Midterm exam: A more detailed look… • 45 multiple-choice questions (1 point each = 45 points) • 5 math questions w/ calculator (1 point each = 5 points) • 1 “long” (3-part) FRQ (30 points = 30 points) • 3 “short answer” FRQ parts (20 pts = 20 points) • TOTAL = 100 points We’ll be doing an in-class review session on Friday and additional review during the scheduled afternoon sessions next week.
Ch. 21: The Genetic Basis of Development The development of an organism from a zygote to an embryo to an adult is a delicate process that involves much genetic control!
Chapter 21: The Genetic Basis of Development • How do we study development in the genetics-based lab? -Model organisms -fruit fly, nematode worm, mouse, etc.
DROSOPHILA MELANOGASTER (FRUIT FLY) CAENORHABDITIS ELEGANS (NEMATODE) Figure 21.2 Model Organisms for Genetic Studies of Development Drosophila - small, easy & cheap to culture - 2 week generation time - 4 chromosomes - LARGE literature of info C elegans - easy to culture - transparent body with few cell types - zygote to mature adult in 3 days 0.25 mm
Chapter 21: The Genetic Basis of Development Zygote 0 First cell division Germ line (future gametes) Outer skin, nervous system Nervous system, outer skin, mus- culature Musculature, gonads Time after fertilization (hours) Musculature 10 Hatching Intestine Intestine Eggs Vulva ANTERIOR POSTERIOR 1.2 mm -The complete cell lineage of the C. elegans nematode is known…
ARABIDOPSIS THAMANA (COMMON WALL CRESS) MUS MUSCULUS (MOUSE) DANIO RERIO (ZEBRAFISH) Mouse - vertebrate - LARGE literature - transgenics & knock-outs
Chapter 21: The Genetic Basis of Development (b) Tadpole hatching from egg (a) Fertilized eggs of a frog Figure 21.3a, b • How does a zygote transform into an organism? • Cell division 2) Cell differentiation 3) Morphogenesis—”creation of form/shape”
(a) Animal development. Most animals go through some variation of the blastula and gastrula stages. The blastula is a sphere of cells surrounding a fluid-filled cavity. The gastrula forms when a region of the blastula folds inward, creating a tube—a rudimentary gut. Once the animal is mature, differentiation occurs in only a limited way—for the replacement of damaged or lost cells. Gut Cell movement Zygote (fertilized egg) Eight cells Blastula (cross section) Gastrula (cross section) Adult animal (sea star) Cell division Morphogenesis Observable cell differentiation (b) Plant development. In plants with seeds, a complete embryo develops within the seed. Morphogenesis, which involves cell division and cell wall expansion rather than cell or tissue movement, occurs throughout the plant’s lifetime. Apical meristems (purple) continuously arise and develop into the various plant organs as the plant grows to an indeterminate size. Seed leaves Shoot apical meristem Root apical meristem Zygote (fertilized egg) Two cells Embryo inside seed Plant Figure 21.4a, b
Chapter 21: The Genetic Basis of Development • How do we study development in the genetics-based lab? • How does a zygote transform into an organism? • How do cells become differentiated? -All cells have the same DNA, so differential gene expression must be the explanation!
Researchers enucleated frog egg cells by exposing them to ultraviolet light, which destroyed the nucleus. Nuclei from cells of embryos up to the tadpole stage were transplanted into the enucleated egg cells. EXPERIMENT Frog tadpole Frog egg cell Frog embryo Fully differ- entiated (intestinal) cell Less differ- entiated cell Donor nucleus trans- planted Enucleated egg cell Donor nucleus trans- planted <2% develop into tadpoles Most develop into tadpoles Figure 21.6 • -Once a cell is differentiated, it’s difficult to “de-differentiate.”
