Chapter 21 Reading Quiz

2. Chapter 21 Reading Quiz • When cells become specialized in structure & function, it is called … • Name 2 of the 5 “model organisms”. • What does it mean to be “totipotent”? • What is the name for programmed cell death during development? • What is a chimera?

3. 1. Distinguish between the patterns of morphogenesis in plants and in animals. • Morphogenesis  development for the overall shape • Animals  movement of cells and tissues are involved in the development of the physical form • Plants  not limited to embryonic and juvenile periods as it is in animals - roots and shoot tips of plants possess apical meristems for continuous growth 

5. 2. List the animals used as models for developmental biology research and provide a rationale for their choice. • Drosophila melanogaster easily grown in lab, short generation time, embryos outside mom’s body • Caenorhabditis elegans  nematode; easily grown, transparent body, cell types arise in same way, hermaphroditic, short generation time • Danio rerio  zebrafish; small and easy to breed, transparent embryo, rapid embryonic development, small genome size • Mus musculus  mouse, more complex organism, yet much is known (background info) 

7. 3. Describe how genomic equivalence was determined for plants and animals. • Genomic equivalence  nearly all of the cells of an organism have the same genes • Because the cells of animals will not often divide in culture, scientists have adopted alternative approaches to examine genomic equivalence: transplanting nuclei of differentiated cells into enucleated egg cells of frogs • Plants’ genomic equivalence was demonstrated by experiments in which entire individuals developed from differentiated somatic cells 

8. 4. Describe what kinds of changes occur to the genome during differentiation. • Earliest changes are subtle and at the molecular level  known as determination • Differences among the cells of a multicellular organism arise from different patterns of gene expression, not differences in the genomes of the cells • Transplantation (frog egg) showed that the nuclei do change in some ways during differentiation • Changes do not occur to the sequence of DNA but rather in chromatin structure 

10. 5. Describe the general processes by which “Dolly” was cloned. • The nucleus of a dedifferentiated mammary cell from one sheep was transplanted into an unfertilized, enucleated egg of another sheep 

12. 6. Describe the molecular basis of determination. • The result of determination is the presence of tissue-specific proteins characteristic of a cell’s structure and function 

14. 7. Describe the two sources of information that instruct a cell to express genes at the appropriate time. • Information in the cytoplasm of the unfertilized egg, in the form of RNA and protein, that is of maternal origin • Chemical signals produced by neighboring embryonic cells; such signals, through a process called “induction” influence the growth and differentiation of adjacent cells 

16. 8. Describe how Drosophila were used to explain basic aspects of pattern formation (axis formation and segmentation). • Pattern formation  the spatial organization of tissues and organs characteristic of a mature organism • Identified how specific molecules influence position and direct differentiation • The life cycle  fruit flies are segmented: head, thorax, abdomen - cytoplasmic determinants provide positional information - after fertilization, orientation of segments and development of associated structures is initiated • Genetic analysis of early development - using mutants identified 1200 genes essential for development of which 120 are for segmentation - various determinants in the cytoplasm control the expression of segmentation genes continued 

17. Number 8 continued…. Axis formation  • Gradients of maternal molecules in the early embryo control axis formation (maternal effect genes) • One set helps to establish anterior-posterior axis of the embryo • Second set is involved with the dorsal-ventral axis • The means by which maternal effect genes influence pattern formation is exemplified by the BICOID gene (essential for the anterior end) 

19. 9. Describe how homeotic genes serve to identify parts of the developing organism. • Homeotic genes  master regulatory genes • Encode for transcription factors that influence the genes responsible for specific structures • Ex: homeotic proteins produced in cells of a particular thoracic segment lead to leg development • Homeotic mutations replace structures characteristic of one part of an animal with structures normally found at some other location 

20. 10. Provide evidence of the conservation of homeobox sequences. • The homeotic genes of Drosophila all contain a 180 nucleotide sequence called the homeobox • Sequences identical or very similar to the homeobox of Drosophila have been discovered in other invertebrates and vertebrates along with yeast and prokaryotes • Such sequence similarity suggests that the homeobox sequence emerged early during the evolution of life • Not all homeobox genes serve as homeotic genes, yet most homeobox genes are associated with some aspect of development 

21. 11. Describe how the study of nematodes contributed to the general understanding of embryonic induction. • Sequential inductions control organ formation • The effect of an inducer can depend on its concentration • Inducers operate through signal systems similar to those in adult organisms • Induction results in the selective activation or inactivation of specific genes within the target cell • Increasing concentration of inducers stimulate division and differentiation 

22. 12. Describe how apoptosis functions in normal and abnormal development. • Apoptosis  selective, programmed cell death • Normal pattern formation depends on apoptosis - occurs 131 times during normal development - chemical signals initiate the activation of a cascade of “suicide genes” • Abnormal  certain degenerative diseases and cancers may have their basis in faulty apoptotic mechanisms 

24. 13. Describe how the study of tomatoes has contributed to the understanding of flower development. • Environmental cues (ex: day length) initiate processes that convert shoot meristems to flower meristems • This induction is exemplified by tomato flowers • Mixing mutant and wild-type plants resulted in floral meristems in which the three cell layers did not all arise from the same “parent” • These layers’ sources were traced and it was determined that the number of organs/flowers depended on genes in the L3 cell layer (the innermost) 

26. 14. Describe how the study of Arabidopsis has contributed to the understanding of organ identity in plants. • Organ-identity genes determine the type of structure that will grow from a meristem - they are analogous to homeotic genes - they are divided into 3 classes: A, B, and C  these 3 genes direct the formation of four types of organs • They appear to be acting like master regulatory genes that control the transcription of other genes directly involved in plant morphogenesis - do Not contain the homeobox sequence 