Info

Gene

Where Expressed

Action

bicoid

Ovary

Regulates expression of genes responsible for anterior structures; stimulates hunchback

nanos

Ovary

Regulates expression of genes responsible for posterior structures; inhibits translation of hunchback mRNA

hunchback

Embryo

Regulates transcription of genes responsible for anterior structures

gene that acts at the posterior end of the axis. The nanos gene is transcribed in the adult female, and the resulting mRNA becomes localized at the posterior end of the egg (I Figure 21.7b). After fertilization, nanos mRNA is translated into Nanos protein, which diffuses slowly toward the anterior end. The Nanos protein gradient is opposite that of Bicoid protein: Nanos is most concentrated at the posterior end of the embryo and is least concentrated at the anterior end. Nanos protein inhibits the formation of anterior structures by repressing the translation of hunchback mRNA. The synthesis of the Hunchback protein is therefore stimulated at the anterior end of the embryo by Bicoid protein and is repressed at the posterior end by Nanos protein. This combined stimulation and repression results in a Hunchback protein concentration gradient along the anterior-posterior axis that, in turn, affects the expression of other genes and helps determine the anterior and posterior structures.

Concepts J"

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