Transverse section of carrot root CONCLUSION EXPERIMENT 2-mg fragments Fragments cul- tured in nutrient medium; stir- ring causes single cells to shear off into liquid. Single cells free in suspension begin to divide. Embryonic plant develops from a cultured single cell. Plantlet is cul- tured on agar medium. Later it is planted in soil. A single Somatic (nonreproductive) carrot cell developed into a mature carrot plant. The new plant was a genetic duplicate(clone) of the parent plant. RESULTS Adult plant At least some differentiated (somatic) cells in plants are totipotent, able to reverse their differentiation and then give rise to all the cell types in a mature plant. Figure 21.5 However, plants behave differently…
Egg cell donor Mammary cell donor APPLICATION This method is used to produce cloned animals whose nuclear genes are identical to the donor animal supplying the nucleus. 1 2 Egg cell from ovary Nucleus removed Nucleus removed Cells fused Cultured mammary cells are semistarved, arresting the cell cycle and causing dedifferentiation 3 TECHNIQUE Shown here is the procedure used to produce Dolly, the first reported case of a mammal cloned using the nucleus of a differentiated cell. Nucleus from mammary cell Grown in culture 4 RESULTS The cloned animal is identical in appearance and genetic makeup to the donor animal supplying the nucleus, but differs from the egg cell donor and surrogate mother. Early embryo Implanted in uterus of a third sheep 5 Surrogate mother Embryonic development 6 Lamb (“Dolly”) genetically identical to mammary cell donor Figure 21.7
Chapter 21: The Genetic Basis of Development Embryonic stem cells Adult stem cells Early human embryo at blastocyst stage (mammalian equiva- lent of blastula) From bone marrow in this example Totipotent cells Pluripotent cells Cultured stem cells Different culture conditions Liver cells Blood cells Nerve cells Different types of differentiated cells Figure 21.9 4. What is a stem cell? -a relatively unspecialized cell -can differentiate into cells of different types under specific conditions -Embryonic = totipotent -Adult = pluripotent (can produce some, but not all, cell types)
Chapter 21: The Genetic Basis of Development 2 1 Nucleus Master control gene myoD Other muscle-specific genes DNA OFF OFF Embryonicprecursor cell Determination. Signals from othercells lead to activation of a masterregulatory gene called myoD, andthe cell makes MyoD protein, atranscription factor. The cell, nowcalled a myoblast, is irreversiblycommitted to becoming a skeletalmuscle cell. OFF mRNA MyoD protein(transcriptionfactor) Myoblast (determined) Differentiation. MyoD protein stimulatesthe myoD gene further, and activatesgenes encoding other muscle-specifictranscription factors, which in turn activate genes for muscle proteins. MyoD also turns on genes that block the cell cycle, thus stopping cell division. The nondividing myoblasts fuse to become mature multinucleate muscle cells, alsocalled muscle fibers. mRNA mRNA mRNA mRNA Myosin, othermuscle proteins,and cell-cycleblocking proteins MyoD Anothertranscriptionfactor Muscle cell(fully differentiated) 5. What type of genetic signal leads to cell differentiation? -Step 1: Cell receives signals from other cells -Step 2: A regulatory gene is turned “on”, and a protein is made that activates other genes. (“point of no return”) -Step 3: Activated genes make proteins that determine cell type/ structure/behavior.
Chapter 21: The Genetic Basis of Development 2 Posterior 1 Anterior Signal protein 4 3 Receptor EMBRYO 3 4 Signal Anterior daughter cell of 3 Posterior daughter cell of 3 Will go on to form muscle and gonads Will go on to form adult intestine (b) Induction by nearby cells. The cells at the bottom of the early embryo depicted here are releasing chemicals that signal nearby cells to change their gene expression. (a) Induction of the intestinal precursor cell at the four-cell stage. 6. What is induction? -signal molecules (proteins or hormones) from embryonic cells cause trancriptional changes in nearby target cells.
Epidermis Signal protein Gonad Anchor cell Vulval precursor cells Outer vulva ADULT Inner vulva Epidermis Figure 21.16b (b) Induction of vulval cell types during larval development.
