Evolutionary Biology 10: Signals n shit 2

Question 1

Explain the french flag hypothesis

Answer 1

Morphogen is secreted by an inducer cell
- diffuses to form a concentration gradient

• Cells in high concentration differentiate to cell fate X
• Cells in medium concentration differentiate to cell fate Y
• Cells in low concentration differentiate to cell fate Z

Different genes are activated based on concentration of morphogen

Cells all have baseline equal potential to become X, Y, Z (red, white, blue french flag)

Question 2

4 Families of paracrine factors?

Answer 2

FGFs (fibroblast growth factors)
TGFs (transforming growth factors)

Hengehogs
Wnts

Question 3

How do hedgehog (Hh) proteins regulate gene transcription?

Answer 3

When Hh receptors are not activated, patched protein binds to inhibit ‘Smoothened’ G-protein receptor

• When smoothened is inhibited, PKA and Slimb are free to target and cleave TF cubitus interruptus (CI)
• Cleaved CI enters nucleus and represses transcription of target genes

When Hh is activated

• Shape change, patched unbinds, smoothened activated
• Smoothened inhibits PKA and Slimb
• No CI cleavage
• Intact CI acts as transcriptional activator

Question 4

Function of Hh signalling? Example?

Answer 4

Controls wide range of developmental processes
- differentiation and cell patterning

e.g. Sonic hedgehog (Shh) patterning of spinal cord neurons - Shh secreted by notocord and floor plate

Question 5

Mutations in Hh can cause?

Answer 5

growth retardation, failure of limb growth, polydactyly etc.

Question 6

How do Wnts signal?

Answer 6

Through (GPCR) frizzled receptors

2 pathways

• canonical (Beta-catenin signalling)
• non-canonical (Rac/Rho signalling)

Question 7

Describe canonical frizzled pathway

Answer 7

When Wnt is not bound to receptor, APC/GSK3 degrade TF beta-catenin

• When Wnt binds, phosphoprotein Dishevelled (Dsh) is activated
• Dsh prevents degradation by APC/GSK3
• beta-catenin translocates to nucleus and acts as transcription factor

Question 8

Describe non-canonical frizzled pathway

Answer 8

Dsh tethered to membrane by Strabismus/prickle

• When Wnt binds, Dishevelled is activated
• Dsh activated Rak/Rho proteins
• Rak/Rho control cytoskeletal rearrangements (via ROCK) and gene transcription ( via JNK)

Question 9

Functions of Wnt signalling?

Answer 9

Regulates wide range of processes e.g.

Cell proliferation
Cell fate determination
Cell polarity

Question 10

Example of canonical Wnt signalling function?

Answer 10

Canonical Wnt signalling essential for control of body axis formation

• During vertebrate development, Wnt low in future anterior region, High in future posterior region
• Disruption of this gradient disrupts anterior -posterior patterning

Question 11

Example of non-canonical Wnt signalling function?

Answer 11

Non-canonical Wnt signalling essential for cytoskeletal rearrangements underlying convergent extension movements

Question 12

What evidence is there to suggest that auxin (IAA) asymmetries specify local control of cell growth?

Answer 12

1) All embryos synthesise auxin
2) IAA transport -> along apical-basal axis - basipetal - from shoot meristem to root tip
3) Chemical inhibitors that block polar transport of auxin block globular stage to heart stage transition

Question 13

PIN protein function?

Answer 13

Mediate auxin movement

Question 14

Example of an apical-basal axis mutant in Arabidopsis? Why does the mutant phenotype form?

Answer 14

GNOM - forms ball-like embryos without apical/basal organs -> lacks ability to establish polarity

GNOM is required to establish polar localisation of PIN1

• gnom mutant shows random localisation of PIN1
• this is responsible for polarity defects