Genetic Basis for Developmnet


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  • Cytoplasmic determinants in the egg. The unfertilized egg cell has molecules in its cytoplasm, encoded by the mother’s genes, that influence development. Many of these cytoplasmic determinants, like the two shown here, are unevenly distributed in the egg. After fertilization and mitotic division, the cell nuclei of the embryo are exposed to different sets of cytoplasmic determinants and, as a result, express different genes.
  • Genetic Basis for Developmnet

    1. 2. The Genetic Basis of Development How do cells with the same genes grow up to be so different?
    2. 3. Three Procceses of Development <ul><li>The transformation from a zygote into an organism </li></ul><ul><ul><li>Results from three interrelated processes: cell division, cell differentiation, and morphogenesis </li></ul></ul>Figure 21.3a, b (a) Fertilized eggs of a frog (b) Tadpole hatching from egg
    3. 4. <ul><li>Through a succession of mitotic cell divisions </li></ul><ul><ul><li>The zygote gives rise to a large number of cells </li></ul></ul><ul><li>In cell differentiation </li></ul><ul><ul><li>Cells become specialized in structure and function </li></ul></ul><ul><li>Morphogenesis encompasses the processes </li></ul><ul><ul><li>That give shape to the organism and its various parts </li></ul></ul>
    4. 5. Some key stages of development in animals and plants Animal 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 gastrula forms when a region of the blastula folds inward, creating a tube—a rudimentary gut. Once the 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) 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 ) (a) (b)
    5. 6. Differential gene expression <ul><li>Nearly all the cells of an organism have genomic equivalence, that is, they have the same genes </li></ul><ul><li>Differences between cells in a multicellular organism </li></ul><ul><ul><li>differences in gene expression </li></ul></ul><ul><ul><li>not from differences in the cells’ genomes </li></ul></ul>
    6. 7. <ul><li>yields a variety of cell types </li></ul><ul><li>each expressing a different combination of genes </li></ul><ul><li>multicellular eukaryotes </li></ul><ul><ul><li>cells become specialized as a zygote develops into a mature organism </li></ul></ul>Cell Differentiation 0
    7. 8. <ul><ul><li>Different types of cells </li></ul></ul><ul><ul><ul><li>Make different proteins because different combinations of genes are active in each type </li></ul></ul></ul>Cell Diferentiation 0 Muscle cell Pancreas cells Blood cells
    8. 9. <ul><li>may retain all of their genetic potential </li></ul><ul><ul><li>Most retain a complete set of genes </li></ul></ul><ul><ul><li>May be totipotent </li></ul></ul>Differentiated cells 0
    9. 10. Totipotency in Plants 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. Plantlet is cultured 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 At least some differentiated (somatic) cells in plants are toipotent, able to reverse their differentiation and then give rise to all the cell types in a mature plant. EXPERIMENT RESULTS CONCLUSION
    10. 12. DNA packing in a eukaryotic chromosome <ul><li>Wound around clusters of histone proteins, forming a string of beadlike nucleosomes </li></ul><ul><li>This beaded fiber is further wound and folded </li></ul><ul><li>DNA packing tends to block gene expression </li></ul><ul><ul><ul><li>Presumably by preventing access of transcription proteins to the DNA </li></ul></ul></ul>DNA double helix (2-nm diameter) Histones Linker “ Beads on a string” Nucleosome ( 10-nm diameter ) Tight helical fiber (30-nm diameter ) Supercoil (300-nm diameter ) Metaphase chromosome 700 nm TEM TEM
    11. 13. <ul><li>X chromosome inactivation in the cells of female mammals </li></ul>An Extreme Example of DNA Packing 0 Early embryo X chromosomes Allele for orange fur Allele for black fur Cell division and random X chromosome inactivation Two cell populations in adult Active X Inactive X Inactive X Active X Orange fur Black fur
    12. 14. Nuclear Transplantation <ul><li>The nucleus of an unfertilized egg cell or zygote is replaced with the nucleus of a differentiated cell </li></ul>
    13. 16. How to Clone a Sheep Bill Ritchie (Produced for the 1997 Royal Agricultural Show) <ul><li>The Egg </li></ul><ul><ul><li>The unfertilized eggs are flushed out of a sheep which has been induced to produce a larger than normal number of eggs. </li></ul></ul><ul><li>The Cell </li></ul><ul><ul><li>Previously a sample of tissue was from the udder of a six year old ewe was taken and cultured in a dish (Dolly 1). </li></ul></ul><ul><ul><li>The cultured cells are starved to send them into a resting or quiescent state . </li></ul></ul><ul><li>The fusion </li></ul><ul><ul><li>A cell is placed beside the egg and an electric current used to fuse the couplet . </li></ul></ul>
