AP Biology
Ch.21.2
Development
21.2 Different cell types result from differential gene expression in cells with the same DNA.
 Cells have equivalent genomes (genes are the same)
 differences between cells come from differences in gene expression.
 exception: antibody-producing cells
Experimental evidence of totipotence:
Totipotence: The ability of cells to give rise to any specialized cell types.
This results in cloning
Used extensively in agriculture.
Example: growing a new plant from a cutting.
Nuclear Transplantation in Animals
Differentiated cells from animals generally do not divide in culture, much less
develop into the multiple cell types of a new organism. A different approach
was necessary.
Reproductive Cloning in animals
Conclusion: At least some differentiated (somatic) cells in plants are
totipotent, able to reverse their differentiation and then give rise to all
the cell types in a mature plant.
Conclusion: The nucleus from a differentiated frog cell
can direct development of a tadpole. However, its
ability to do so decreases as the donor cell becomes
more differentiated, presumably because of changes in
the nucleus.
Reproductive cloning of mammals.
Can a nucleus from a fully differentiated cell be “reprogrammed” to be totipotent?
Yes, 1997, Scotland. Dolly the sheep.
Mrs. Loyd 
cloyd@waukee.k12.ia.us
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Conclusion: The cloned animal is identical in appearance and
genetic makeup to the donor animal supplying the nucleus, but
differs from the egg cell donor and surrogate mother.
ESSENTIAL KNOWLEDGE
2.e.1 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.
ILLUSTRATIVE EXAMPLE: Morphogenesis of
fingers and toes, development of C. elegans
Effect of apoptosis during paw development in the mouse. In mice, humans, and other mammals, as well as land birds,
the embryonic region that develops into feet or hands initially has a solid, platelike structure. Apoptosis eliminates the cells in
the interdigital regions, thus forming the digits.
ESSENTIAL KNOWLEDGE
Mrs. Loyd 
cloyd@waukee.k12.ia.us
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4.A.3: Interactions between external stimuli and regulated
gene expression result in specialization of cells, tissues
and organs. No illustrative example given.
C. elegans: The Role of Cell Signaling
 Ultimate basis for the differences between cells is
transcriptional regulation, turning on and off of specific
genes.
 Induction: signaling from one group of cells to an adjacent
group, that brings about differentiation.
 May also lead to apoptosis, programmed cell death.
 Image below traces a cell lineage.
 Image right: A signal protein on the surface of cell 4
induces events in cell 3 that determine the fate of the
posterior daughter cell of cell 3. The fates of the cells
arising from the anterior daughter cell are determined by
later events.
Induction of the intestinal precursor cell during nematode
development illustrates a number of important concepts that
apply elsewhere in the development of C. elegans and many
other animals:
 In the developing embryo, sequential inductions drive the
formation of organs.
 The effect of an inducer can depend on its concentration.
 Inducers produce their effects via signal transduction
pathways similar to those operating in adult cells.
Mrs. Loyd 
cloyd@waukee.k12.ia.us
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ESSENTIAL KNOWLEDGE
3.B.2 A variety of intercellular and intracellular signal transmissions mediate gene expression.
ILLUSTRATIVE EXAMPLE: Hox genes and their role in development.
“Evo-devo”
Biologists in the field of evolutionary developmental biology,
evo-devo, compare developmental processes of different
multicellular organisms.
 How do developmental processes evolve and how do
changes in these processes modify existing organismal
features or lead to new ones.
 Genomes of organisms that look very different may have
only minor differences in gene sequence or regulation
 Evidence sheds light of evolution
Widespread Conservation of Developmental Genes Among
Animals.
Homeotic genes or Hox genes control the overall body plan of
animals and plants by controlling the developmental fate of
groups of cells.
 Homeotic genes in Drosophila have shown they all include a
180-nucleotide sequence called a homeobox
 which specifies a 60-amino acid homeodomain in the protein.
 Very similar to homeotic genes of invertebrates and
vertebrates.
 Related sequences have been found in regulatory genes of
plants and yeasts, even prokaryotes.
 Similarities show that Homeobox DNA sequence evolved very
early in the history of life and was valuable enough to
organisms to be conserved in animals and plants virtually
unchanged for hundreds of millions of years.
Homeotic genes (Hox genes) that control the form of anterior
and posterior structures of the body occur in the same linear
sequence on chromosomes in Drosophila and mice.
 Each colored band on the chromosomes shown here
represents a homeotic gene.
 In fruit flies, all homeotic genes are found on one
chromosome.
 The mouse and other mammals have the same sets of
genes on four chromosomes.
 The color code indicates the parts of the embryos in which
these genes are expressed and the adult body regions
that result.
 All of these genes are essentially identical in flies and mice
 Except for those represented by black bands, which are less
similar in the two animals.
Mrs. Loyd 
cloyd@waukee.k12.ia.us
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