Team Members: Skip Vasquez & Kat
Mosman
Team Members: Skip Vasquez & Kat
Mosman
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Comments :
The anterior-posterior axis is established in the Drosophila embryo
before the embryo's nuclei has even begin to function. Maternal
effect genes and the oocyte's cytoplasm play roles in regulating the
anterior-posterior polarity of the embryo. Ovarian nurse cells
surrounding the oocyte deposit mRNA's into the developing egg cell.
These mRNA's accumulate into various regions within the oocyte. There
are four maternal mRNA's that are critical in the formation of the
anterior-posterior polarity in the developing embryo and they are
bicoid, nanos, hunchback and caudal.
While hunchback and caudal mRNA's are distributed evenly throughout the oocyte's cytoplasm, bicoid and nanos mRNA's are sequestered into specific regions of the oocyte. The bicoid mRNA's (transcribed from the mother's bicoid genes) are secreted into the oocyte where they are strictly localized in the anterior region of the egg cell. A 3' untranslated portion of the bicoid mRNA is anchored to the cytoskeleton in the oocyte's anterior region. The bicoid mRNA then remains dormant until fertilization. Meanwhile, the maternal nanos gene generates nanos mRNAs that are also secreted into the oocyte by way of the ovarian nurse cells. Nanos mRNAs are then transported to the posterior region of the oocyte where they are bound to the cytoplasm by their 3' untranslated sequence and their association with the products of several other maternal genes (oskar, valois, vasa, staufen, and tudor). If nanos or the associated products necessary for the localization of nanos are absent, the future embryo's abdomen will not form. While the bicoid and nanos maternal mRNAs are localizing, the oocyte's nucleus is also migrating to a dorsal anterior position.
Upon fertilization, the bicoid mRNA is activated and can then be translated due to the addition of a polyadenylate tail. Once translated, the Bicoid protein begins to form a concentration gradient. The highest concentration of Bicoid protein is, and will remain, in the anterior portion of the egg. Similarly, at fertilization, the nanos mRNA is no longer repressed by its association with other gene products and begins to be translated. After translation, the Nanos protein forms a concentration gradient, with its highest concentration in the posterior portion of the egg cell.
In the same manner that bicoid and nanos mRNA's enter the egg cell, hunchback and caudal mRNAs are secreted into the oocyte by the ovarian nurse cells. However, unlike bicoid and nanos, hunchback and caudal mRNA's are evenly distributed through out the oocyte's cytoplasm. The even distribution of hunchback and caudal mRNA's is eventually altered through the interference of Bicoid and Nanos proteins. By suppressing the translation of caudal mRNA and interacting with the nucleus to produce additional hunchback mRNA's, the Bicoid protein disrupts the distribution of the hunchback and caudal products and thereby distinguishes the anterior section of the embryo. The translation of caudal is suppressed in the anterior region because the Bicoid protein adheres to the 3' untranslated region of the caudal mRNA. If Caudal protein were produced in the anterior region, the head and thorax of the embryo would form improperly. The Bicoid protein also passes through the nuclear membrane and stimulates the transcription of hunchback mRNA. The embryo-derived hunchback mRNA's, along with the maternally derived hunchback mRNA's, are translated into Hunchback proteins. The Hunchback protein is a crucial component in the differentiation of the anterior region and works in concert with Bicoid to develop the head and thorax structures.
In the posterior region of the egg cell, the Nanos protein prevents the translation of hunchback mRNA into protein. Throughout the embryo, the 3' untranslated region of the hunchback mRNA is bound to the Pumilio protein. The Pumilio protein alone is not capable of inhibiting the translation of hunchback. When the Nanos protein in the posterior region of the embryo binds to the Pumilio protein, the hunchback mRNA becomes deadenylated. This deadenylation prevents the translation of hunchback mRNA into Hunchback protein. The blockage of Hunchback in the posterior region along with its activation in the anterior region creates a concentration gradient of Hunchback proteins. Similarly, a concentration gradient of the Caudal protein results from its suppression by Bicoid in the anterior region of the embryo. As a result of the interactions between Bicoid, Nanos, hunchback and caudal, opposing concentration gradients are established with Bicoid and Hunchback accumulating in the anterior portion and Nanos and Caudal favoring the posterior.
The regulation of hunchback and caudal expression by Bicoid and Nanos is essential in the creation of an embryo's anterior-posterior polarity. However, bicoid and nanos mRNA's must be localized for the Bicoid and Nanos concentration gradients to form. Clearly, the differential placement of maternally derived bicoid and nanos mRNA's is a crucial component of anterior-posterior axis formation. Without the sequestering of these two mRNA's in the oocyte, any further steps in anterior-posterior axis formation would be thwarted; however, concentration gradients of all four proteins are necessary for the proper progression of anterior-posterior axis development.
