10/ invertebrate models sea urchin and c. elegans Flashcards
why are sea urchins a good model
large number of embryos, open to experimental manipulation, transparent embryo - can easily follow gastrulation
categorizing sea urchins
sea urchins are echinoderms, which are deuterostomes
protostomes vs deuterostomes
- protostomes: blastopore forms mouth (so this forms first), then anus forms second
- deuterostomes: blastopore forms anus (this forms 1st), then mouth forms second
- note evolution has tinkered with this
sea urchin life cycle
- fertilised egg, cleaves until 64 cell blastula
- gastrulation at day 1 into gastrula then late gastrula
- hatching into pluteus larva
- then metamorphosis into adults
mosaic vs regulative development models
- mosaic: nucleus of egg would contain all determinants that specify cell fates by specific segregation to those cells
- regulative: developmental cells are communicating and difs brought about by cell-cell interactions
why were sea urchins a good model to determine which of the mosaic or regulative models were best?
- all embryos undergo cell divisions in the exact same way
- 2 divisions at right angles along animal-vegetal (yolky bit) axis, 1 perpendicular to these divisions separating animal from vegetal half
how did sea urchins prove regulative development
- driesch
- separated 2 blastomeres at the 2 cell stage and got 2 small but complete embryos, not half an embryo
how did sea urchins ALSO prove mosaic model of development
- when animal and vegetal half were separated (8 cell stage) an animalized and vegetalized half formed
- some mosaicism, but mostly regulative
ideal characteristics of a model organism
small, large batches of embryos, short generation time, easy to breed, easy scoring of phenotypes +/-, sequenced genome
background to c elegans as an organism
- super simple, basically just eats and breeds
- gut and reproductive organs
- some muscles
- neurons and sensory cells - info about environment
c elegans as hermaphrodites
- first male then female - uses own sperm to fertilise own eggs
- occasionally only males arise, these can breed w females and prevent only inbreeding
c elegans cleavage
- ENTIRELY fixed and stereotypic - fate of each early cell mapped to completion
- 1st cleavage is asymmetric (smaller posterior cell)
how can experimental manipulation change cell fates in c elegans
- moving cells to a new position
- shows cells don’t contain determinants
how many cells make up c elegans, how many are programmed to die by apoptosis
- 1090
- 131
what is apoptosis important for
- proper development: formation of male/female reproductive organs
- homeostasis: maintain constant number of cells, remove damaged cells
- when not regulated can lead to autoimmune disease and cancer
notes on RNA interference: in what organisms was it discovered, what is it
- largely worked out in c elegans while studying their muscle activity
- double stranded RNA triggers a biochemical process that degrades identical mRNAs, so blocks gene activity after transcription has happened
siRNA and its exploitation
- double stranded RNA cut by dicer enzyme into siRNAs
- these combine w RISC complex, which finds mRNAs comp to siRNAs and degrades them
- siRNAs can be chemically synthasised and we have a library that can target every gene in the human and c elegans genome
- can use this to deregulate a gene involved in a process of choice or in medicine to downregulate overactive gene
