2011_Questions_Week_15_Answers

2011_Questions_Week_15_Answers - 1. Why is Sex lethal (Sxl)...

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1. Why is Sex lethal (Sxl) in fact lethal when mutated in female flies? Because females will then have twice as much transcription from genes on their X- chromosome. 2. How is Sxl null mutant female similar or different to a null mutant in Transformer (Tra)? -Sxl lethal is upstream of Tra; both have the same effect that in their absence, they will generate male-specific isoforms of the sex determination transcription factor Doublesex (Dsx). They are different because Sxl has an additional role in inhibiting Msl genes, thereby preventing amplification of transcription in female cells. Thus, transformer only effects the sex determination pathway and not the doseage compensation pathway. Because of this, Tra mutants are not lethal, and XX females will display male morphology and male courting behavior (since they will have a male isoform of Fruitless). 3. What is the functional effect of mammalian cells lacking Xist? -There will be no inactivation of X-chromosomes and there will therefore be a lethal increase in X-chromosome genes. 4. What is Xist? How is it controlled? How does it inactivate X-chromosome transcription? -Xist is a long non-coding RNA that binds to the X-chromosome at a specific region (Xic) and randomly initiates inactivation of one of the X-chromosomes. Xist is only transcribed and presence on the inactive X-chromosome. Prior to X-inactivation, Xist is transcribed at low levels on both X-chromosomes, but is inhibited by being bound by an antisense non-coding transcript called Tsix. After initiating inactivation of one X- chromosome, Xist transcription is highly upregulated, eventually binding to sites along the entire X-chromosome. Xist facilitates the recruitment of histone modification enzymes that are markers of silenced regions, and also bind along the inactive X- chromosome. 5. Why don't XX mice expressing an Sry transgene produce functional sperm? -Sry is sufficient to initiate the male sexual development pathway. However, additional proteins necessary for making sperm are also present on the Y-chromosome.
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2011_Questions_Week_15_Answers - 1. Why is Sex lethal (Sxl)...

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