bicoid (bcd)

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A gene that is essential for normal axis formation in Drosophila. The transcription of bcd takes place in the nurse cells (q.v.), and these are of maternal genotype. This mRNA is transported in a cytoplasmic stream that is pumped by the nurse cells (q.v.) into the oocyte, and here it is localized at the anterior pole. The sequence required for this localization resides in the trailer portion of the mRNA. This bcd mRNA is not translated until after the egg is fertilized. The product is a homeodomain protein that functions as a morphogen for anterior structures. Mothers who lack a functional allele of bcd produce embryos with aberrant or missing head and thoracic structures and anterior abdominal segments. Transplantation of anterior polar cytoplasm from wild type embryos or injection of purified bcd mRNA into bcd-deficient embryos can rescue (q.v.) the mutant phenotype. bcd protein is distributed in an anterioposterior concentration gradient in the zygote (q.v.) and controls the expression of several zygotic genes, depending on its local concentration. The bcd gene is a member of the anterior class of maternal polarity genes, and it lies within the Antennapedia (q.v.) complex. See Chronology, 1988, Macdonald and Struhl; 1988, 1989, Driever and Nüsslein-Volhard; cytoplasmic localization, cytoplasmic determinants, goosecoid, homeobox, hunchback, maternal polarity mutants, nanos (nos), trailer sequence, zygotic gene.

Subjects: Genetics and Genomics.

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