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Chapter 21 The Genetic Basis of Development PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • When the primary research goal is to understand broad biological principles, the organism chosen for study is called a model organism • Researchers select model organisms that are representative of a larger group, suitable for the questions under investigation, and easy to grow in the lab Video: C. elegans Crawling Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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”) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-3 Fertilized egg of a frog Tadpole hatching from egg • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-4 Animal development Cell movement Zygote (fertilized egg) Eight cells Gut Blastula Gastrula Adult animal (cross section) (cross section) (sea star) Cell division Morphogenesis Observable cell differentiation Seed leaves Plant development Zygote (fertilized egg) Two cells Shoot apical meristem Root apical meristem Embryo inside seed Plant 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 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-5 Transverse section of carrot root 2-mg fragments Fragments cultured in nutrient medium; stirring causes single cells to shear off into liquid. Single cells free in suspension begin to divide. Embryonic plant develops from a cultured single cell. 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 Plantlet is cultured on agar medium. Later it is planted in soil. 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-6 Frog embryo Frog egg cell Frog tadpole UV Fully differentiated (intestinal) cell Less differentiated cell Donor nucleus transplanted Most develop into tadpoles Enucleated egg cell Donor nucleus transplanted <2% develop into tadpoles 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-7 Mammary cell donor Egg cell donor Egg cell from ovary Cultured mammary cells are semistarved, arresting the cell cycle and causing dedifferentiation Nucleus removed Cells fused Nucleus from mammary cell Grown in culture Early embryo Implanted in uterus of a third sheep Surrogate mother Embryonic development Lamb (“Dolly”) genetically identical to mammary cell donor • Since 1997, cloning has been demonstrated in many mammals, including mice, cats, cows, horses, and pigs • “Copy Cat” was the first cat cloned Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-9 Embryonic stem cells Totipotent cells Adult stem cells Pluripotent cells Cultured stem cells Different culture conditions Different Liver cells types of differentiated cells Nerve cells Blood cells 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-10_1 Nucleus Master control gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF LE 21-10_2 Nucleus Master control gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF mRNA OFF Determination Myoblast (determined) MyoD protein (transcription factor) LE 21-10_3 Nucleus Master control gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF mRNA OFF Determination MyoD protein (transcription factor) Myoblast (determined) Differentiation mRNA MyoD Muscle cell (fully differentiated) mRNA Another transcription factor mRNA mRNA Myosin, other muscle proteins, and cell-cycle blocking proteins 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-11a Unfertilized egg cell Sperm Molecules of another cytoplasmic determinant Molecules of a Nucleus cytoplasmic Fertilization determinant Zygote (fertilized egg) Mitotic cell division Two-celled embryo Cytoplasmic determinants in the egg • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-11b Early embryo (32 cells) NUCLEUS Signal transduction pathway Signal receptor Signal molecule (inducer) Induction by nearby cells 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 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-12 Follicle cell Egg cell developing within Nucleus ovarian follicle Egg cell Nurse cell Fertilization Laying of egg Fertilized egg Egg shell Nucleus Embryo Multinucleate single cell Early blastoderm Plasma membrane formation Yolk Late blastoderm Body segments Cells of embryo Segmented embryo 0.1 mm Hatching Larval stages (3) Pupa Metamorphosis Adult fly Head Thorax Abdomen 0.5 mm Dorsal BODY AXES Anterior Posterior Ventral 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-13 Eye Leg Antenna Wild type Mutant 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 eggpolarity genes because they control orientation of the egg and consequently the fly 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-14a Tail Head Wild-type larva Tail Tail Mutant larva (bicoid) Drosophila larvae with wild-type and bicoid mutant phenotypes LE 21-14b Nurse cells Egg cell Developing egg cell bicoid mRNA Bicoid mRNA in mature unfertilized egg Fertilization Translation of bicoid mRNA 100 m m Bicoid protein in early embryo Anterior end Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo Animation: Development of Head-Tail Axis in Fruit Flies 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 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-15 Zygote Time after fertilization (hours) 0 First cell division Nervous system, outer skin, musculature Musculature, gonads Outer skin, nervous system Germ line (future gametes) Musculature 10 Hatching Intestine Intestine Eggs Vulva ANTERIOR POSTERIOR 1.2 mm 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-16a 2 Anterior 1 4 Posterior Signal Receptor protein 3 EMBRYO 4 3 Signal Anterior daughter cell of 3 Will go on to form muscle and gonads Posterior daughter cell of 3 Will go on to form adult intestine Induction of the intestinal precursor cell at the four-cell stage • Induction is also critical later in nematode development, as the embryo passes through three larval stages prior to becoming an adult Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-16b Epidermis LARVA Gonad Anchor cell Signal protein Vulval precursor cells ADULT Inner vulva Outer vulva Epidermis Induction of vulval cell types during larval development • 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-17 2 mm • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-18 Ced-9 protein (active) inhibits Ced-4 activity Mitochondrion Death signal receptor Ced-4 Ced-3 Inactive proteins No death signal Cell forms blebs Ced-9 (inactive) Death signal Active Active Ced-4 Ced-3 Activation cascade Death signal Other proteases Nucleases • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-19 Interdigital tissue 1 mm 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mechanisms of Plant Development • In general, cell lineage is much less important for pattern formation in plants than in animals • The embryonic development of most plants occurs inside the seed, and thus is relatively inaccessible to study • However, other important aspects of plant development are observable in plant meristems, particularly apical meristems at the tips of shoots • In meristems, cell division, morphogenesis, and differentiation give rise to new organs 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) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-20 Carpel Stamen Cell layers Petal L1 L2 L3 Sepal Floral meristem Anatomy of a flower Tomato flower • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-21 Sepal Petal Carpel Stamen Fasciated (ff), extra organs Wild-type, normal Graft Chimeras L1 Key Wild-type (FF) Fasciated (ff) Plant Flower L2 L3 Floral meristem Phenotype Wild-type parent Wild-type Fasciated (ff) parent Fasciated Chimera 1 Fasciated Chimera 2 Fasciated Chimera 3 Wild-type Floral Meristem • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-22 Wild type Mutant 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-23 Adult fruit fly Fruit fly embryo (10 hours) Fly chromosome Mouse chromosomes Mouse embryo (12 days) Adult mouse • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 21-24 Thorax Thorax Genital segments Abdomen Abdomen • 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings