Complete question:
In Drosophila the anterior-posterior axis is established by
a. The distribution of the RNA-binding transcription factor called bicoid and the protein nanos localized in the anterior and posterior poles of the embryo, respectively
b. The distribution of a gene that codes for an RNA-binding protein called nanos localized in the anterior pole of the embryo and a transcription factor called bicoid localized in the posterior pole of the embryo
c. The distribution of a gene that codes for an RNA-binding protein called nanos localized in the posterior pole of the embryo and a transcription factor called bicoid localized in the anteror pole of the embryo
d. The distribution of the RNA-binding transcription factor called bicoid and the protein nanos localized in the posterior and anterior poles of the embryo, respectively
Answer:
c. The distribution of a gene that codes for an RNA-binding protein called nanos localized in the posterior pole of the embryo and a transcription factor called bicoid localized in the anterior pole of the embryo
Step-by-step explanation:
When the Drosophila zygote develops, it goes through different stages until it gets to the blastula stage. At this point, there are pole cells that determine the axis of the organism -the anteroposterior and the dorsoventral axis-.
A two hours old embryo has already determined its two axes. A ten hours old embryo has already determined its body segments. And from there, each of them develops a structure that turns to be their identity.
Different genes determine the polarity of the egg.
Dorsoventral genes place in the superior and inferior parts of the egg and determine the development of dorsal and ventral tissues.
The same happens with anteroposterior genes.
- The BICOID genes place in the anterior part of the egg and activate the hunchback genes that determine the development of the head and the thorax. The Bicoid mRNA distributes in the anterior region of the egg.
- The NANOS gene reprimes the hunchback genes, which promoted the development of the posterior structures of the body. The Nano mRNA places in the posterior region of the egg, and Nano proteins distribute in this area.
The polarity genes of the egg are genes of maternal effects.
While the bicoid and nano genes are expressed in the ovary, the hunchback gene is expressed in the embryo. This last one is activated or inhibited by the foreign ones.
Bicoid and nano genes gradient regulate the expression of the gap, pair-rule, and segment polarity genes.