Following statements are regarding to signals that regulate axis extension in chick embryos.

A. In the stage 10 chick embryo, Fgf8 inhibits expression of the retinoic acid (RA) synthesizing enzyme Raldh2 in the presomitic mesoderm and the expression of the RA receptor RARβ in the neural ectoderm, thus preventing RA from triggering differentiation in the caudal-lateral epiblast cells and the caudal-most paraxial mesoderm.

B. Fgf8 inhibits Sonic hedgehog (Shh) expression in the neural tube floorplate, controlling the onset of ventral patterning genes.

C. FGF signaling is also required for expression of Delta1 in the medial portion of the caudal-lateral epiblast cells and promotes expression of Wnt8c.

D. Fgf8 decays in the caudal paraxial mesoderm, Wnt signaling, most likely provided by Wnt8c, now acts to promote Raldh2 in the adjacent paraxial mesoderm. RA produced by Raldh2 activity expresses Fgf8 and Wnt8c.

Which of the following statements are correct?

#Question id: 4074

#Unit 3. Fundamental Processes

The promoter sequences of RNA polymerase III-transcribed genes are located

#Question id: 4075

#Unit 3. Fundamental Processes

Pre-mRNA molecules

a. exist as free RNA molecules in eukaryotic cells.

b. are associated with an abundant set of nuclear proteins.

c. are mostly located in the cytoplasm.

d. are located in the nucleus.

#Question id: 4076

#Unit 3. Fundamental Processes

RNA-binding proteins

a. can be identified by chromatography of UV irradiated nuclear extracts over oligo-dT columns.

b. can be identified by sequence homology to known RNA-binding domains.

c. have a conserved structure as seen by X-ray crystallographic analysis.

d. alter the secondary structure of pre-mRNAs, which decreases interactions with other RNAs or proteins.

e. always remain in the nucleus.

#Question id: 4077

#Unit 3. Fundamental Processes

Which of the following is (are) associated with most polyadenylation events?

a. cleavage of the 5´ end of the RNA molecule

b. an assembly of a large, multiprotein complex

c. the AAUAAA site upstream of where polyadenylation will occur

d. a GU-rich or U-rich region near the cleavage site

e. slow addition of A residues followed by rapid addition of 200 to 250 more A residues

#Question id: 4078

#Unit 3. Fundamental Processes

Which of the following is (are) required for splicing to occur?

a. two transesterification reactions

b. intact, naturally occurring introns

c. formation of a lariat like structure

d. a branch-point G residue

#Question id: 4079

#Unit 3. Fundamental Processes

What is the evidence that pre-mRNAs are not synthesized unless machinery for processing is properly positioned?