Week 11 - CHAPTER 18 Flashcards
What is development?
The building of a multicellular organism
- begin life as single cell followed by many rounds of cell division and differentiation
- Gene expression patterns within and among cells form the basis for development
What can forward genetics approach identify
Mutagenesis, abnormal phenotype, genetic crosses, mapping, cloning and functional confirmation
Fundamentals of development
- Cell differentiation
- Positional information and gradient model
- Pattern formation
Cell differentiation as a fundamental of development
- A single celled zygote undergoes several mitoses, starting with stem cells
- Cells divide and differentiate
- differentiated cells take on different morphologies, physiological activities and specialized gene expression profiles
Embryonic stem cells
Totipotent, capable of forming any tissue of cell type
Somatic cells in adult animals
- Most fully differentiated and locked into a specific cell fate
- Exceptions: Pluripotent stem cells in several tissues. Each PS cell can be differentiated into a limited number of cell types
What are morphogen gradients formed by?
- Partitioning of cellular contents within cells
- Asymmetric cell divisions
- Daughter cells inherit distinct subsets of the factors present in the original cell
Morphogen
Substance whose presence in different concentrations directs developmental fates
Pattern formation
The interacting event that organize the differentiating cells to establish the body-plan axes
Organizers
Cells that determine their neighbor’s identity
- Can be through induction or inhibition
Induction organizers
Induces the neighboring cells to adopt a specific fate
Inhibition organizers
Prevents its neighbors from adopting a certain fate
Drosophila Development
- Anterior-Posterior and dorsal-ventral polarities are acquired during its production in the female
- During early development, nuclear cell division occurs without division of cytoplasm
- Forms Syncytium: multinucleate cell where nuclei are not separated by membranes
- After laying egg, cellularization occurs by assembling membranes that separate nuclei into individual cells forming cellular blastoderm
- Cells become restricted in their developmental potential
- Cells develop into tissues reflecting their original region
How is drosophila larval development controlled
Five classes of mutations influencing Drosophila development in a cascade
- Coordinator Gene: defines axis of embryo
- Gap Gene: Defines board region of the embryo
- Pair-rule gene: defines segments of the embryo
- Segment Polarity gene: defines anterior and posterior regions of individual segments
- Homeotic (Hox) gen: defects alter the identity of one or more segments (influenced by other 4)
Interactions of gens involved in Drosophila development
- Coordinate genes activate gap gene expression patterns
- Combinatorial Coordinate and Gap Genes determine the expression patterns of all pair-rule and segment polarity genes
- Individual segments acquire their unique identities through homeobox genes
Mutations in coordinate genes
defects affect an entire pole of the larva
Mutations in gap genes
missing large, contiguous groups of segments
Mutations in Pair-rule genes
Mutants are missing parts of adjacent segment pairs, in alternating patterns
Mutations in segment polarity gene
defects affecting patterning within each of the 14 segments
Mutations in Homeotic genes
defects affect the identity of one or more segments
Organization of Hox genes on chromosome
Order reflects location of expression
Hox complexes in Metazoans
- Exist in all animals
- Arranged in clusters
- Duplications, deletions, pseudogenizations among linages
How are human digits determined
Each digit has its own unique identity, specified by hox gene expression
- Hoxd9 only on thumb
- increase with additional one per finder
- up to Hoxd9 - 13 for pinky
Mutations that alter digit formation
- Ectopic expression of Hoxd genes can alter digit identity
- Mutations that expand or increase expression of shh gene result in formation of extra digits
- Separation of the limb bud into individual digests in humans requires programmed cell death between digits
- Loss of limbs in snakes and cetaceans is due to alterations in Hox and/or shh gene expression
caenorhabditis elegans
- Free-living transparent animal
- 1,032 cells in adult male
- 959 cells in adult hermaphrodite
- self-insemination: 300 eggs
- Cross-fertilized by a male: 1000 eggs
- life-span: 2-3 week; generation time: 3-4 days
- Five pair of autosomes; one pair of sex chromosomes
- Male: XO, hermaphrodite: XX
Vulva development in C. elegans
- Six vulval precursor cells (P3 to P8)
- only 3 of which included in vulva formation (3 closest to anchor cell (P5 to P7)
- Cell closest to anchor is identified as the primary cell and adjacent cells are secondary cells forming peripheral parts
Genes and signal transduction in Volva development
- LIN-3 from anchor cell activates receptor on vulva precursor cell at LET-23 receptor
- LET-60 acts as transduction molecule to induce gene expression of primary cell
- lateral inhibition signal set to neighboring cells to convert to secondary cells (lag-2 to lin-12)
What are the major differences between plant and animal development
- Early separation of germ cells in animals
- Plants can add new organs after maturity
Meristems
Pluripotent cells in plants
- Root meristems
- Shoot meristems: left, axillary, reproductive (inflorescence or flower)
Arabidopsis Thaliana
A model plant for genetics research
- composed for four concentric whorls of organs
How is plant flower development genetically controlled?
- ABC gene expression precedes floral organ formation
- Distribution of the ABC gene products in the four floral whorls determines the identity of organs in the whorl
- The four SEPALLATA proteins provide transcriptional activator activity
Sepals: AP1 and SEP
Petals: AP1 AP3 PI and SEP
Stamens: AP3, PI, SEP AG
Carpels: SEP AG, SEP, AG
Flower development A-class Mutation
Homeotic transformations in the outer two whorls, where carpels develop in the positions normally occupied by sepals and stamens replace petals
Flower development B-class mutation
Homeotic transformation in the 2 middles whorls, where sepals replace petals and carpels replace stamens
Flower development C-class mutation
Homeotic transformation in inner whorls, where petals replace stamens and the cells that would normally give rise to the carpels behave as if they were another flower meristem that reiterates the development cycle
Flower development BC double mutant
All sepals
Flower development AC double mutant
outer and inner whorl is leaf-like carpels and middle is petal-like stamens
Flower development ABC triple mutant
All whorls are leaf-like carpels
MADS-box gene family
- MCM1 forming the budding yeast
- AGEMOUS from the thale cress
- DEFICIENS from the snapdragon
- SRF from homo sapiens
- Homeotic genes that mind to the consensus motif CC[A/T]6GG
- located in ABC genes and interact with SEPALLATAS
