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Chapter 21

Chapter 21. The Genetic Basis of Development. 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

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Chapter 21

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  1. Chapter 21 The Genetic Basis of Development

  2. 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. 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

  4. 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”)

  5. LE 21-3 Fertilized egg of a frog Tadpole hatching from egg

  6. 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

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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

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

  14. 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

  15. 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

  16. LE 21-9 Embryonic stem cells Adult stem cells Totipotent cells Pluripotent cells Cultured stem cells Different culture conditions Different types of differentiated cells Liver cells Nerve cells Blood cells

  17. 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

  18. 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

  19. LE 21-17 2 mm

  20. 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

  21. 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 Ced-4 Active Ced-3 Other proteases Nucleases Activation cascade Death signal

  22. 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

  23. LE 21-19 Interdigital tissue 1 mm

  24. 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

  25. 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

  26. Widespread Conservation of Developmental Genes Among Animals • Homeotic genes are genes that control the overall body plan of animals and plants by controlling the developmental fate of groups of cells • 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

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

  28. 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

  29. LE 21-24 Genital segments Thorax Abdomen Thorax Abdomen

  30. 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

  31. 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

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