Chapter 21

1. Chapter 21 The Genetic Basis of Development

2. 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

7. Concept 21.1: Embryonic development involves cell division, cell differentiation, and morphogenesis • 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

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

9. 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

10. 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

11. 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. 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. 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

15. 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 https://www.youtube.com/watch?v=tELZEPcgKkE Grown in culture Early embryo Implanted in uterus of a third sheep Surrogate mother Embryonic development Lamb (“Dolly”) genetically identical to mammary cell donor

16. Since 1997, cloning has been demonstrated in many mammals, including mice, cats, cows, horses, and pigs • “Copy Cat” was the first cat cloned, in College Station, TX at TAMU. • http://www.youtube.com/watch?v=qIHq2pR6UxA&feature=related

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. Follicle cell Egg cell developing within ovarian follicle Nucleus 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 Thorax Abdomen Head 0.5 mm Dorsal BODY AXES Anterior Posterior Ventral

25. Eye Leg Antenna Wild type Mutant

26. 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

27. 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

28. LE 21-14b Egg cell Nurse cells 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

29. 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

30. 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

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

32. Induction • As early as the four-cell stage in C. elegans, cell signaling directs daughter cells along the appropriate pathways, a process called induction

33. 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

34. 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

35. LE 21-17 2 mm

36. 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

37. 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

38. Interdigital tissue 1 mm

39. Adult fruit fly Fruit fly embryo (10 hours) Fly chromosome Mouse chromosomes Mouse embryo (12 days) Adult mouse

40. Genital segments Thorax Abdomen Thorax Abdomen