Chapter 19Cellular 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 cleavageAs 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 Cells1. 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 – embryothese 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 experimentCells 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 – DrosophilaEmbryogenesis – fertilization to larva
Development begins before fertilizationNurse Cells move mRNA into the eggAfter 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 blastodermnuclei 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 messnegers1. 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 hunchback2.Pair-rule genes – 8+ genes such as hairy
produce 7 distinct bands of protein3. 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 regulate1. number, timing and orientation of divisions2. cell growth and expansion3. changes in cell shape4. cell migration5. 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 differentiation1. 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
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