AP Biology, Chapter 21
The Genetic Basis of Development

FROM SINGLE CELL TO MULTICELLULAR ORGANISM
	Introduction
	Embryonic development involves cell division, cell differentiation, and 	morphogenesis
		1. Distinguish between the patterns of morphogenesis in plants and in animals.
			a. Movements of cells and tissues are required in animals
			b. Plants continue growth and morphogenesis throughout life
	Researchers study development in model organisms to identify general 	principles
		2. List the animals used as models for developmental biology research and 			provide a rationale for their choice.
			a. Drosophila melanogaster, fruit fly
				i. Long used for genetics
				ii. Small with a short generation time
				iii. Complete sequence since 2000
			b. Caenorhabditis elegans, nematode
				i. 959 cell adults; strict cell lineage/cell fates
				ii. Hermaphrodites; easier to recognize recessive mutants
				iii. Complete sequence in hand
			c. Mus musculus, mouse
				i. Long used in medical research
				ii. Complete "knockout" mutagenesis in progress
			d. Danio rerio, zebrafish
				i. Easily grown
				ii. Hatch in two days
			e. Arabidopsis thaliana. a mustard
				i. Miniature, 1000s of offspring in 8-10 weeks
				ii. It’s a plant
				iii. Complete sequence in hand

DIFFERENTIAL GENE EXPRESSION
	Intro
	Different types of cells in an organism have the same DNA
		Intro
			3. Describe how genomic equivalence was determined for plants and 				animals.
		Totipotency in Plants
				a. Genomic equivalence = adult cells have the same genome as 					the zygote
				b. Plant tissue culture
					i. Single plant cell can grow into an adult
					ii. Each cell is totipotent, can form all adult cell types
		Nuclear Transplantation in Animals
				c. Frog cloning
					i. Kill the egg nucleus with UV
					ii. Replace it with another nucleus
					iii. Early embryonic nuclei give the best results
					iv. Tadpole intestinal cell nuclei work 2% of the time
			4. Describe what kinds of changes occur to the genome during 					differentiation.
				a. Chromatin structure changes, for example by methylation
				b. Some genes may be edited like the antibody genes
				c. "Most nuclei probably have all of the genes"
			5. Describe the general process by which the ewe Dolly and the first 				mice were cloned.
				a. Mammary donor cells grown in vitro were arrested in G0
				b. Egg cells had their nuclei removed
				c. Electric current was used to fuse the two cells
				d. Grown in vitro to early embryo stage
				e. Implanted into a surrogate mother
		The Stem Cells of Animals
			6. Describe the two important properties of stem cells. Explain their 				significance to medicine.
				a. Properties
					i. Continually divide
					ii. Embryonic stem cells are immortal, like cancer cells
					iii. Under appropriate conditions will differentiate
					iv. Pluripotent but not totipotent
				b. Significance to medicine
					i. For transplantation, hemocytoblasts in bone marrow 						give all blood cells
					ii. Gives a way to study differentiation
					iii. May be implanted to repair body parts 
	Different cell types make different proteins, usually as a result of 	transcriptional regulation
		7. Describe the molecular basis of determination.
			a. Determination = "irreversible" commitment to cellular fate
			b. Molecular basis
				i. Changes in transcription
				ii.  tissue-specific mRNAs
				ii.  tissue-specific proteins
			c. Example: embryonic precursor cell  myoblast  muscle cell
				i. Precursors become determined when they express myoD
				ii. The myoD protein is a transcription factor
				iii. It turns on other transcription factors for muscle proteins
			d. Technique
				i. Myoblast mRNAs were used to make a cDNA library
				ii. The cDNAs were put next to a promoter
				iii. The cDNAs were individually put in precursor cells
				iv. Some cDNAs, like the one with myoD, caused determination
	Transcriptional regulation is directed by maternal molecules in the cytoplasm 	and signals from other cells
		8. Describe the two sources of information that instruct a cell to express genes at 		the appropriate time.
			a. Cytoplasmic determinants in the egg
				i. mRNAs and proteins are stored in the unfertilized egg
				ii. Many are unevenly distributed, giving a pattern to 						development
			b. Induction by neighboring cells
				i. Signal molecule may bind to a cell-surface receptor
				ii. Induction may result from cell-to-cell contact
GENETIC AND CELLULAR MECHANISMS OF PATTERN FORMATION
	Intro
	Genetic analysis of Drosophila reveals how genes control development: an 	overview
		Intro
		The Life Cycle of Drosophila
		Genetic Analysis of Early Development in Drosophila
	Gradients of maternal molecules in the early embryo control axis formation
		9. Describe how Drosophila were used to explain the basic aspects of pattern 			formation (axis formation and segmentation).
			a. Axis formation
				i. Maternal effect genes (egg-polarity genes) encode the 						cytoplasmic determinants
				ii. Example bicoid
					.Homozygous bicoid mothers have embryos with two 						tails
					.Normal bicoid mRNA and protein are found in the 						anterior
					.Injection of normal bicoid mRNA causes anterior 						structures to form wherever it is injected
				iii. Normal bicoid protein is a morphogen determining the 					anterior end by its gradient
	A cascade of gene activations sets up the segmentation pattern in Drosophila: 	a closer look
			b. Segmentation pattern
				i. Egg-polarity gene proteins are transcription factors for gap 					genes
				ii. Sequence of activations
					.Gap genes map out major blocks of segments
					.Pair-rule genes define adjoining segments
					.Segment polarity genes define anterior-posterior axis of 					each segment
	Homeotic genes direct the identity of body parts
		10. Describe how homeotic genes serve to identify parts of the developing 			organism.
			a. Code for transcription factors that act after segment polarity genes
			b. Trigger the formation of structures in specific segments
			c. Mutants form normal looking parts in the wrong segments 
	Homeobox genes have been highly conserved in evolution
		11. Provide evidence of the conservation of homeobox patterns.
			a. Homeobox = 180-base sequence common to all homeotic genes
			b. Yields the homeodomain in the finished protein
			c. Mammalian Hox genes have a very similar sequence
			d. Vertebrate homeotic genes have the same linkage as the fly genes
	Neighboring cells instruct other cells to form particular structures: cell 	signaling and induction in the nematode
		Intro
			12. Describe how the study of nematodes contributed to the general 				understanding of embryonic formation.
				a. Sequential inductions drive organ formation
				b. Effect of an inducer can depend on its concentration
				c. Inducers use signal-transduction pathways
				d. Induced cell's response is gene activation
		Induction in Vulval Development
		Programmed Cell Death (Apoptosis)
			13. Describe how apoptosis functions in normal and abnormal 					development.
				a. Apoptosis = programmed cell death
				b. 131 specific occurrences in nematode development
					i. Mediated by a cell surface receptor and signal 							transduction pathway
					ii. Ultimately, proteases and nucleases are activated
				c. Very common in other organisms and for other purposes 
					i. Between fingers and toes; frogs lose their tails
					ii. Apoptosis kills virus-infected and cancer cells
	Plant development depends on cell signaling and transcriptional regulation
		Intro
		Cell Signaling in Flower Development
			14. Describe how the study of tomatoes has contributed to the 					understanding of flower development.
				a. Mutant tissue was transplanted to floral meristems
				b. Fasciated mutant cells can induce increases in flower part 					number from the third cell layer down
		Organ Identity Genes in Plants 
			15. Describe how the study of Arabidopsis has contributed to the 				understanding of organ identity in plants.
				a. Arabidopsis flower mutants allowed identification of organ 					identity genes
				b. Organ identity genes are analogous to animal homeotic genes
				c. Both encode transcription factors