1. 2.E.1 timing and coordination
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“timing and coordination”
Timing and coordination of specific events are necessary for the normal development of an organism, and these events are regulated by a variety of mechanisms.
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2. The determination of different cell types (cell fates) precedes differentiation and involves the expression of genes for tissue-specific proteins. Tissue-specific proteins enable differentiated cells to carry out specific tasks.

3. Cell differentiation follows determination as the cell continues on a specific developmental path. Differentiation results in cell types such as nerve cells, blood cells, and muscle cells.

4. A single-celled zygote can develop into a multicellular adult organism that contains hundreds of different cell types.  

5. Totipotent cells are stem cells that can form all the cell types in a body, plus placental cells. Embryonic cells within the first couple of cell divisions after fertilization are the only cells that are totipotent. 

6. Pluripotent cells can give rise to all of the cell types that make up the body; embryonic stem cells are considered pluripotent.

7. The transformation from a zygote into an organism results from three interrelated processes:
Cell division: the zygote gives rise to a large number of cells through a succession of mitotic cell divisions
Cell differentiation: cells become specialized in structure and function
Morphogenesis: the processes that give shape to the organism and its various parts

8. Box #1
Compare cell differentiation and cell division.

9. Genomic Equivalence: Nearly all the cells of an organism have the same genome. Cells differentiate through differential gene expression.

10. Hierarchy of Gene Activity in Early Drosophila Development
Observable cell differentiation results from the expression of genes for tissue-specific proteins.
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

12. Box #2
Describe the function of tissue specific proteins.

13. Regulation of transcription factors during development results in sequential gene expression. Transcription factors are proteins that bind to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA.

14. Cell signaling helps direct daughter cells down the appropriate pathways, a process called induction. Cells induce neighboring cells to differentiate. 4
Anterior
EMBRYO
Posterior
Receptor
Signal
protein
daughter
cell of 3
Will go on to
form muscle
and gonads
form adult
intestine 1 2 3

15. During induction, signal molecules from embryonic cells cause transcriptional changes in nearby target cells.
Early embryo
(32 cells)
Signal
transduction
pathway
NUCLEUS
Signal
receptor
Signal
molecule
(inducer)

16. An inducing signal produced by one cell in the embryo can initiate a chain of inductions that results in the formation of a particular organ.

17. Homeotic genes are involved in developmental patterns and sequences.

18. Box #3
Describe how induction can lead to the development of an organism.

19. Hox genes are a group of related homeotic genes that control the body plan of the embryo.

20. 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.
Occurs continually in plants
Is mostly limited to embryos and juveniles in animals

21. An identical or very similar nucleotide sequence has been discovered in the homeotic genes of both vertebrates and invertebrates.
Adult
fruit fly
Fruit fly embryo
(10 hours)
Fly
chromosome
Mouse
chromosomes
Mouse embryo
(12 days)
Adult mouse

25. Embryonic induction in development results in the correct timing of events.

27. Box # 4
Describe the function of homeotic genes

28. Temperature and the availability of water determine seed germination in most plants.

29. Genetic mutations can result in abnormal development.
Snake with one limb.

30. Box # 5
Describe environmental and mutations effect on development

31.   Genetic transplantation experiments support the link between gene expression and normal development.

32. In nuclear transplantation, the nucleus of an unfertilized egg cell or zygote is replaced with the nucleus of a differentiated cell.

33. Experiments with frog embryos have shown that a transplanted nucleus can often support normal development of the egg.
Frog embryo
Frog egg cell
Frog tadpole
Less differ-
entiated cell
Donor
nucleus
trans-
planted
Enucleated
egg cell
Fully differ-
entiated
(intestinal) cell
Most develop
into tadpoles
<2% develop

34. In 1997, Scottish researchers cloned a lamb from an adult sheep by nuclear transplantation.
Dolly the Sheep

35. Reproductive Cloning

36. “Copy Cat” was the first cat ever cloned.

37. In most nuclear transplantation studies, few cloned embryos develop normally.

38. Genetic regulation by microRNAs plays an important role in the development of organisms and the control of cellular functions.

39.  MicroRNA (miRNA) are a small, non-coding RNA molecules that function in post-transcriptional regulation of gene expression. They generally bind to their target mRNAs and repress protein synthesis by destabilizing the mRNA. 

40. Programmed cell death (apoptosis) plays a role in the normal development and differentiation. Cell signaling is involved in programmed cell death.

41. Example: Morphogenesis of fingers and toes
Apoptosis plays a critical role in the sculpting of digits in vertebrate limbs. The death of the cells that would otherwise form inter‐digital webbing enables individual fingers and toes to be formed.
Example: Morphogenesis of fingers and toes

42. Example: Development in the nematode C. elegans
As early as the four-cell stage in C. elegans, cell signaling helps direct daughter cells down the appropriate pathways. Induction is also critical later in nematode development as the embryo passes through three larval stages prior to becoming an adult. A protein in the outer mitochondrial membrane serves as a master regulator of apoptosis .
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) 10
Hatching
Intestine
Eggs
Vulva
ANTERIOR
POSTERIOR
1.2 mm
Example: Development in the nematode C. elegans

43. Example: Flower development
Floral meristems contain three cell types that affect flower development.
Carpel
Petal
Stamen
Sepal
Floral meristem
Tomato flower
Cell
layers L1 L2 L3
Example: Flower development

44. Box #6
How does apoptosis aid in development?

45. Learning Objectives:
LO 2.31 The student can connect concepts in and across domains to show that timing and coordination of specific events are necessary for normal development in an organism and that these events are regulated by multiple mechanisms. [See SP 7.2]
LO 2.32 The student is able to use a graph or diagram to analyze situations or solve problems (quantitatively or qualitatively) that involve timing and coordination of events necessary for normal development in an organism. [See SP 1.4]
LO 2.33 The student is able to justify scientific claims with scientific evidence to show that timing and coordination of several events are necessary for normal development in an organism and that these events are regulated by multiple mechanisms. [See SP 6.1]
LO 2.34 The student is able to describe the role of programmed cell death in development and differentiation, the reuse of molecules, and the maintenance of dynamic homeostasis. [See SP 7.1]