17 18 fate
DESCRIPTION
TRANSCRIPT
set aside early in devel, not arise from particular germ layer
How cells become diff: Gastrulation: 3 germ layers
Find fate of cell: cell lineage tracing, fate mapping. Dye injections can be used for fate mapping. To derive fate map, multiple experiments required Decisions cell fate most part irreversible Determination: Cells committed irreversibly to particular devel fate Differentiation expressing cell-type specific characters. Determination typically precedes overt differentiation Standard test: Cells determined if transplantation from normal to new position in embryo no ∆ normal fate
fate mapping early stage embryos
fate map: single cells /groups cells marked at particular time pt in devel, & at later time pts, location & identity of labeled cells noted. require many individual experiments to see range of fates that can be produced by particular cell. Cell fate specification: Bc determined (or committed) cells don’t look diff from non-determined cells, distinguishing 2 is based on experimentation (operational definition). cell determined, transplanting to diff region embryo (usually host embryo of same species & stage) not affect devel fate. cell not yet determined, develop according to new environ.
Cell /devel fate: for particular/group cells. Tracing cell lineages requires cells transparent, division patterns reproducible animal to animal (cells always divide same # times, same orientations, have same fates
cell lineage nematode C. elegans complete known, powerful tool devel analysis. most animals, esp vert,
cond tracing lineages high resolution not met.
fate mapping later stage embryos
Frog blastula
frog fate map at beginning of gastrulation
forms toes
Mlcs in cytoplasm (cytoplasmic determinants)Influence fate of cells receive them
Cytoplasmic determinants become asymm localized
Cell division gives 1 daughter most cytoplasmic determinants
commitment to particular fate progressive: cells have memory C. H. Waddington’s epigenetic landscape
Mech determined Cell autonomous (intrinsic): cells acquire fates independent of interactions w neighboring cells. asymmetric division events in cell in question. mother cell prod cytoplasmic determinants become localized unequally along 1 axis of mother cell. When cell divides across axis, determinants unequally distributed to daughter cells where influence cell fate Conditional (extrinsic): cells acquire fates based on interactions w neighboring cells
Conditional (extrinsic) determinationInduction, morphogenic gradients, pos feedback
induction
asymm division. Cond determination, cells acquire fates based on sigs from neighboring cells. Cell sig determines cell fates: induction. Sequential induction can generate progressively more complicated patterns
Morphogen gradients: Morphogen: diffusable mlc secreted at distance from target. Cells spec
fates of target cells. graded distribution, cells respond directly to diff amts mlc & become spec
to diff fates. ∆ gradient ∆ pattern
Specialized induction mediated by morphogen gradients. Morphogens: high conc near source, lower at greater distances. Cells w approp devel history can respond to morphogen in conc dependent way; high conc will induce different cell fates than low or intermediate conc of morphogen. pos feedback/lateral inhibition, system starts off homogeneous & symm can pattern itself spont, even where no organized external sig
OFF
Threshold reached
ON
1 S
IGN
AL,
1
OU
TCO
ME
Source of signal
diff threshold levels for diff devel decisions
threshold 2 reached1 S
IGN
AL,
MU
LT
OU
TCO
MES
threshold 1 reachedFATE CFATE A FATE B
Source of signal
FRENCH FLAG MODEL Wolpert ‘60s
morphogen gradient outcomes
positive feedback aka lateral inhibition
threshold 2reached
threshold 1reached
DIGIT 4 DIGIT 3 DIGIT 2
1 signal, multiple outcomes
threshold level for devel decision
Group equiv cells
cells compete by inhib neighbors
Some predominate
normal fly
Fly w patch of cells mutant for Delta
example: Notch-mediated pos feedback
intestinal epithelial cells
neurons
all cells in embryo/adult: same genomic DNA seq. not all cells within embryo or adult express same set genes. cells become different bc express diff genes. proteins confer cells ability to be diff (morphologically & functionally)
Same genome, diff cells: reg gene expression = cell specification
components differential gene expression nature switches that turn genes on/off (post)transcriptional controls events turn particular switches on /off devel events: asymm divisions, cell sig
gene expression can be controlled at many levels
