Download 1. Distinguish between the patterns of morphogenesis in plants and

Chapter 21 Reading Quiz
1.
2.
3.
4.
5.
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?
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.
2.
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 
8. Describe how Drosophila were used to
explain basic aspects of pattern formation
(axis formation and segmentation).


1.
2.
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 
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 