A) B-catenin is present in the nuclei of dorsal blastomeres and is necessary for

Question

A) B-catenin is present in the nuclei of dorsal blastomeres and is necessary for the activity of disheveled, and is necessary to impose dorsal identity.

B) Vg1 is ade in and relased by vegetal blastomeres, and is necussary to induce prospective ectoderm to become mesoderm.

C) The transcriptional activators siamois/twin (downstream of b-catenin) and activated SMAD4 (downstream of vg1) bind to enhancer noggin and are necessary for its expression.

Describe the experiments you would do to prove these to be true on a Xenopus and the results you see.

Explanation / Answer

a) B-catenin is present in dorsal blastomeres and necessary for activity of deshveled.

Desheveled is recruited onto Fzd -- destruction complex(APC/GSK3B/CK1) inhibited--- b catenin- maintained--- enters nucleus- activates Wnt taget genes.

This can be proved by expressing Beta-catenin on the ventral side ectopically promotes formation of second /another dorsal axis.

B) Vg 1mRNA are found on the vegetal hemisphere . Vg1 and Activin are involved in mesodermal induction in marginal zone lying adjacent to this.

This can be proved by creating a fusion protein of Vg1(Vg1 c-terminal domain fused to BMP4), When the Xenopus embryos are injected with this , the animal caps are isolated and found to be mesodermalised and a second dorsal axis formation will occur.

C) Beta- catenin activates transcription of siamois and twin ,which act as transcriptional activator , they relay beta catenin signal by binding to proximal element and the distal DNA elements binds to SMAD4.

By blocking the expression of siamois/twin and activated SMAD4 by LOSS OF FUNCTION OR USING MORPHOLINO(ANTISENSE OLIGOMERS) will lead to blocked expression of noggin.

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