Embryology

Three interrelated precesses are crucial to development:

1. Cell Division- through a large number of mitotic divisions, the zygote produces many cells
2. Cell differentiation- cells become specialized in structure and function
3. Morphogenesis- "creation of form" different cells are organized into tissues and organs

• In animals, movements of cells and tissues are necessary to transform an early embryo into the 3D form of an organism
• In plants, morphogenesis and growth occur throughout the entire life of the plant

• Be small and easily grown
• Have a short generation time
• Produce many offspring
• Embryos develop outside of mother
• No ethical issues

Differental Gene Expression

• Cells in multicellular organisms are copied via mitosis and therefore, are exactly identical
• Differences between cells are due not to differeneces ingenomes, but rather, due to differences in gene expression (which genes are turned on/off)
• Expression differences occur during development as reglatory mechanisms turn specific genes on and off

• FC. Steward
• Took differentiated cell roots, planted them, and noted they grew into normal plants
• A mature cell dedifferentiated, therefore differentiation does not necessarily involve irreversible changes in the DNA

• Destroyed the nucleus of a frog cell, and transplanted the nucleus of another frog cell into the egg cells
• Nuclei of undifferentiated cells usually resulted in new tadpoles
• Nuclei of differentiated cells resulted in very few new tadpoles
• Conclusion: Nuclei do change in some ways as cells differentiate: base sequence is still the same of the DNA, but the chromatin (DNA and proteins that make up a chromosome) structure is altered

• Ethical backlash
• Safety issues with cloning of farm animals- lack of diversity
• Only a tiny percentage of cloned embryos develop normally (DNA of embryos seems to be improperly  methylated, which helps to regulate gene expression)

1. Continually reproduce
2. Yet to differentiate

1. Existing stem cell lines
2. Miscarried or aborted embryos
3. Unused embryos from in vitro fertilization
4. Cloned embryos

• In cytoplasm of egg cell
• Contains RNA and proteins encoded for by mother's DNA
• Once mitosis begins, new cells are exposed to different cytoplasmic enviroments and, as a result, expresses different genes
• Collectively the materval substances are referreed to as cytoplasmic determinants

• Enviroment outside of the cell
• Most importantly, the signals from neighboring ebryonic cells

The development of spatial organization

• Connected developmental abnormalities to mutations in specific genes
• First evidence that genes direct developmetal processes

Nusslein-Volhard and Wieschaus

• Substances that initially establish the axes of the Drosophila body
• Present in unfertilized egg
• Coded for by the genes of the mother- MATERNAL EFFECT GENES

• Genes of the Embryo that directs the formation of the segments of the embryo
• Consists of three different sets of genes:

1. Gap Genes- map out basic subdivision
2. Pair-Rule Genes- define the segment pairs
3. Segment Polarity Genes- set the front-back axis

• Product of one set of genes directly activated next set
• Net result: boundaries and axes of the segments of the embryo are set
• The next genes to be expressed determine the anatomy of each segment of the embryo

• Set the anatomical identity of each segment
• Specify the types of apendages and other structures that each segment will form
• Mutations in the homeotic genes cause a certain part of an animal to arise in the wrong place
• Many homeotic genes encode transcription factors that control the expression of genes responsible for anatomical structures

1. Maternal Effect Genes
2. Gap Genes
3.  Pair Rule Genes
4. Segement Polarity Genes
5. Homeotic Genes of the Embryo
6. Other Genes of the Embryo

• Specific 180 nucleotide sequence of Drosophila
• Master reglaroty genes which specify the type of appendages and structures that form on each segment
• Highly conserved in evolution: identical or similar sequence found in genes of many other organisms
• Crucial for the proper placement of structures
• Produces a 60 amino acid homeodomain- part of a protein that binds to DNA when the protein acts as a transcription factor

Apoptosis

• Programmed cell death
• essential to development of:

1. Nervous system
2. Immune system
3. Morphogenesis of hands/ feet in humans

• Failure of apoptosis of mutated cells can lead to cancer
• Inappropriate activation of it can lead to certain degereative nercous system disorders