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 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 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 Gap genes – 9 genes including hunchback 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

Bithorax complex – controls the development of the rear end of the fly and all of the abdomen Antennapedia complex – controls the development of the anterior end of the fly

Compare animal and plant pattern formation 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