Interactions of zygotic genes

Question 1

What is the general hierarchy of zygotic genes?

Answer 1

Differing concentrations of gap genes cause transcription of pair rule genes which in turn activates transcription of segment polarity genes.

Question 2

How are gap genes initially expressed?

Answer 2

Maternal effect genes and gap genes determine initial gap gene expression. hunchback creates a gradient important for establishing initial gap gene expression. High levels of hunchback, along with maternal bicoid result in expression of giant. Low levels allow expression of kruppel.

Question 3

How are gap gene interactions stabilised and maintained, and give examples.

Answer 3

By interactions between gap gene products themselves. Stabilised by mutual repression between pairs of nonadjacent gap genes - e.g Giant vs. Kruppel, Hunchback vs. Knirps

Question 4

How do gap genes regulate pair rule genes?

Answer 4

Each stripe determined by pair rule genes is activated by specific gap gene combination. For exmaple even-skipped is under the control of a series of enhancer elements able to bind Bicoid and Kruppel TFs. Stripe 2 is activated by hunchback, repressed by Giant and Kruppel.

Question 5

How do pair rule genes activate segment-polarity genes?

Answer 5

Combinatorial action of pair-rule genes determine expression of segment polarity genes after cellularisation.

Question 6

How is engrailed activated?

Answer 6

By high levels of even-skipped, fushi tarazu or paired. Repressed by odd-skipped, run or sloppy-paired. Therefore engrailed is expressed in all 14 segments.

Question 7

How is wingless activated?

Answer 7

Activated when little even-skipped so anterior to engrailed.

Question 8

How does wingless regulate hedgehog and engrailed?

Answer 8

1. Wingless diffusible protein binds to frizzled receptor,
2. Activates dishevelled
3. Inhibits ZW3
4. Releases armadillo to nucleus
5. Activates hh and en

Question 9

How does hedgehog activate Wg?

Answer 9

1. Hedgehog diffusible protein binds to patched
2. Smoothened no longer inhibited
3. Smoothened activates cubitus interruptus
4. Enters nucleus, activates Wg

Question 10

How is segment polarity maintained?

Answer 10

By feedback loop between Wg and Hh signalling.

Later, both act as morphogens to control parasegmental identities