In a Toll null mutant, in which cells will Dorsal protein be localized to the nucleus?...

In a Toll null mutant, in which cells will Dorsal protein be localized to the nucleus? Why? What is the resulting phenotype?

Expert Solution

A null mutation is a mutation or change in a gene that results in its complete lack of transcription or translation. ie; as a result of null mutation the product of a gene is not at all produced or a non functional product is obtained.

Dorsal is a protein in drosophila that distinguishes ventral region from dorsal. ie; the presence of dorsal protein in a cell makes it ventralized while the absence of dorsal protein make the cell dorsalized. The dorsal protein is a product of dorsal gene. When dorsal gene is transcribed, it is actually found throughout the embryo and is not specified to either dorsal or ventral side. But the fact is that, when dorsal protein is produced at first, it is combined with cactus ( a protein in cytoplasm of the syncytial blastoderm). But when it is held with cactus protein, dorsal is in an inactive state and cannot enter any of the cells. The seperation of dorsal from cactus is initaited by the ventral activation of toll receptor. (ie; when spatzle protein whose concentration is highest in ventral side binds to toll receptor, the receptor get activated. It recruites pele,a protein kinase and another protein called tube. This pele protien, as it is a kinase, it phosphorelates cactus. Once cactus is phosphorelated, it gets degraded and dorsal enters the nucleus. Toll is activated by spatzle gradient which is highest in ventral side. So, dorsal enters the nucleus of ventral side and makes it ventralized.

Here, in this case, as it is a toll null mutant, it means that either toll receptor is not produced or a receptor that is functionless is produced. The results of both will be the same. So, as a result the Toll receptors located within the embryo will be absent or non functional. Therefore, spatzle protein cannot activate toll and hence cactus is not cleaved from dorsal. As a result, dorsal protein cannot enter any of the cells. If dorsal doesnot enter the nucleus, the genes responsible for specifying ventral cell types are not transcibed,and at the same time the genes responsible for specifying dorsal cell types are not repressed. Hence all the cells of embryo become specified as dorsal cells ( presence of dorsal protein is required to establish ventralization of the embryo).

gladiator answered 1 year ago

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