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Development > Segment differentiation in drosophila > Flashcards

Segment differentiation in drosophila Flashcards

1
Q

Define a homeotic selector gene.

A

Genes responsible for segment identity.

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2
Q

What are nanos and bicoid?

A

AP egg polarity genes.

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3
Q

How do egg polarity and segment polarity genes interact?

A

To determine segment boundaries.

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4
Q

What does a homeotic mutation do?

A

Transforms one segment into another.

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5
Q

Antennapedia is a homeotic mutation. What happens?

A

It turns the antennae into legs.

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6
Q

What kind of mutation is antennapedia?

A

A dominant mutation.

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7
Q

Give an example of a homeotic double mutant?

A

bx and pbx: causes the fourth segment to also produce wings, so the fly has 2 sets.

N.B. normal wings are on the 3rd thoracic segment.

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8
Q

Larvae and adults looks vastly different. Are their segments equivalent?

A

Yes.

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9
Q

What is the basic segmental structure from anterior to posterior?

A

Head, T1-3, A1-9.

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10
Q

Which segments are the genitalia located on?

A

A7-9.

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11
Q

Define a parasegment.

A

Larval segments that are equivalent to adult segments, but are out of phase with the adult boundary.

Essentially its the same as the adult segment but isn’t in the right place yet.

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12
Q

How was engrailed expression used to identify parasegments?

A

Engrailed is switched on permanently.
En in larvae was found at the anterior of the segments, but en in adults was found at the posterior.
The segments were said to be 1/4 out of phase in larvae.

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13
Q

Which pair rule genes establish segmental boundaries?

A

Ftz and eve, the first two pair rule genes to be expressed.

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14
Q

Why are the parasegments important to larval development?

A

Allows the larva to move, ‘facilitates coordination of muscle blocks’ relative to the adult nerve cord.

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15
Q

Which 2 genes determine parasegment identity?

A
  1. ANT-C: antennapedia complex

2. BX-C: bithorax complex

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16
Q

Where are ANT-C and BX-C located?

A

On chromosome 3 in two separate clusters.

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17
Q

By how many base pairs are the ANT-C and BX-C complexes split by?

A

9.6Mb

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18
Q

Which parasegments does ANT-C specify?

A

PS 0-5 (basically anterior segments)

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19
Q

Which parasegments does BX-C specify?

A

PS 5-14 (basically posterior segments)

20
Q

The ANT-C and BX-C complexes are regulated by the same mechanisms. True or false?

A

False: they are regulated independently, although due to close proximity do share some regulatory mechanisms.

21
Q

What does Lewis’ Model of Mutation look at?

A

Homeotic BX-C mutants.

22
Q

Under Lewis’ model of mutation there are 2 types of mutant. What are they?

A
  1. Recessive

2. Dominant

23
Q

For recessive mutants in Lewis’ model of mutation:

a) What are they caused by?
b) What do they result in?
c) What kind of transformations do they cause?

A

a) 6 genes
b) Loss of function
c) Anterior-ward transformations

24
Q

For dominant mutants in Lewis’ model of mutation:

a) What are they caused by?
b) What do they result in?
c) What kind of transformations do they cause?

A

a) 3 genes
b) Gain of function
c) Posterior-ward transformations

25
Q

Define a point mutation.

A

A mutation affecting only one or a few nucleotides in a sequence.

26
Q

Both recessive and dominant BX-C mutations are point mutations. True or false?

A

True.

27
Q

What happens if you BX-C?

A

It results in the conversion of all parasegments to PS4, the anterior-most PS.

28
Q

Which parasegmental region does ANT-C specify?

A

Anterior

29
Q

Which parasegmental region does BX-C specify?

A

Posterior

30
Q

The way the genes map through the segments is reflective of their location on the chromosomes. What does this mean in terms of expression?

A

Effectively they are ‘read’ in a list: the first genes in the sequence are active in all the segments, whereas the genes towards the end are only active in the posterior segments.

31
Q

Thus what can be said about parasegment identity?

A

‘It is reflective of the posterior-most homeotic gene active in that segment’.

Basically each segment is identifiable by the last gene active in its specification, as this is the only part that varies.

32
Q

In Lewis’ model of mutation he hypothesised repressor, a chemical that varies in concentration across the AP axis. What level is the repressor at a) the anterior and b) the posterior of the embryo?

A

a) High conc.

b) Low conc.

33
Q

In Lewis’ model of mutation, the a) anterior and b) posterior homeotic genes vary in sensitivity to the repressor. How?

A

a) Least sensitive

b) Highly sensitive

34
Q

Why is there varying sensitivity to the repressor molecule across the homeotic genes?

A

The promoter for each gene has varying affinity.

35
Q

What happens to all the parasegments if you delete the polycomb complex? Why?

A

It results in the conversion of all parasegments to PS14, the posterior-most parasegment.
This is because the deletion inactivates the repressor molecule, so all genes are active in all segments. As iab8 is the last gene in the list and specifies PS14, they all become PS14.

36
Q

What assumption does the Lewis model of mutation act under?

A

Each mutation controls a separate gene and thus transcriptional unit.

37
Q

There are 3 major transcription units. What are they and where are they active?

A
  1. Ubx: PS5-14
  2. Abd-A: PS7-10
  3. Abd-B: PS10-14
38
Q

What can be said about transcriptional units in terms of PS identity?

A

PS identity depends on the most posterior-acting TU.

39
Q

In PS mutation, which part of the transcriptional unit is affected?

A

The cis-regulatory region.

40
Q

Define a compartment.

A

A section of a parasegment.

41
Q

Which gene is important in defining the anterior and posterior compartments of each segment?

A

Engrailed.

42
Q

In homeotic mutations, numerous parasegments must be mutated in order to affect a single adult segment. Why?

A

The parasegments overlap with the adult segments as they are not yet properly aligned.

43
Q

What is the vertebrate equivalent of ANT-C and BX-C?

A

All genes are conserved in a homeodomain, also called a hox sequence.

44
Q

In vertebrates, how are hox genes a) arranged and b) expressed?

A

a) Arranged in clusters

b) Expressed in distinct domains along the AP axis

45
Q

Vertebrate hox genes display co-linearity. What does this mean?

A

They are expressed in the order they are found in along the chromosome. E.g. genes at the start of the sequence are expressed anteriorly etc.

Development (18 decks)

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