Morphogens are signals that form concentration gradients to specify multiple cell types during development. Cells respond to different morphogen thresholds to activate distinct gene expression. Bicoid is a transcription factor that acts as a morphogen gradient in Drosophila, turning on different genes in different embryo regions based on its concentration. It forms a gradient from anterior to posterior, with high concentrations specifying head structures and lower concentrations specifying more posterior structures. This gradient establishes the basic body plan of the fruit fly embryo.

1. Morphogen gradients
By
Mrs Sanchita Choubey
(M.Sc., PGDCR, Pursuing Ph. D)
Assistant Professor of Microbiology
Dr. D Y Patil Arts Commerce and Science College Pimpri, Pune

2. science-img.com

3. science-img.com
**Morphogenesis is the outcome of correct pattern
formation

4. •A morphogen is a signal (usually secreted from a
subset of cells) that elicits different cellular responses
at different concentrations.
•A morphogen specifies more than one cell type by
forming a concentration gradient, ie- it diffuses from
its site of synthesis to become progressively less
concentrated farther from the source of its synthesis.
•Cells respond to different, or threshold,
concentrations, of the morphogen by activating
expression of distinct sets of genes.
•Thresholds can represent the amount of the
morphogen required to bind to receptors for activation
intracellular signaling, or concentrations of
transcription factors required to activate certain genes

5. *The very first step in patterning the
embryo of the fruit
fly, Drosophila melanogaster, is a good
example of pattern formation by a
gradient.
*Bicoid is a transcription factor which turns on different genes in different
levels - acting as a morphogen gradient.

6. The four genes shown in part
A (tailless, empty spiracles,
hunchback, and kruppel) are
found in different locations
within the Drosophila
embryo, as a result of the
amount of Bicoid protein at a
particular location in the
embryo.

7. The empty spiracles gene is necessary for proper head formation
Kruppel :Termed as gap gene .. Meant for development of Centre of embryo Hunchback
are the maternal effect genes that are most important for patterning of anterior parts
(head and thorax) of the Drosophila embryo
Tailles : Posterior part

8. After fertilization, bicoid mRNA from the mother fly
begins to be translated into Bicoid protein in the
Drosophila zygote.
image B shows how the Bicoid protein diffuses through
the egg forming a gradient.
High concentrations of Bicoid protein are shown in white on the left (anterior) end of the
zygote, and low concentrations are shown in blue on the right (posterior) end.

9. Image C shows Bicoid protein in
the nuclei of a Drosophila embryo
after a number of rounds of
mitosis. The nuclei in the
anterior end (left) have more
Bicoid protein than those in the
posterior end (right)

10. Image D shows Kruppel protein in
orange and Hunchback protein in
green. The region where the two
proteins overlap is yellow. The
colors come from fluorescent dyes
attached to antibodies that bind
specifically to these proteins.

11. studentreader.com
•The asymmetric segregation of cytoplasmic determinants
is due to asymmetric localization of molecules (usually
proteins or mRNAs) within a cell before it divides.
•During cell division, one daughter cell receives most or all
of the localized molecules, while the other daughter cell
receives less (or none) of these molecules.
•This result in two different daughter cells, which then
take on different cell fates based on differences in gene
expression.
•The localized cytoplasmic determinants are often mRNAs
encoding transcription factors, or the transcription factors
themselves.