    14. 17. How to Clone a Sheep <ul><li>Culture </li></ul><ul><ul><li>The reconstructed embryo is put into culture and grows for seven days. </li></ul></ul><ul><li>Development </li></ul><ul><ul><li>Embryos which grow successfully are taken and transferred to a sheep which is at the the same stage of the oestrus cycle as the egg. </li></ul></ul><ul><ul><li>The sheep becomes pregnant and produces a lamb after 21 weeks (Dolly). </li></ul></ul>
    15. 18. “Copy Cat” <ul><ul><li>Was the first cat ever cloned </li></ul></ul>Figure 21.8
    16. 19. The Stem Cells of Animals <ul><li>A stem cell </li></ul><ul><ul><li>Is a relatively unspecialized cell </li></ul></ul><ul><ul><li>Can reproduce itself indefinitely </li></ul></ul><ul><ul><li>Can differentiate into specialized cells of one or more types, given appropriate conditions </li></ul></ul>
    17. 20. Embryonic and Adult Stem Cells <ul><li>Stem cells can be isolated </li></ul><ul><ul><li>From early embryos at the blastocyst stage </li></ul></ul><ul><li>Adult stem cells </li></ul><ul><ul><li>pluripotent, able to give rise to multiple but not all cell types </li></ul></ul>Totipotent cells Liver cells Nerve cells Early human embryo at blastocyst stage (mammalian equiva- lent of blastula ) From bone marrow in this example Pluripotent cells Cultured stem cells Different culture conditions Different types of differentiated cells Blood cells Embryonic stem cells Adult stem cells
    18. 21. <ul><li>Complex assemblies of proteins control eukaryotic transcription </li></ul><ul><ul><li>A variety of regulatory proteins interact with DNA and with each other </li></ul></ul><ul><ul><ul><li>To turn the transcription of eukaryotic genes on or off </li></ul></ul></ul>Transcriptional Regulation of Gene Expression During Development 0
    19. 22. <ul><li>Assist in initiating eukaryotic transcription </li></ul>Transcription Factors 0 Enhancers Promoter Gene DNA Activator proteins Other proteins Transcription factors RNA polymerase Bending of DNA Transcription
    20. 23.   Determination and differentiation of muscle cells DNA OFF OFF Master control gene myoD Other muscle-specific genes Nucleus Embryonic precursor cell
    21. 24.   Determination and differentiation of muscle cells DNA OFF OFF OFF mRNA MyoD protein (transcription factor) Master control gene myoD Determination. Signals from other cells lead to activation of a master regulatory gene called myoD , and the cell makes MyoD protein, a transcription factor. The cell, now called a myoblast, is irreversibly committed to becoming a skeletal muscle cell. 1 Other muscle-specific genes Nucleus Myoblast (determined) Embryonic precursor cell
    22. 25.   Determination and differentiation of muscle cells DNA OFF OFF OFF mRNA mRNA mRNA mRNA mRNA Another transcription factor MyoD Muscle cell (fully differentiated ) MyoD protein (transcription factor ) Myoblast (determined ) Embryonic precursor cell Myosin, other muscle proteins, and cell-cycle blocking proteins Other muscle-specific genes Master control gene myoD Nucleus Determination. Signals from other cells lead to activation of a master regulatory gene called myoD , and the cell makes MyoD protein, a transcription factor. The cell, now called a myoblast, is irreversibly committed to becoming a skeletal muscle cell. 1 Differentiation. MyoD protein stimulates the myoD gene further, and activates genes encoding other muscle-specific transcription 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, also called muscle fibers. 2
    23. 26. Cytoplasmic Determinants and Cell-Cell Signals in Cell Differentiation <ul><li>Cytoplasmic determinants in the cytoplasm of the unfertilized egg </li></ul><ul><ul><li>Regulate the expression of genes in the zygote that affect the developmental fate of embryonic cells </li></ul></ul>Sperm Molecules of another cyto- plasmic deter- minant Unfertilized egg cell Molecules of a a cytoplasmic determinant Fertilization Zygote (fertilized egg) Mitotic cell division Two-celled embryo Nucleus Sperm
    24. 28. Induction <ul><li>Signal molecules from embryonic cells cause transcriptional changes in nearby target cells </li></ul>Early embryo (32 cells) NUCLEUS Signal transduction pathway Signal receptor Signal molecule (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)
