Chapter 21 Reading Quiz 1. When cells become specialized in structure & function, it is called … 2. Name 2 of the 5 “model organisms”. 3. What does it mean to be “totipotent”? 4. What is the name for programmed cell death during development? 5. What is a chimera?

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

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)

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

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

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

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

7. Describe the two sources of information that instruct a cell to express genes at the appropriate time. 1. Information in the cytoplasm of the unfertilized egg, in the form of RNA and protein, that is of maternal origin 2. Chemical signals produced by neighboring embryonic cells; such signals, through a process called “induction” influence the growth and differentiation of adjacent cells

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 1. 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 2. 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 

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)

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

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

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

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 

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)

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