Transcript Differentiation

Development
AP Biology
Big Questions:
1. How does a multicellular organism
2.
3.
4.
5.
develop from a zygote?
How is the development of an animal
different or similar from the development
of a plant?
How is the position of the parts of an
organism determined?
How does differentiation of cell type
occur in animals and plants?
How are genetics and development
connected? Environment?
Essential Knowledge:
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.
AP Biology
Gamete formation
Fertilization
Four processes of development:
Cleavage
• Determination sets the fate of
Gastrulation
the cell.
Organogenesis • Differentiation is the process
by which different types of cells
Growth
arise.
• Morphogenesis shapes
differentiated cells into organs,
etc.
• Growth is an increase in body
size by cell division and cell
expansion.
A program of differential gene expression leads to the
different cell types in a multicellular organism
 Embryonic development--fertilized eggs give rise to different
cell types
 Tissuesorgansorgan systemswhole organism
 Gene expression orchestrates development; zygoteadult
results from cell division, cell differentiation, morphogenesis
and then growth

(a) Fertilized eggs of a frog
(b) Newly hatched tadpole
Fertilization
 Formation of zygote – union of male &

female gametes
Purpose? Variation/creates a diploid cell
Triggers development of the egg
Cytoplasmic Determinants
and Inductive Signals
 Unfertilized egg—uneven distribution of
RNA, proteins, other substances
 Cytoplasmic determinants—molecules
in oocyte that influence early
development
–
Zygote dividescells have different
determinantsdifferential gene
expression
 Another source of developmental
info--environment (e.g. signals from
nearby embryonic cells)
 Induction--signal molecules from
embryonic cells cause transcriptional
changes in nearby target cells
Interactions between cells
induce differentiation of
specialized cell types
blastomere-cytoplasmic
determinants distributed/
create first “totipotent” cells
Cleavage
 Embryo divides into cells


called “blastomeres”
Hollow ball forms of
somatic cells called a
blastula
These cells are also called
germ cells (totipotent
stem cells!!!)
**NO growth – just cell
division
Morula….Blastula
Gastrulation
“conversion of blastula into germ layers”
 Invagination – blastula forms
horseshoe shape
-Gastrocoel forms (stomach
cavity)
-Blastopore forms (“germ
hole”)
Gastrula stage forms 2 germ
layers: Endoderm & Ectoderm
(incomplete gut)
“Diploblastic”
Comb Jelly
Gastrulation: Formation of
complete gut
 Cavity extends through animal
(endoderm and ectoderm layers join)
Gastrulation: Formation of
Mesoderm (3rd germal layer)
 Usually formed by pieces of endoderm
breaking away
 Animals are termed “Triploblastic”
 Triploblastic animals can be
Protostomes (blastopore forms mouth
first) or Deuterostomes (blastopore
forms the anus 1st and mouth is formed
2nd)
Endoderm – inner layer
forms the digestive system, inner lining of
respiratory system
Ectoderm – outer layer
forms the skin and nervous system
Mesoderm – middle layer
forms muscle, bone, reproductive system,
circulatory system, etc.
Acoelomate-no body
cavity
Pseudocoelomatefalse body cavity
Coelomate-true body
cavity (cavity lined
with mesoderm)
Organogenesis
 Cell differentiation
 Formation of coelom

(cavity)
Body organs form
Development is Regulated
 The development
of an organism is
coordinated by
sequential
changes in gene
expression.
One Cell  Organism
Cellular Differentiation
 Accomplished by the expression of cell typespecific proteins.
Stem Cells
Transcriptional Regulations
 Determination vs. Differentiation
 Determination sets the fate of the cell.
 Differentiation is the process by which
different types of cells arise.
Sequential Regulation of Gene Expression During Cellular
Differentiation
Myoblasts—make muscle-specific proteins, form
skeletal muscle cells.
MyoD--“master regulatory genes”; makes proteins
that commit cell to becoming skeletal muscle (50+)
MyoD—TF--binds to enhancers of target genes
Pattern Formation
 Cells need to establish their position in
the developing organism. This is
established through protein gradients.
Embryonic Induction
 Local signals communicated among
populations of cells to control their
development
Apoptosis
 “Programmed cell death”: Important
role in defining borders and openings
in the developing
organism.
Environmental Cues
 The presence of particular molecules
and conditions in the local environment
is required for development to proceed
properly.

Ex. The role of temperature and
moisture in seed development.
Experimental Evidence
 Mutations in normal developmental
pathways lead to malformations in
embryonic development
Experimental Evidence
 Transplantation Experiments: Moving
regions of a developing embryo affects
normal pattern formation.
Essential Knowledge:
Interactions between external
stimuli and regulated gene
expression result in specialization
of cells tissues and organs.
AP Biology
Control of Differentiation
 Differentiation

is regulated by
internal and
external cues.
These cues
“switch”
specific genes
“on” and “off”
at specific
times.
Turning On AND Turning Off
 Genes must be both turned

on and turned off at
appropriate times and
locations in the developing
organism.
Transcription factors, and
micro RNA’s both function
in regulating gene
expression.
Pattern Formation: Setting Up the
Body Plan/Axis Establishment
 Pattern formation-development of spatial
organization of tissues
and organs—studied in
fruit flies
 Animal pattern formation
starts with formation of
major axes
 Positional information-molecular cues that
control pattern formation- gives cell location
relative to the body axes
and neighboring cells
The Life Cycle of Drosophila
Head
Thorax
0.5 mm
 Pre-fertilization--cytoplasmic determinants
determine axes
 After fertilization--embryo develops into
segmented larva with 3 larval stages
 Edward Lewis, Christiane NüssleinVolhard, and Eric Wieschaus--Nobel 1995
Prize--decoding pattern formation in
Drosophila
discovered that homeotic genes direct the
developmental process; studied segment
formation
–
discovered embryonic lethals with lethal
mutations
–
found 120 genes essential for normal
segmentation
Antenna
Abdomen
Dorsal
BODY Anterior
AXES
Left
Right
Posterior
Ventral
(a) Adult
Follicle cell
1 Egg cell
Nucleus
developing within
ovarian follicle
Egg
cell
Nurse cell
Egg
shell
2 Unfertilized egg
–
Depleted
nurse cells
Fertilization
Laying of egg
3 Fertilized egg
Embryonic
development
4 Segmented
embryo
0.1 mm
Body
segments
Leg
Wild type
5 Larval stage
Leg
Hatching
Eye
(b) Development from egg to larva
Mutant
Hox
Genes
Hox Genes
 A family of related
genes that serve
as master
regulators of
animal
development in all
animals on the
planet.
AP Biology
Internal Cues
 Various transcription factors must be

present inside a cell to allow for
specific genes to be expressed.
Transcription factors can be either
stimulatory or inhibitory.
External Cues
 External cues signal to cells, causing
cells to alter their gene expression.
Differentiation leads to Divergence
 As cells differentiate, the proteins that
they express commit them to particular
“fates”. These fates are normally
irreversible.
Don’t Forget The Environment!
 The environment of the cell can also
play a major role by contributing to
gene expression in mature cells.