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