Lecture 8

Question 1

What are the functions of the homeotic genes?

Answer 1

They control the identity of the segments in the drosophila and are generated by segmentation genes, transcribed just before cellularisation

Question 2

What is the expression pattern of the homeotic genes?

Answer 2

Their expression is transient unless it becomes fixed which involves alteration of the chromatin conformation which is caused by the polycomb and trithorax genes

Question 3

What occurs if there is a mutation in the homeotic genes?

Answer 3

There is a substitution of one body part for another

Question 4

What is the hom cluster?

Answer 4

The drosophila homeotic genes are in two complexes on chromosome 3 known as the antennapedia complex and the bithroax complex these two complexes are collectively known as the hom cluster

Question 5

What is the function of the bithorax complex?

Answer 5

Controls differences in the posterior second thoracic segment and the abdominal segments
it is made up of the members Ubx, abd-A, Abd-B which all contain a homeobox and act combinatorially

Question 6

What is the function of ultrabithorax (Ubx)?

Answer 6

Expresed in parasegment 5-14 specifying parasegment 5 and 6

Question 7

What is the effect on drosphila of a deletion of ultrabithorax?

Answer 7

T3 is transformed to default T2 with a four winged fly developing

Question 8

What is the function of abdominal-A (abd-A)?

Answer 8

It is expressed in parasegment 7-14 and works with ultrabithorax to specify parasegment 7-9

Question 9

What is the function of abdominal-B (Abd-B)?

Answer 9

It is expressed in parasegment 10-14 and works in combination with ubx and abd-A to specify most of the posterior parasegments (10-13)

Question 10

What prevents abd-A and abd-B from being expressed in the head and thorax of drosphila?

Answer 10

Gap genes hunchback and kruppel restricts expression in the head and thorax so anterior parasegments must be identified by different genes

Question 11

How does the hom box in drosphila demonstrate co-linearity?

Answer 11

the 3’ to 5’ order of the homeotic genes on chromosome 3 correlates with anterior to posterior expression

Question 12

What is the relationship between the homeobox and the homeodomain?

Answer 12

Hom genes contain a homeobox motif which is 180 base pairs long and is a conserved sequence, genes with this sequence typically make proteins that remain in the nucleus
The homeobox motif code for a 60 amino acid homeodomain which bind to DNA suggesting they act as regulators of transcription
(Not all homeobox motifs are homeotic genes)

Question 13

What are Hom-C Cofactors?

Answer 13

Some homeodomain containing proteins require co-factors in order to function for example Ubx will specify the identity of A1 but unless extradenticle is present Ubx will transform the segment into A3

Question 14

What is the function of realisator genes?

Answer 14

These genes from the tissue or organ primordium specified by the homeotic genes

Question 15

What is an example of a homeotic gene controlling realisator genes?

Answer 15

In the formation of T2 the antennapedia protein represses the enhancers of homothorax and eyeless preventing there expression ensuring that T2 forms a leg

Question 16

How is the dorsal-ventral plan formed in drosophila?

Answer 16

initiated when dorsal protein enters cell nuclei on the ventral side
where it acts as the transcriptional regulator, if this does not occur the fly will have two dorsal sides as the dorsal genes will not be repressed

Question 17

What does positional information specify across the dorsal-ventral axis of the drosophila?

Answer 17

Mesoderm which is specified by the ventral midline
Mesectoderm 
neurogenic ectoderm
epidermis
amnioserosa

Question 18

Where does the dorsal protein which specifies the dorsal-ventral axis come from?

Answer 18

Maternal mRNA which is only translated 90 min after fertilization this is found throughout the entire syncytium but during the 10th division it enters ventral cell nuclei

Question 19

What are the signalling pathways that play a role in drosophila embryo dorsal-ventral patterning?

Answer 19

The gurken-torpedo pathway which acts during oogenesis
The spatzle-toll pathway that operates after fertilization
The decapentaplegic signalling pathway

Question 20

How does the gurken-torpedo pathway specify the dorsal-ventral pathway?

Answer 20

during oogenesis the egg moves posteriorly so the nucleus takes up a posterior position causing posterior location of gurken mRNA which signals the torpedo receptor of posterior follicular cells which signal back to activate protein kinase A to trigger reorganisation of the microtubules of the oocyte so the growing ends are posterior
the nucleus then relocates to an antero-dorsal location via these microtubules and gurken is activated at this position activating dorsal follicle cell fate so the other side must become ventral by default

Question 21

How does dorsal signalling of gurken form the dorsal-ventral axis?

Answer 21

The dorsal expression of gurken activates torpedo in dorsal follicle cells, this activated torpedo then inhibits the expression of pipe resulting in pipe only being found in the ventral surface

Question 22

How does the spatzle-toll pathway form the dorsal-ventral axis of drosophila?

Answer 22

After fertilization pipe which is only found in the ventral cells will sulfate vitelline membrane proteins which will bind to gastrulation-defective which splites snake, cleaving easter spliting spatzle in a serine protease cascade
spatzle then binds to the toll receptor in the oocyte membrane resulting toll only being activated ventrally
toll which then relays in the embryp to pelle which phosphorylates cactus to degrade it freeing dorsal from it allowing it to enter the ventral nuclei

Question 23

What is the spatzle-toll pathway homologous to in mammals?

Answer 23

Homologous to lymphocyte differentiation

Question 24

What is the decapentaplegic signalling protein?

Answer 24

Signalling pathway which establishes patterning of the amnioserosa and dorsal ectoderm, by acting at the dorsal side of the embryo
it includes zygotic genes such as twist, snail and decapentaplegic

Question 25

What effect does dorsal make on zygotic gene expression in the drosophila embryo?

Answer 25

While dorsal is present throughout the cytoplasm only the ventral cells have it enter their nuclei
There it acts as a transcription factor positively regulating twist and snail, these are also transcription factors which establish the ventral mesoderm with twist activating mesodermal genes and snail repressing non-mesodermal genes

Question 26

What is the effect of high concentrations of dorsal?

Answer 26

The activation of rhomboid, but this will be repressed by snail causing it to only be expressed in the venterolateral cells which specify neurogenic ectoderm

Question 27

What is the relationship between dorsal and decapentaplegic and zerknult?

Answer 27

Dorsal negatively regulates these two proteins so they are only expressed on the dorsal side of the embryo causing them to influence the dorsal-lateral and amnioserosa respectively