Most often used control pt in devel
Promoter contains seq recognized & bound by RNA pol, ~50 nuc upstream of transcription start site
“upstream” “downstream”
promoters vary in strength of binding RNA pol. promoters w strong bind reg by transcriptional repressors. w weak binding reg by transcriptional activators. activators & repressors can act in concert to provide highly sensitive transcriptional reg
of enz that make tryptophan
constitutivelypresent
prok genes can be reg via repressor proteins
negative feedback loop
Tryptophan low
Tryptophan high
Biosynth genes ONBiosynth genes OFF
RepressorOFF
Repressor ON
prok genes can be reg via activator proteins
activators enhancer RNA pol binding promoter
Prok transcriptional reg
E. coli use gluc as E source but if no gluc available, can get by breaking down lactose. genes Lac operon allow E. coli to utilize lactose. E. coli wants to express Lac genes when: lactose is in the medium, glucose is not around. Lac genes: gluc only OFF lactose only ON gluc & lactose OFF neither OFF
repressors and activators can work together to reg Lac operon
inactive repressor
Lac repressor inhib
transcription when lactose not present
allolactose
Allolactose: metabolite of lactose; levels reflect lactose levels
inactive
CAP activates transcription when
gluc not present
cAMP
Gluc low, cAMP ^
inactive repressor
inactive
Lac operon is controlled by
2 signals
euk cells contain 3 distinct RNA pol: I, II, III (II transcribes mRNAs), euk RNA pol require assistance from general transcription factors, euk contain large amnts untranscribed DNA bw genes, euk DNA packaged into complex chromatin structures can prevent access to RNA pol
euk RNA pol II requires set of GENERAL TRANSCRIPTION
FACTORS
But this is not enough!
euk activators & repressors: euk transcriptional regulators: can activate /repress transcription, bind reg elements enhancers, bind enhancers thousands of base pairs away from gene they reg, bind enhancers up/ downstream of gene they reg
transcription regulators contain variety DNA binding motifs transcription regulators usually bind major groove DNA
transcription regulators work together as committee to control gene expression
combinatorial control transcriptional status of any gene result of combo transcriptional reg
Transcription initiation: RNA Pol II can only bind at euk promoters w help of large group GTFs. Activation large complexes promoted by transcription activators, prevented by transcription repressors, bind to reg seq at large distances from promoter
example of single contact bw
transcription reg & 1 base pair DNA
small # of transcription factors acting in combo can specify variety
of cell fates
single gene can be turned on by diff transcriptional reg
Transcribed region
Transcribed region
Cell A
Cell B
transcriptional reg: possible to express genes where/when we want
GFP expressed under control of gamma-crystallin reg seq
single gene can organize entire organ thru cascade transcriptional reg
single transcription regulator can coord expression many diff genes
Each gene has region DNA adjacent promoter where RNA pol binds. Genes controlled by proteins (transcriptional regulators) bind to reg DNA seq via noncov bonding to nuc within the major groove of DNA. Repressors function by blocking binding RNA pol to promoter. Activators function by facilitating binding RNA pol to promoter. repressor & activator combine to reg Lac operon in response to C source availability. Euk transcriptional regulation: specialized RNA pol for mRNA (RNA pol II); much more non-coding DNA bw genes; DNA packaged in chromatin. RNA pol II doesn’t bind directly promoter region. promoter contains seq (most commonly, TATA box) that bind to one or more of the general transcription factors that recruit RNA polymerase and associate with it to facilitate transcription and its proper regulation. complex RNA pol w general transcription factors, not sufficient activate transcription. Specific activators /repressors bind to enhancer sequences (enhancers) that can be quite far from the start site of transcription. activators can facilitate looping out of the DNA to allow the activator or repressor to interact w transcriptional machinery. Enhancers can occur up/downstream of euk genes. Each gene reg by combo of enhancer seq, & whether given gene on /off result of combinatorial action of multiple transcriptional reg. single transcriptional reg (or particular combo of reg) can activate many genes. single transcriptional regulator can simult activate some genes repress others. difference is combo of transcription factors that bind diff genes.