Drosophila

Question 1

Define a morphogen.

Answer 1

A compound that, in varying concentrations, can cause cells to differentiate. The same molecule can produce various cell types depending on concentration.

Question 2

What kind of information does a morphogen convey to a cell?

Answer 2

Positional.

Question 3

Explain Wolpert’s French Flag model.

Answer 3

There are source (high) and sink (low) levels of morphogen at opposite ends of the organism. This creates a gradient across the embryo. Cells respond in strength accordingly, i.e. highest conc. of morphogen gives the strongest response etc.

Question 4

In which organism was the first ever morphogen discovered?

Answer 4

Drosophila.

Question 5

What is the drosophila ovary called?

Answer 5

The ovariole.

Question 6

How many eggs does the ovariole produce everyday?

Answer 6

~100, regardless of prospective fertilisation.

Question 7

What are the oocytes formed from?

Answer 7

Multipotent stem cells.

Question 8

In oocyte formation, is cleavage asymmetric or symmetrical?

Answer 8

Asymmetric.

Question 9

What happens to the oocyte at the 16-cell stage?

Answer 9

15 of the cells become nurse cells and 1 continues as the oocyte.

Question 10

What do the nurse cells do?

Answer 10

Feed the oocyte.

Question 11

What connects the nurse cells and oocyte?

Answer 11

Cytoplasmic ridges

Question 12

What does the follicle cell do?

Answer 12

Produces the shell around the egg.

Question 13

What happens to the nurse cells further in to development?

Answer 13

They transfer all the RNA and yolk proteins to the oocyte and shrivel up and die.

Question 14

What kind of cleavage does the embryo undergo? What happens?

Answer 14

Superficial: only the nuclei cleave at first, there is no cleavage of cells. This forms syncytial blastoderm. The nuclei then migrate to the surface and cellularisation occurs.

Question 15

How is the drosophila endoderm formed?

Answer 15

Furrows at the anterior and posterior ends of the embryo invert to form the endoderm.

Question 16

How is the drosophila mesoderm formed?

Answer 16

The ventral furrow forms the mesoderm.

Question 17

What happens in germ-band extension?

Answer 17

During the formation of the germ layers the tail flexes up over the head and back.

Question 18

How long does it take for the larvae to hatch after the eggs are laid?

Answer 18

~24 hours.

Question 19

What do the larval cuticular denticles indicate? Why is this useful?

Answer 19

Internal segmentation. This is useful in experiments: scientists create mutants to deduce the function of segment genes which are visible from the surface.

Question 20

Define an egg polarity gene.

Answer 20

Genes that are expressed in gradients and specify the AP axis across the syncytium.

Question 21

There are various types of egg polarity gene. What are the 3 main mutants?

Answer 21

1. Anterior: have no head
2. Posterior: have no abdomen
3. Terminal: have no telson

Question 22

What is arguably the most important anterior gene?

Answer 22

Bicoid is a nuclear transcription factor.

Bicoid RNA is anteriorly localised, although it diffuses across the embryo following the FF model.

Question 23

What do bicoid mutants lack?

Answer 23

Anterior segments.

Question 24

What is an important posterior gene?

Answer 24

Nanos.

Question 25

How do bicoid and nanos interact in AP axis formation?

Answer 25

They create opposing gradients across the embryo to specify the AP axis.

Question 26

What happens in experiments when scientists increase the concentration of morphogen in the wrong place? Give an example.

Answer 26

Ectopic body parts form, e.g. injecting bicoid mRNA at the posterior greated a second head.

Question 27

What happens if you increase bicoid copy number at the source?

Answer 27

The cephalic furrow shifts posteriorly with the gradient.

Question 28

There are 3 main types of zygotic segmental gene. What are they?

Answer 28

1. Gap genes
2. Pair rule genes
3. Segment polarity

Question 29

The names of segmental genes refer to the mutant phenotype. What are gap gene mutants like?

Answer 29

There is a lack of contiguous (touching) body segments, resulting in ‘gaps’ in the body plan.

Question 30

What do gap genes all code for?

Answer 30

Transcription factors.

Question 31

How is the expression of gap genes described?

Answer 31

Transient.

Question 32

The names of segmental genes refer to the mutant phenotype. What are pair rule gene mutants like?

Answer 32

They lack every other segment.

Question 33

What do pair rule genes code for?

Answer 33

Transcription factors

Question 34

When are pair rule genes at maximum expression?

Answer 34

During cellularisation of the syncytium.

Question 35

Pair rule genes can be observed at blastoderm stage as what?

Answer 35

7 evenly spaced, transverse stripes.

Question 36

Which genes are the first to be expressed in the zygote?

Answer 36

Gap genes.

Question 37

The names of segmental genes refer to the mutant phenotype. What are segment polarity mutants like?

Answer 37

Parts of each segment are missing and replaced with mirror images.

Question 38

When are segment polarity genes expressed?

Answer 38

Later on in development and into adulthood.

Question 39

What do segment polarity genes code for?

Answer 39

Some are transcription factors, other are signalling molecules.