AP Biology

Big Questions:
1.
How does a multicellular organism develop from a zygote?
2.
How is the development of an animal different or similar from the development of a plant?
3.
How is the position of the parts of an organism determined?
4.
How does differentiation of cell type occur in animals and plants?
5.
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
Gastrulation
• Determination sets the fate of
Organogenesis the cell.
• Differentiation is the process
Growth by which different types of cells 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 (3 rd 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 1 st and mouth is formed
2 nd )


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

Pre-fertilization--cytoplasmic determinants determine axes

After fertilization--embryo develops into segmented larva with 3 larval stages

Edward Lewis, Christiane N üsslein-
Volhard, 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
Head
3 Fertilized egg
Thorax
Nurse cell
2 Unfertilized egg
Depleted nurse cells
Abdomen
0.5 mm
BODY
AXES
(a) Adult
Dorsal
Right
Anterior
Left
Ventral
Posterior
1 Egg cell developing within ovarian follicle
Follicle cell
Nucleus
Egg cell
Egg shell
Fertilization
Laying of egg
Embryonic development
4 Segmented embryo
0.1 mm
Hatching
Body segments
Leg
Wild type
Leg
Mutant
5 Larval stage
Eye
(b) Development from egg to larva

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.