    25. 30. Pattern Formation <ul><li>Pattern formation in animals and plants results from similar genetic and cellular mechanisms </li></ul><ul><li>Pattern formation </li></ul><ul><ul><li>Is the development of a spatial organization of tissues and organs </li></ul></ul><ul><ul><li>Occurs continually in plants </li></ul></ul><ul><ul><li>Is mostly limited to embryos and juveniles in animals </li></ul></ul>
    26. 31. Cell Positioning <ul><li>Positional information </li></ul><ul><ul><li>Consists of molecular cues that control pattern formation </li></ul></ul><ul><ul><li>Tells a cell its location relative to the body’s axes and to other cells </li></ul></ul>
    27. 32. <ul><li>Cascades of gene expression and cell-to-cell signaling direct the development of an animal </li></ul><ul><ul><li>Early understanding of the relationship between gene expression and embryonic development </li></ul></ul><ul><ul><ul><li>Came from studies of mutants of the fruit fly Drosophila melanogaster </li></ul></ul></ul>THE GENETIC CONTROL OF EMBRYONIC DEVELOPMENT 0 Eye Antenna Leg SEM 50  Head of a normal fruit fly Head of a developmental mutant
    28. 33. Key Developmental Genes are Very Ancient <ul><ul><li>Homeotic genes contain nucleotide sequences, called homeoboxes </li></ul></ul><ul><ul><ul><li>That are very similar in many kinds of organisms </li></ul></ul></ul>0 Fly chromosome Mouse chromosomes Fruit fly embryo (10 hours ) Mouse embryo (12 days) Adult fruit fly Adult mouse
    29. 34. <ul><li>Key developmental events in the life cycle of Drosophila </li></ul>Figure 21.12 Follicle cell Nucleus Egg cell Fertilization Nurse cell Egg cell developing within ovarian follicle Laying of egg Egg shell Nucleus Fertilized egg Embryo Multinucleate single cell Early blastoderm Plasma membrane formation Late blastoderm Cells of embryo Yolk Segmented embryo Body segments 0.1 mm Hatching Larval stages (3 ) Pupa Metamorphosis Head Thorax Abdomen 0.5 mm Adult fly Dorsal Anterior Posterior Ventral BODY AXES
    30. 36. Bicoid 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 T2 T3 A1 A2 A3 A4 A5 A6 A7 A8 A8 A7 A6 A7 A8 Tail
    31. 37. Summary of Gene Activity During Drosophila Development Hierarchy 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 genes of the embryo
    32. 38. C. elegans : The Role of Cell Signaling <ul><li>The complete cell lineage </li></ul><ul><ul><li>Of each cell in the nematode roundworm C. elegans is known </li></ul></ul>Figure 21.15 Zygote Nervous system, outer skin, mus- culature Musculature, gonads Outer skin, nervous system Germ line (future gametes) Musculature First cell division Time after fertilization (hours) 0 10 Hatching Intestine Intestine Eggs Vulva ANTERIOR POSTERIOR 1.2 mm
    33. 39. Induction <ul><li>As early as the four-cell stage in C. elegans </li></ul><ul><ul><li>Cell signaling helps direct daughter cells down the appropriate pathways, a process called induction </li></ul></ul>Figure 21.16a 4 Anterior EMBRYO Posterior Receptor Signal protein Signal Anterior daughter cell of 3 Posterior daughter cell of 3 Will go on to form muscle and gonads Will go on to form adult intestine 1 2 4 3 3 (a)
    34. 40. Induction <ul><li>also critical later in nematode development </li></ul><ul><ul><li>As the embryo passes through three larval stages prior to becoming an adult </li></ul></ul>Figure 21.16b Epidermis Gonad Anchor cell Signal protein Vulval precursor cells Inner vulva Outer vulva Epidermis ADULT
    35. 41. Programmed Cell Death (Apoptosis) <ul><li>In apoptosis </li></ul><ul><ul><li>Cell signaling is involved in programmed cell death </li></ul></ul>2 µm Figure 21.17
    36. 42. <ul><li>In vertebrates </li></ul><ul><ul><li>Apoptosis is essential for normal morphogenesis of hands and feet in humans and paws in other animals </li></ul></ul>Figure 21.19 Interdigital tissue 1 mm
    37. 43. Plant Development: Cell Signaling and Transcriptional Regulation <ul><li>Thanks to DNA technology and clues from animal research </li></ul><ul><ul><li>Plant research is now progressing rapidly </li></ul></ul>
    38. 44. Mechanisms of Plant Development <ul><li>In general, cell lineage </li></ul><ul><ul><li>Is much less important for pattern formation in plants than in animals </li></ul></ul><ul><li>The embryonic development of most plants </li></ul><ul><ul><li>Occurs inside the seed </li></ul></ul>
    39. 45. Pattern Formation in Flowers <ul><li>Floral meristems </li></ul><ul><ul><li>Contain three cell types that affect flower development </li></ul></ul>Figure 21.20 Carpel Petal Stamen Sepal Anatomy of a flower Floral meristem Tomato flower Cell layers L1 L2 L3
    40. 46. Organ Identity Genes <ul><li>Organ identity genes </li></ul><ul><ul><li>Determine the type of structure that will grow from a meristem </li></ul></ul><ul><ul><li>Are analogous to homeotic genes in animals </li></ul></ul>Figure 21.22 Wild type Mutant