BIOL 303 Study Questions Set 7
1. The Kruppel gene is expressed in a single broad stripe in the mid region of the
Drosophila syncytial blastoderm embryo. What does the expression of Kruppel look
like in a bicoid mutant? What accounts for this expression pattern?
Gap genes: mutants lack large regions of the body – several contiguous segments.
Activated or repressed by maternal genes. Where we see expression of gap gene
in mutant is exactly where we see that part of the body deleted
Gap genes include Hunchback, Giant, Knirps and Kruppel.
These genes are DNA binding proteins, short lived.
Initial pattern of gap gene expression is stabilized by repressive interactions
between different gap genes – gap gene expression can overlap but there is
strong mutual repression between pairs of nonadjacent gap genes.
Depending on concentration, Hunchback may either repress (when high) or
activate (when low) the expression of Kruppel.
Bicoid mutant only has maternal Hunchback protein. There is no additional zygotic
expression of hunchback in the bicoid mutant, so levels are not sufficient enough
to repress Kruppel. 2. What is the difference between primary and secondary pair-rule genes?
Pair-rule genes: pair rule gene expression establishes the segmental body plan
Primary pair-rule genes: stripe expression pattern is set up by the action of
different gap genes with enhancer elements – each stripe expression is
established “from scratch” i.e., expression within each stripe is associated with a
different enhancer-gap gene combination. There is one enhancer for each stripe.
Primary pair-rule genes depend on maternal and gap transcription factors for its
activation. There is one enhancer for one stripe.
Secondary pair-rule genes: primary pair-rule genes influence expression. Ftz gene
– secondary pair-rule gene.Get turned on by primary gene Ftz transcript is
uniformly expressed at low levels throughout entire embryo. Primary pair rule
will interact with ftz and repress expression in certain areas. Can autoregulate
(ftz makes more ftz).
Secondary pair-rule genes depend on primary pair-rule genes for their activation.
There is one enhancer for all stripes.
3. Where and when are the pair-rule genes expressed? Where and when are the
segment polarity genes expressed?
Pair-rule genes: expression begins during cycle 13 (just before cellularization) –
giving 7 transverse stripes. Pair rule gene expression establishes the segmental
Segment polarity genes: are expressed after cellularization; reinforce the
parasegment pattern of gene expression and establish cell fates within each
4. How are stripes of engrailed and wingless expression maintained?
The expression patterns of the segment polarity genes engrailed (en) and wingless (wg) are established through positive and negative transcriptional regulation by
the pair-rule genes
For example, the expression of en is activated by either ftz or eve in each
parasegment, whereas wingless is repressed by either ftz or eve in each
Overall - wingless is transcribed only in the row of cells anterior to the row of cell
transcribing engrailed. Engrailed expression marks the anterior compartment of
Engrailed and wingless are both required for each other’s expression. They
maintain each other’s expression but since they are in different cells, cell-cell
communication must occur. Engrailed encodes a homeodomain protein;
wingless encodes a secreted peptide. Wingless is secreted from the cells which
make it. When the wingless protein binds