EXPERIMENT RESULTS CONCLUSION Spemann and Mangold transplanted a piece of the dorsal lip of a pigmented newt gastrula to the ventral side of the early gastrula of a nonpigmented newt. Pigmented gastrula (donor embryo) Dorsal lip of blastopore Nonpigmented gastrula (recipient embryo) During subsequent development, the recipient embryo formed a second notochord and neural tube in the region of the transplant, and eventually most of a second embryo. Examination of the interior of the double embryo revealed that the secondary structures were formed in part from host tissue. Primary embryo Primary structures: Secondary (induced) embryo Secondary structures: Neural tube Notochord Notochord (pigmented cells) Neural tube (mostly nonpigmented cells) The transplanted dorsal lip was able to induce cells in a different region of the recipient to form structures different from their normal fate. In effect, the dorsal lip “organized” the later development of an entire embryo. Figure 47.25
Chapter 21: The Genetic Basis of Development Unfertilized egg cell Molecules of a a cytoplasmic determinant Fertilization Nucleus Zygote (fertilized egg) Mitotic cell division Two-celled embryo (a) Cytoplasmic determinants in the egg. The unfertilized egg cell has molecules in its cytoplasm, encoded by the mother’s genes, that influence development. Many of these cytoplasmic determinants, like the two shown here, are unevenly distributed in the egg. After fertilization and mitotic division, the cell nuclei of the embryo are exposed to different sets of cytoplasmic determinants and, as a result, express different genes. Figure 21.11a -cytoplasmic determinants in the unfertilized egg regulate gene expression in the zygote that affects differentiation/development
Chapter 21: The Genetic Basis of Development Tail Head T1 A8 T2 A7 T3 A6 A1 A5 A2 A4 A3 Wild-type larva Tail Tail A8 A8 A7 A6 A7 Mutant larva (bicoid) (a) Drosophila larvae with wild-type and bicoid mutant phenotypes. A mutation in the mother’s bicoid gene leads to tail structures at both ends (bottom larva). The numbers refer to the thoracic and abdominal segments that are present. Figure 21.14a -Cytoplasmic determinants from mother’s egg initially establish the axes of the body of Drosophila. -bicoid gene
Egg cell Nurse cells Developing egg cell 1 bicoid mRNA Bicoid mRNA in mature unfertilized egg 2 Fertilization 100 µm Translation of bicoid mRNA Bicoid protein in early embryo 3 Anterior end (b) Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo. Figure 21.14b
Chapter 21: The Genetic Basis of Development Hierarchy of Gene Activity in Early Drosophila Development Maternal effect genes (egg-polarity genes) Gap genes Segmentation genes of the embryo Pair-rule genes Segment polarity genes Homeotic genes of the embryo Other genes of the embryo -7. How does morphogenesis (pattern formation) occur in animals? After the body’s axes are determined (by cytoplasmic determinants)… -Segmentation genes produce proteins that direct formation of body segments. -Then, the development of specific features of the body segments is directed by HOMEOTIC GENES (Hox genes.)
Chapter 21: The Genetic Basis of Development Adult fruit fly Fruit fly embryo (10 hours) Fly chromosome Mouse chromosomes Mouse embryo (12 days) Adult mouse Figure 21.23 8. What is the relationship among the genetic basis of development across organisms? -Molecular analysis of the homeotic genes in Drosophila has shown that they all include a sequence called a homeobox -An identical (or very similar) DNA sequence has been discovered in the homeotic genes of vertebrates and invertebrates
Chapter 21: The Genetic Basis of Development Ced-9 protein (active) inhibits Ced-4 activity Mitochondrion Ced-4 Ced-3 Death signal receptor Inactive proteins Cell forms blebs (a) No death signal Ced-9 (inactive) Death signal Active Ced-3 Active Ced-4 Other proteases Nucleases Activation cascade (b) Death signal 9. What is apoptosis? -programmed cell death (cell suicide) Figure 21.18 Molecular basis of apoptosis in C. elegans
Chapter 21: The Genetic Basis of Development Interdigital tissue 1 mm Figure 21.19 8. What is apoptosis? -programmed cell death (cell suicide) -necessary for development of hands/feet in vertebrates
Unit 6 Test (2012-13) Please take your test folder and learning log from the table by the door! • Average: 19 / 30 • Range: 8 – 28 • Learning Logs: 8/10 needed for test corrections • 5 points for completion • 5 points for accuracy • Test Corrections: Due Wednesday, January 22nd • FOCUS ON MIDTERM PREP!!!
Welcome! • Before you begin… • 1) Please turn in food under computer tables and sign the log sheet. Thank you! • 2) Leave your test folder on top of your desk. I’ll collect them during the testing period and record your 10 point “learning log” bonus. • 3) Sign the honor code sheet on the front of your test packet. You may NOT write in the multiple-choice test booklet. (You may write on the math sheet and on the FRQ sheet.) • 4) Use the green side of your scantron, and make sure that you mark your KEY ID as either B or C. • 5) Make sure that your name is on your scantron, math sheet, and FRQ answer sheet before you turn them in. • 6) Good luck! I’m proud of your hard work this semester and look forward to finishing out the year! • *I have your Unit 6 Test FRQs graded if you’re interested in seeing them at the end of the period. • *MIDTERM test grades will be available on Tuesday, Jan. 22nd…we need to calculate the test average for ALL students before assigning a curve!