L18, 19 & 20: Developmental Axes

Question 1

How do morphogen gradients work?

Answer 1

1. Cells secrete morphogen
2. Gradient formed
3. Cells in different positions perceive different morphogen concentrations
4. Levels of activated transcription factors vary
5. Different sets of genes are expressed

Question 2

Describe the 3 axes of the body for drosophila and for a human.

Answer 2

Question 3

What are the 4 proteins that interact to determine anterior-posterior axis?

Answer 3

1. Bicoid
2. HB-M (Hunchback)
3. Nanos
4. Caudal

Question 4

Describe the concentration of bicoid and nanos protein at the anterior and posterior end.

Answer 4

Anterior end has a high concentration of bicoid

Posterior end has a high concentration of nanos.

Question 5

How do bicoid, nanos and hunchback interact to form gradients?

Answer 5

mRNA is MATERNALLY deposited in the oocyte.

Bicoid mRNA stimulates hunchback expression

Nanos mRNA inhibits hunchback expression

Thus hunchback regulates genes for anterior structures.

Question 6

How is HB-M regulated by nanos.

Answer 6

In the anterior end, hunchback mRNA is adenylated, and is translated to form the hunchback protein.

In the posterior end, the nanos protein interacts with pumilio, causing deadenylation of the hunchback mRNA resulting in NO translation.

Question 7

Describe the interactions of the caudal gene.

Answer 7

Caudal gene is only translated after fertilisations.

Caudal mRNA is evenly distributed, but an anterior-posterior gradient is formed by the inhibition of caudal protein by bicoid.

Thus, bicoid inhibits caudal.

Nanos inhibits hunchback.

Question 8

Describe the process of cleavage in drosophila embryo.

Answer 8

1. Nuclei undergo cleavage
2. Nuclei move to edges of embryo and pole cells form
3. Syncytial blastoderm forms
4. Cellularisation of nuclei (membranes form around them) forms the cellular blastoderm.

Question 9

What are the signals involved in dorsal-ventral axis formation?

Answer 9

Dorsal transcription factor gradient is formed through the interaction of Spatzle and Toll gene products.

Dorsal is evenly distributed through the embryo.

Spatzle-Toll complex triggers signal transduction pathway that releases dorsal.

• HIGH concentration of SPZ-ligand on ventral side (results in activated dorsal on ventral side)
• LOW concentration of SPZ-ligand on dorsal side (no activated dorsal)

Question 10

Describe the process of activation of dorsal.

Answer 10

Spaztle ligand bind to TOLL on embryo cell surface, forming the Spatzle-Toll complex.

Cactus phosphorylation releases dorsal

Active dorsal moves to the nucleus.

Question 11

What are the 5 steps involved in forming the A-P body axis plan.

Answer 11

1. Maternal effect genes function
2. Gap genes divide embryo into broad regions
3. Pair-rule genes divide embryo into strips, defining segment borders
4. Segmentation polarity genes divide segments into anterior-posterior halves
5. Homeotic selector genes identify specific fatesof each segment.

Question 12

What are gap genes

Answer 12

Gap genes are the first zygotic genes to be expressed along the A-P axis. All before this are maternal genes.

maternal bicoid mRNA activates hunchback. Hunchback activates other gap genes.

E.g. Kruppel responds to hunchback in concentration dependent manner.

• High levels of HB repress Kruppel
• Medium levels of HB activate Kruppel
• Low levels of HB fail to activate Kruppel

Question 13

What are homeotic selector genes?

Answer 13

Determine specific fates of the segments.

Homeotic selector genes contain the homeobox domain. These activate/repress certain genes.

Example: Deletion of a homeotic selector gene turned what would be a 3rd thoracic segment into one with wings - resulted in a fly with 4 wings rather than two.