1. Differential Gene Expression
Development
Differential Gene Expression

2. Development development consists of a series of changes
zygote => embryo => immature => mature
developmental changes continue until death

3. development in multicellular eukaryotes Figure 19.1

4. The Processes of Development
development consists of
growth
cell division + cell expansion
differentiation
cellular specialization
morphogenesis
pattern formation
due to the regulated expression of appropriate sets of genes

5. The Processes of Development
determination
for “early” embryonic cells
developmental potential is greater than developmental fate
“later” embryonic cells become determined
developmental fate is fixed
determined cells eventually differentiate into mature forms
determination precedes differentiation

6. Cell Differentiation: Differential Gene Expression
the zygote is totipotent
contains the entire genetic constitution (information)
capable of forming all adult tissues (expression)
differentiated cells retain genetic constitution
some cells remain totipotent

7. plant parenchyma cells remain totipotent in mature, fully developed tissues Figure 19.3

8. Cell Differentiation: Differential Gene Expression
early embryonic animal cells are naturally totipotent
nuclei transferred to unfertilized eggs can direct normal development
genomic equivalence
nuclear “fate swapping”
separation of early embryo cells - naturally or artificially - produces identical twins

9. Cell Differentiation: Differential Gene Expression
adult animals do not possess naturally totipotent cells
some differentiated adult cell nuclei can be “made” totipotent again

10. some differentiated animal cells can be “reprogrammed”… Figure 19.4

11. Figure 19.5

12. culture of undifferentiated embryonic stem cells Figure 19.6

13. Cell Differentiation: Differential Gene Expression
embryonic stem cells are naturally nearly totipotent
can be cultured in an undifferentiated state
appropriate treatment causes differentiation
therapeutic cloning would produce replacement tissues from stem cell cultures
nuclear transfer would produce compatible stem cell cultures

14. Cell Differentiation: Differential Gene Expression
adult stem cells can be isolated from differentiated tissues
exhibit varying degrees of determination
pleuripotent (or pluripotent)
able to form all cell types belonging to their source tissue

15. Cell Differentiation: Differential Gene Expression
as cells differentiate, different genes are expressed
molecular tools can demonstrate genomic equivalence and differential gene expression
Southern blotting can locate any gene in any cell type
Northern blotting locates only certain mRNAs in each cell type

16. Cell Differentiation: Differential Gene Expression
as cells differentiate, different genes are expressed
MyoD1 encodes a transcription factor that directs myoblasts to develop into skeletal muscle fibers
experimentally, MyoD1 can cause other precursor cells to become muscle fibers

17. Cell Differentiation: Differential Gene Expression
as cells differentiate, different genes are expressed
each cell type has a master gene that begins its specific expression program
expression of a cell’s master gene is regulated by signals peculiar to its location

18. Polarity Contributes to Cell Determination
eggs, zygotes, and embryos have unequal distributions of cellular components
one end differs from the other
different developmental events occur at different locations

19. sea urchin: apical/basal polarity… Figure 19.7

20. unequal distribution of cytoplasmic determinants Figure 19.8
initial divisions
segregate
materials of the egg cytoplasm into different embryo cells

21. Embryonic Induction Contributes to Cell Determination
inducers secreted by some embryonic tissues direct the development of neighboring cells
induction may be reciprocal between neighboring tissues

22. reciprocal induction in the embryonic development of the frog’s eye Figure 19.9

23. Embryonic Induction Contributes to Cell Determination
Caenorhabditis elegans
developmental model system
fertilized egg => 959 cell adult
pattern of division & development is mapped

24. a full millimeter of developmental excitement!! Figure 19.10

25. Embryonic Induction Contributes to Cell Determination
Caenorhabditis elegans
developmental model system
fertilized egg => 959 cell adult
pattern of division & development is mapped
induction directs vulval development
anchor cell secretes inducer
the nearest neighbor receives the signal; activates gene 1
adjacent neighbors receive two signals; activate genes 1 and 2

26. Pattern Formation in Organ Development
apoptosis contributes to pattern formation by removing groups of cells
programmed cell death
mediated by specific gene products

27. apoptosis in development of the human hand Figure 19.11

28. flower organ identity genes
Figure 19.12

29. Pattern Formation in Organ Development
organ identity genes interact to form flower parts
mutations produce inappropriate organs in a flower

30. leafy activates transcription of A, B & C
Figure 19.13

31. Pattern Formation in Organ Development
cells of a developing tissue receive positional information in the form of morphogens
signals are secreted by specific groups of cells
diffusion of signal molecules produces a morphogen gradient
the concentration of morphogen at a particular cell determines its developmental response

32. Establishing Body Segmentation: Differential Gene Expression
the Drosophila adult consists of dissimilar body segments that develop from similar larval segments
maternal effect genes establish polarity
express morphogens in nurse cells
deposited locally in an egg
gradients of morphogens develop
some determine dorsal-ventral axis
others determine anterior-posterior axis

33. maternal effect genes Figure 19.14
Bicoid

34. Establishing Body Segmentation: Differential Gene Expression
segmentation gene products organize larval segmentation
gap genes, activated by maternal effect genes, organize anterior-posterior regions
pair rule genes define pairs of segments
segment polarity genes define boundaries & anterior-posterior organization of segments
homeotic genes determine roles of segments

35. maternal effect genes induce gap genes induce pair rule genes induce segment polarity genes & homeotic genes Figure 19.15

36. antennapedia is a homeotic mutation Figure 19.16

37. Establishing Body Segmentation: Differential Gene Expression
homeotic gene mutations can produce bizarre developmental effects
bithorax mutants produce two winged segments
antennapedia produces legs in place of antennae
homeotic genes are homologous
share the 180-bp homeobox
encode the 60-a.a. homeodomain in their polypeptide products