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Homeotic protein bicoid is encoded by the ''bcd''
Bicoid is one of the few proteins which uses its 
Bicoid protein gradient formation is one of the earliest steps in fruit fly embryo A-P patterning. The proper spatial expression of downstream genes relies on the robustness of this gradient to common variations between embryos, including in the number of maternally-deposited ''bicoid'' mRNAs and in egg size. Comparative phylogenetic and experimental evolution studies suggest an inherent mechanism for robust generation of a scaled Bicoid protein gradient. Mechanisms that have been proposed to effect this scaling include non-linear degradation of Bicoid, nuclear retention as a size-dependent regulator of Bicoid protein's effective diffusion coefficient, and scaling of cytoplasmic streaming.
maternal effect
A maternal effect is a situation where the phenotype of an organism is determined not only by the environment it experiences and its genotype, but also by the environment and genotype of its mother. In genetics, maternal effects occur when an org ...
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
in ''Drosophilia''. Homeotic protein bicoid concentration gradient
Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size (mass) of ...
patterns the anterior-posterior (A-P) axis during ''Drosophila'' embryogenesis. Bicoid was the first protein demonstrated to act as a morphogen
A morphogen is a substance whose non-uniform distribution governs the pattern of tissue development in the process of morphogenesis or pattern formation, one of the core processes of developmental biology, establishing positions of the various ...
. Although bicoid is important for the development of ''Drosophila'' and other higher dipterans, it is absent from most other insects, where its role is accomplished by other genes.
Role in axial patterning
''Bicoid'' mRNA is actively localized to the anterior of the fruit fly egg during oogenesis alongmicrotubules
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...
by the motor protein dynein
Dyneins are a family of cytoskeletal motor proteins that move along microtubules in cells. They convert the chemical energy stored in ATP to mechanical work. Dynein transports various cellular cargos, provides forces and displacements importa ...
, and retained there through association with cortical actin. Translation of ''bicoid'' is regulated by its 3′ UTR and begins after egg deposition. Diffusion and convection within the syncytium
A syncytium (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus) ...
produce an exponential gradient of Bicoid protein within roughly one hour, after which Bicoid nuclear concentrations remain approximately constant through cellularization. An alternative model proposes the formation of a ''bicoid'' mRNA gradient in the embryo along cortical microtubules which then serves as template for translation of the Bicoid protein to form the Bicoid protein gradient. Bicoid protein represses the translation of caudal mRNA and enhances the transcription of anterior gap gene
A gap gene is a type of gene involved in the development of the segmented embryos of some arthropods. Gap genes are defined by the effect of a mutation in that gene, which causes the loss of contiguous body segments, resembling a gap in the norm ...
s including hunchback, orthodenticle, and buttonhead.
Structure and function
Bicoid is one of the few proteins which uses its homeodomain
A homeobox is a DNA sequence, around 180 base pairs long, that regulates large-scale anatomical features in the early stages of embryonic development. For instance, mutations in a homeobox may change large-scale anatomical features of the full-g ...
to bind both DNA and RNA targets to regulate their transcription and translation, respectively. The nucleic acid-binding homeodomain of Bicoid has been solved by NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
. Bicoid contains an arginine-rich motif (part of the helix shown axially in this image) that is similar to the one found in the HIV protein REV and is essential for its nucleic acid binding.
Bicoid protein gradient formation is one of the earliest steps in fruit fly embryo A-P patterning. The proper spatial expression of downstream genes relies on the robustness of this gradient to common variations between embryos, including in the number of maternally-deposited ''bicoid'' mRNAs and in egg size. Comparative phylogenetic and experimental evolution studies suggest an inherent mechanism for robust generation of a scaled Bicoid protein gradient. Mechanisms that have been proposed to effect this scaling include non-linear degradation of Bicoid, nuclear retention as a size-dependent regulator of Bicoid protein's effective diffusion coefficient, and scaling of cytoplasmic streaming.
See also
*Maternal effect
A maternal effect is a situation where the phenotype of an organism is determined not only by the environment it experiences and its genotype, but also by the environment and genotype of its mother. In genetics, maternal effects occur when an org ...
References
{{Reflist Animal developmental biology Drosophila melanogaster genes