchers know the entire ancestry of every cell of an adult C. elegans—the organism’s complete cell lineage Video: C. elegans Embryo Development (time lapse) Video: C. elegans Embryo Development (time lapse)

35. LE 21-15 Nervous system, outer skin, mus- culature Time after fertilization (hours) Musculature, gonads 0 10 Zygote First cell division Hatching Outer skin, nervous system Intestine Musculature Germ line (future gametes) Intestine EggsVulva ANTERIOR 1.2 mm POSTERIOR

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Induction As early as the four-cell stage in C. elegans, cell signaling helps direct daughter cells down the appropriate pathways, a process called induction

37. LE 21-16a Anterior EMBRYO Posterior Receptor Signal protein 1 2 3 4 4 3 Signal Posterior daughter cell of 3 Anterior daughter cell of 3 Will go on to form muscle and gonads Will go on to form adult intestine Induction of the intestinal precursor cell at the four-cell stage

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Induction is also critical later in nematode development, as the embryo passes through three larval stages prior to becoming an adult

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A number of important concepts apply to C. elegans and many other animals: – In the developing embryo, sequential inductions drive organ formation – The effect of an inducer can depend on its concentration – Inducers produce their effects via signal transduction pathways, as in adult cells – The induced cell often responds by activating genes that establish a pattern of gene activity characteristic of a particular kind of cell

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Programmed Cell Death (Apoptosis) Cell signaling is involved in apoptosis, programmed cell death During apoptosis, a cell shrinks and becomes lobed (called “blebbing”); the nucleus condenses; and the DNA is fragmented

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In C. elegans, a protein in the outer mitochondrial membrane is a master regulator of apoptosis Research on mammals has revealed a prominent role for mitochondria in apoptosis A built-in cell suicide mechanism is essential to development in all animals

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The timely activation of apoptosis proteins in some cells functions during normal development and growth in both embryos and adult In vertebrates, apoptosis is part of normal development of the nervous system, operation of the immune system, and morphogenesis of hands and feet in humans and paws in other mammals

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plant Development: Cell Signaling and Transcriptional Regulation Genetic analysis of plant development, using model organisms such as Arabidopsis, has lagged behind that of animal models because fewer researchers work on plants Plant research is now progressing rapidly, thanks to DNA technology and clues from animal research

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pattern Formation in Flowers A floral meristem has three layers of cells (L1-L3), all of which participate in forming a flower with four types of organs: carpels (containing egg cells) stamens (containing sperm-bearing pollen) petals sepals (leaflike structures outside the petals)

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings To examine induction of the floral meristem, researchers grafted stems from a mutant tomato plant onto a wild-type plant and then grew new plants from the shoots at the graft sites The new plants were chimeras, organisms with a mixture of genetically different cells The number of organs per flower depended on genes of the L3 (innermost) cell layer

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In contrast to genes controlling organ number in flowers, genes controlling organ identity (organ identity genes) determine the types of structures that will grow from a meristem Organ identity genes are analogous to homeotic genes in animals Mutations cause plant structures to grow in unusual places, such as carpels in place of sepals

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 21.4: Comparative studies help explain how the evolution of development leads to morphological diversity Evolutionary developmental biology (“evo-devo”) compares developmental processes of different multicellular organisms

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Widespread Conservation of Developmental Genes Among Animals Molecular analysis of the homeotic genes in Drosophila has shown that they all include a sequence called a homeobox An identical or very similar nucleotide sequence has been discovered in the homeotic genes of both vertebrates and invertebrates

49. LE 21-23 Adult fruit fly Fruit fly embryo (10 hours) Fly chromosome Mouse chromosomes Mouse embryo (12 days) Adult mouse

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Related genetic sequences have been found in regulatory genes of yeasts, plants, and even prokaryotes In addition to developmental genes, many other genes are highly conserved from species to species Sometimes small changes in regulatory sequences of certain genes lead to major changes in body form, as in crustaceans and insects

51. LE 21-24 ThoraxAbdomen Genital segments ThoraxAbdomen

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In other cases, genes with conserved sequences play different roles in different species In plants, homeobox-containing genes do not function in pattern formation as they do in animals

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparison of Animal and Plant Development In both plants and animals, development relies on a cascade of transcriptional regulators turning genes on or off in a finely tuned series But genes that direct analogous developmental processes differ considerably in sequence in plants and animals, as a result of ancestry