Chapter 19
Cellular Mechanisms of
Development
AP Biology 2011
Development
The process of systematic, gene directed
changes through which an organism forms the
successive stages of its life cycle
Broken into 4 subprocesses
1. Growth (Cell Division)
Controlled by cyclins and cyclin-dependent
kinases (Cdks)
control and direct cell division through the
mitotic checkpoints
Fertilization is followed by cleavage
As division proceeds cells become smaller and
smaller – each cell is known as a blastomere.
These cells can develop into any tissue
Each tissue has its own tissue-specific stem
cell
Stem Cells
1. totipotent – become any cell type
2. pluripotent – become multiple different cell
types
In mammals the cleavage stage lasts five or six
days forming a blastocyst
outer layer – placenta
inner layer – embryo
these cells can be removed and grown in
culture – embryonic stem cells
ES cells can be stimulated to form various
tissues
In groups distinguish plants and animals – refer
to “Plant growth occurs in specific areas called
meristems” pages 372-373
2. Cell Differentiation
Cell determination – commits a cell to a
particular developmental pathway
Determined by experiment
Cells are moved from one place in an embryo
to another, if develop as they would have then
they are determined, if not then they were
not yet determined
Determination depends on intrinsic and extrinsic
events
This all depends on the organism that is to
develop
Many times this depends on stages
Cells can be partially committed – receives
marker that determine will it will be located in
the embryo
Read chicken example – Pages 373 – 374
Inquiry
You will need to read the sections on pages 374
–375
What dictates whether Macho-1 acts as a
transcriptional repressor or a transcriptional
repressor?
3. Pattern Formation
Example Organism – Drosophila
Embryogenesis – fertilization to larva
Development begins before fertilization
Nurse Cells move mRNA into the egg
After fertilization this maternal mRNA directs
development until several mitotic divisions
have taken place
After 12 nuclear divisions without cytokinesis
around 4000 nuclei are formed
Structure is known as a syncytial blastoderm
nuclei can communicate with each other
Nuclei then evenly space themselves and form
membranes – cellular blastoderm
Within a day a segmented tubular body is
formed
The anterior and posterior axis forms during
maturation of the oocyte
Bicoid proteins form the anterior end
Nanos proteins form the posterior end
Other maternal messnegers
1. hunchback – activates genes for the
anterior structures
2. caudal -- activates genes for posterior
(abdominal) structures
The dorsal and ventral axis is formed by the
dorsal gene product
Gurken RNA accumulates to one side of the cell
forming the dorsal side of the cell
The other side without the gurken RNA
developes into the ventral side
The segmented body plan is determined by
segmentation genes
1. Gap genes – 9 genes including hunchback
2. Pair-rule genes – 8+ genes such as hairy
produce 7 distinct bands of protein
3. Segment polarity genes – 9+ genes such as
engrailed which distinguishes A/P of the 7
bands
Next identity needs to be established
Homeotic genes – genes that can produce
similar body parts – understanding came from
mutants
1. Bithorax complex – controls the development
of the rear end of the fly and all of the
abdomen
2. Antennapedia complex – controls the
development of the anterior end of the fly
Compare animal and plant pattern formation.
Look at the homeobox and the MADS-box
4. Morphogenesis
Generation of ordered form and structure
To achieve this animals regulate
1. number, timing and orientation of divisions
2. cell growth and expansion
3. changes in cell shape
4. cell migration
5. cell death
Number, timing and orientation of
divisions
Position of the spindle will determine the size of
each daughter cell – unequal cytokinesis
Changes in cell shape
Shape will lead to differentiation
1. axons (nerve cells) – connect the big toe to
the spinal chord
2. myoplasts – differentiate into
multinucleated muscle cells
Cell death
Some cells are planned to die – apoptosis
Examples
-- webbing between fingers
-- numerous neurons
-- tail of a tadpole