Exam #3

Question 1

Describe the syncytial blastoderm & type of cleavage it has

Answer 1

Syncytium - 1st 13 cell cycles, the DNA replicates & divides, but the cytoplasm does not divide. It has meroblastic (incomplete cleavage), centrolecithal, and superficial cleavage

Question 2

Syncytial Cell Fate Specification

Answer 2

• Involves interactions between parts of the cell, rather than between different cells
• Cell fates are determined by gradients of substances (morphogens) within the cytoplasm
• Morphogens - diffusible molecules that affect cell fate in a concentration-dependent manner

Question 3

Importance of enhancers

Answer 3

• Reporter gene is only expressed in a stripe that’s regulated by an enhancer to which it is fused
• Mutation of a particular enhancer deletes its particular stripe
• Placement of stripes can change if delete gap genes that regulate them

Question 4

Genes that establish the A-P (Anterior-Posterior) axis

Answer 4

1. Maternal-effect
2. Gap genes
3. Pair-rule genes
4. Segment polarity genes
5. Homeotic genes

Question 5

Examples of Maternal-effect genes

Answer 5

1. Bicoid & 2. Hunchback → make up the head & thorax (anterior) of the Drosophila

• Bicoid mRNA moves on MTs to anterior
• Nanos mRNA moves TSs to posterior

3. Nanos & 4. Caudal → make up the abdomen (posterior) of the Drosophila

Question 6

What are the rules of Bicoid?

Answer 6

1. Represses posterior formation
2. Activate hunchback transcription

Question 7

Homeotic (hox) gene function & characteristics

Answer 7

Hox genes characterize structures of each body segment of Drosophila

Question 8

Is Bicoid a morphogen?

Answer 8

Yes, an anterior morphogen

Question 9

Evidence of Bicoid being a morphogen

Answer 9

• Bcd mRNA is strictly localized to the anterior & Bcd protein forms gradient with highest levels in the anterior (“find it”)
• Bcd mutants lack anterior structures (“lose it”)
• If gradient is disrupted, anterior structures do not assemble correctly (“move it)
• Injected purified Bcd mRNA creates anterior structures

Question 10

Evidence of Nanos being a posterior morphogen

Answer 10

• Nanos localizes mRNA to the posterior region & nanos protein forms gradient highest at posterior end (“find it”)
  Nanos mutants lack posterior structures (abdomen) (“lose it”)

Question 11

Dorsal-Ventral Polarity

Answer 11

• Dorsal is a key player
• Dorsal = transcription factor
• Different concentrations of Dorsal = different cell fates
• Dorsal is present throughout the embryo, only goes into nuclei on ventral side

Question 12

Gap Gene Characteristics

Answer 12

• Activated or repressed by the maternal effect genes
• Divide the embryo into broad regions (e.g. Kruppel)

Question 13

Pair-rule Genes Characteristics

Answer 13

• Activated or repressed by gap genes
• 7 stripes in alternating parasegments (e.g. fushi tarazu)

Question 14

Segment Polarity Genes

Answer 14

• Activated or repressed by the pair-rule genes
• Has 14 stripes aligning with the segments (e.g. engrailed), & 14 segment-wide units

Question 15

Primary Pair-rule Genes

Answer 15

Hairy, runt, even-skipped

Question 16

Function of Segment Polarity Genes

Answer 16

• Mediate intercellular interactions (signaling)

Question 17

Parasegments

Answer 17

• Wg (wingless) is expressed in the posterior region of each parasegment (ps)
• En Hh (hedgehog) is expressed in the anterior region of each parasegment
• Wg % Hh positively regulate, maintain boundary, diffuse-making gradients taht lead to different cellular structures