Developmental Biology Chapter 16

I. Evolutionary significance  A) All animals and plants do sexual reproduction the same way  B)Gamete nuclei fuse  (fertilization) to form a zygote  C) Embryo grows from zygote by mitosis

II. Vertebrate Zygotes undergo:  1.Cell division: mitosis & cytokinesis of heterogeneous cytoplasm  2.Cell differentiation: specialization of structure & function  3. Morphogenesis: development of form by organizing cell types into tissues & organs

III. Cytoplasmic Determinants: substances in eggs, influence development  A. cytoplasmic mRNA & proteins unevenly distributed in egg  B. After 1 st cytokinesis… nuclei of diff. cells exposed to diff. cytoplasmic determinants  C. cytoplasmic determinants influence gene expression & thereby differentiation

IV. Cell Differentiation (specialization)  A) Determination: molecular events determine what differentiation will take place (pre-determined in protostome)  B) Differentiation  1. expression of tissue-specific genes  2. sets of genes sequentially expressed  3. tissue-specific proteins give the cell its characteristic structure & function  4. transcription control most important  (induction of transcription factors)

 C) Skeletal Muscle example  1. determination creates myoblasts by activating ‘master genes’ (example myoD)  2. these regulatory genes create regulatory proteins called…  (transcription factors) that activate groups of tissue specific genes  3. muscle specific version of genes for contractile proteins create lots of  a. actin b. myosin  4. cells fuse to form multinucleate elongate muscle cells

V. Embryonic Induction:  A) an interaction btwn an (inducing) tissue and another (responding) tissue  B) direct timing of developmental steps & differentiation  C) most important mechanism leading to differences between cells & to the  organization of cells into tissues & organs.  D) caused by  1) direct contact btwn cell-surface proteins  2) secreted growth factors (signal mol.)

Vertebrate camera eye development

VI. Apoptosis: programmed cell death (morphogenesis mechanism)  A) signal transduction pathways activate cascade of suicide proteins  B) cell ‘blebs’: partitions into vesicles.  C) vesicles engulfed by phagocytes  D) genetic similarity in apoptosis genes indicate its evolution early in eukaryote line (yeast, animals)

V. Pattern Formation: correctly place tissues (morphogenesis)  A. 3 major body axes  1. anterior-posterior  (head – tail)  2. dorsal – ventral  (back – belly)  3. left – right

 B. pattern formation is determined  1. before fertilization by…  a. Maternal cytoplasmic determinants  b. control anterior/posterior  c. control dorsal/ventral  2. After fertilization pattern formation  controlled by Homeotic genes

 D. morphogen gradient hypothesis  1. maternal effect genes make mRNA that is concentrated in one region of egg  2. mRNA makes its pattern forming protein (morphogen) after fertilization  3. High concentration of that morphogen at one end causes diffusion toward opposite end of embryo setts up gradient  4. cell determination governed by relative concentrations of morphogens

 E) Embryonic Genes Take over  1. after ant/post and dors/vent have been established  2. mRNA of egg-polarity genes broken down by miRNA  3. embryo’s own genes take over  a. homeotic genes  b. transcription factor regulatory genes play major role in pattern formation  c. evo-devo …evolutionary developmental biology

VI. Cloning  A. Organism develops from single cell  1. no meiosis  2. no fertilization  3. genetically identical to 1 st cell  4.first done with carrots  a. differentiated plant cells can dedifferentiate and give rise to all types of cells  b. totipotent cells can become any cell

 B. Differentiated Animal cells  1. don’t divide in culture  2. can not make other cell types  C. Nuclear Transplant  1. dedifferentiates nucleus  2. nucleus placed in enucleated egg  3. egg develops into organism  4. Dolly

 D. Reproductive cloning problems  1. high embryo mortality  2. DNA methylations affect gene regulation  3. Telomeres not restored = premature aging

 E. Therapeutic Cloning – stem cells to treat disease (eggs donated IF clinics)  1. nucleus from person with disease used for nuclear transfer  a. cells to study  b. cells to treat patient  2. Ethical issues

VII. Stem Cells: reproduce indefinitely & can differentiate into specific cell types  A. Adult Stem cells – from fully developed organism  1. can differentiate into a few cell types  2. can reproduce indefinitely

 B. Embryonic Stem cells –from blastula  1. blastula (blastocyst in humans)  2. hollow ball of about 150 cells  3. Pluripotent = differentiate into many types of cells

 C. iPS – induced pluripotant stem cells  1. adult cells given stem cell ‘master genes’  2. genes inserted by retro-virus  3. very similar to embryonic stem cells  4. potential to make genetically identical repair tissues  5. genetically engineer cells & reintroduce to body  6. NO egg NO embryo No problem!!

VII. Cancer & gene regulation  A. Somatic cell mutations can =cancer  1. caused by chemical carcinogens  2. high energy radiation  3. ex. translocation  a. chromosome breakage & relocation  1. moved near very active promotor  2. broken gene inactive  4. ex.gene amplification(many copies)  5. ex. gene mutations

 B) Proto-oncogenes = normal genes  1) if mutated become oncogenes  2) code for proteins that stimulate cell division  3) ras gene example (30% of cancers)  a. codes for a G protein called ras 

G protein review (p 110) (p325)  G proteins use GTP energy  G proteins send signals from receptor proteins to transduction cascade  not working = too much or too little signal

 4) ras G-protein relays growth hormone signal to cascade of protein kinases.  a) transduction of signal results in production of protein that stimulates cell cycle  b) mutated ras gene is an oncogene  c) mutated ras protein constantly triggers cascade of protein kinases regardless of presence of growth factor  d) protein product stimulates cell division (p 325, fig )

 C. tumor-suppressor genes  1. some code for DNA repair proteins  2. some code for adhesion proteins  3. some code proteins that inhibit cell division  4. example:p53 tumor-suppressor gene  a. 50% of cancers show this mutation  b. p53 codes for transcription factor  that promotes production of  cell cycle inhibiting proteins

 c. p53 transcription factor activates several different genes  1. gene p21 makes a protein that binds cyclin-dependent kinases, stopping cell division  2. miRNAs activated by p53 inhibit c.c.  3. genes for DNA repair also activated  4. activates apoptosis genes if DNA can not be repaired 

D. Multi-step model  1. More than one somatic mutation needed to produce cancer cells  2. Explains increased cancer risk with age

E. Inherited Cancer Risk  1. inheriting one oncogene or mutated tumor suppressor gene  a. won’t automatically cause cancer  b. puts you one step closer  2. BRCA1 and BRCA2 are inherited mutations associated with Breast Cancer  a. second most common cancer in USA  b. tumor suppressing genes  c. DNA tests developed

F. Virus caused Cancers  1. implicated in 15% of cases  2. Human Papillomavirus  a. cervical cancer  b. vaccination

G.Cyclin Dependent Kinase Review  CDKs =enzymes. turn on or off processes in cell division  CDKs only active when bound to cyclin proteins  Different cyclins activate different CDKs at each stage of cell division  Tumor suppressor genes may activate genes that block CDK action  Mutated CDK or Cyclin genes can be oncogenes

H. Check points – regulated by CDKs  1) G1 checkpoint – cycle initiation  a)controlled by cell size  b) growth factors  c) environment  2) G2 checkpoint – transition to M  a) DNA replication complete  b) DNA damage/mutations  3) M-spindle checkpoint  a) spindle attachment

 Embryonic induction = timing  Homeotic genes = pattern formation in late embryo  Hox genes = animal homeotic genes (very highly conserved  Morphogenesis = gives organism its shape