Lecture 10 - Invertebrate models - sea urchin & C. elegans

Question 1

Why is a sea urchin a good genetic model?

Answer 1

• large number of embryos
• experimental manipulation

Question 2

Why are genetic models good?

Answer 2

• bred easily in a lab
• genes within the genome can be altered to study their effect on development

Question 3

What are sea urchins?

Answer 3

echinoderms

Question 4

What are echinoderms?

Answer 4

deuterostomes

Question 5

In protostomes, where does the blastopore form from?

Answer 5

the mouth

Question 6

In deuterostomes, where does the blastopore form from?

Answer 6

the anus

Question 7

Why are sea urchin’s embryos useful?

Answer 7

transparent embryo, which is large & easily accessible for manipulation

Question 8

How long does it take for a blastula to develop in a sea urchin?

Answer 8

4 hours after cell division - meaning they have a strict order & orientation

Question 9

When is the primitive gut formed in a sea urchin?

Answer 9

during gastrulation movements

Question 10

How does the extensions on the plateaus larva form?

Answer 10

the larva has internal calcareous skeleton that is responsible for the extensions on the plateaus larva

Question 11

What were the 2 types of development discussed surrounding sea urchins?

Answer 11

mosaic model and regulative development

Question 12

What is the mosaic model (Weissman)?

Answer 12

the nucleus of the egg contain determinants that specify different fates to different cells by specific segregation to these cells.

Question 13

What is the regulative development model?

Answer 13

cells are communicating and differences can be generated DE NOVO by cell-to-cell communication

Question 14

What divisions do sea urchins have in their early stages?

Answer 14

• 2 divisions along the animal-vegetal axis
• 1 perpendicular to these divisions, separating the animal from the vegetal half

Question 15

In what way are the first 2 cleavages in the development of a sea urchin made?

Answer 15

First 2 cleavages are perpendicular

Question 16

In what way is the 3rd cleavage cut?

Answer 16

Perpendicular - and separates the 4 animal cells from 4 vegetal cells

Question 17

Where does gastrulation occur?

Answer 17

at the vegetal pole

Question 18

How was the sea urchin’s development suggested to be regulative?

Answer 18

if determinants were present, as suggested in the mosaic model, then you wouldn’t get a normal embryo, from a single blastomere

• separated 2 blastomeres at the 2 cell stages
• didn’t get 2 half embryos, but rather got 2 smaller complete embryos

Question 19

How was the sea urchin development suggested to be mosaic?

Answer 19

• splitting the embryo at the 4 cell stage indicated that development was regulative.
• splitting the 8 cell embryo in a ventral & dorsal half showed that there is at least this axis a degree of mosaicism is present.
• normal embryo not found - instead there was an animalised and vegetalised incomplete larve - meaning there appears to be some form of mosaicism.
• now known that the localised determinists for this are in the cytoplasm rather than the nucleus.

Question 20

Why do we use genetic model organisms?

Answer 20

by looking at what ‘goes wrong’, you can infer the normal role of that gene on development

Question 21

What are the ideal characteristics for ease of genetic analysis?

Answer 21

• small organism (because they need to keep large numbers)
• large batches of embryos
• short generation time
• easy to breed
• easy scoring of phenotypes of +/-
• sequenced genome

Question 22

What are the main anatomical parts of C elegans?

Answer 22

mainly consists of a gut & reproductive organs. Also has muscles that allow it to move & neurons and sensory cells that can convey information to the animal about the environment is navigating

Question 23

Why are C. elegans good gene model organisms?

Answer 23

• C. elegans develop rapidly
• hatches from egg within 24 hours at normal temperatures
• develops as a hermaphrodite (first male, then female) - use its own sperm to fertilise eggs
• occasionally male only individuals occur - these can mate with females.
• thus prevents continuous inbreeding, which would be detrimental for their fitness

Question 24

Describe the cleavages of a C. Elegans

Answer 24

1st cell division create: AB & P1 cell
- P1 cell - P2 & EMS
- AB cell - ABa & ABp

ABa (anterior)
ABp (posterior)

Question 25

What does the AB cell do?

Answer 25

make hypodermis & neurones

Question 26

What does the EMS cell do?

Answer 26

make mesoderm & endoderm

Question 27

What do P2 & P3 do?

Answer 27

make C & D cells, which somatic tissues

Question 28

What does P4 do?

Answer 28

form the germline - it also receives cytoplasmic granules named P-granules

Question 29

What causes the first division to be asymmetrical?

Answer 29

Par proteins (par from partitioning) - par genes also found in Drosophila

Question 30

Is cell fate deterministic?

Answer 30

lineage is invariant, but cell fate isn’t necessarily absolutely determined
- this can be seen by changing the cell position.
- e.g. ABa & ABp swapped and lineage produced will be according to their NEW position - this would be impossible if they would contain determinants.
- EMS cell is formed by what originally .would’ve been the P2 cell, so there is no fixed determinant for this either

Question 31

What has C. elegans development study contributed to biomedical science?

Answer 31

• apoptosis = cell death
• RNA interference –> switching off genes

Question 32

How many cells are in C. elegans & what do they do?

Answer 32

C. elegans is made up of 1090 cells but precisely 131 cells are programmed to die - apoptosis

Question 33

When does apoptosis occur?

Answer 33

this happens during development but is also essential in a variety of biological processes in adult

Question 34

Why is apoptosis essential for proper development?

Answer 34

• formation of reproductive organs: male/female
• skin between digits
• immune system maturation

Question 35

Why is apoptosis essential for homeostasis?

Answer 35

• mitosis/apoptosis to maintain constant number of cells
• removal of damaged cells (DNA damage, viral infections)

Question 36

What diseases can improper regulation of apoptosis can to?

Answer 36

• autoimmune disease
• cancer

Question 37

What are the 4 factors involved in programmed cell death?

Answer 37

• BID
• Bcl2
• APAF1
• Caspase

Question 38

What is RNA interference (RNAi)?

Answer 38

a powerful mechanism to control gene activity:
- double-stranded RNA triggers a biochemical process that degrades identical mRNAs, thus it blocks gene activity AFTER transcription has happened.

Question 39

How was RNA interference (RNAi) found?

Answer 39

discovered while studying muscle development in C. elegans

Question 40

How does RNA interference (RNAi) work?

Answer 40

• Double-stranded RNA is taken up by cells & recognised and cut into smaller pieces by an enzyme named Dicer creating short interfering (si) RNAs
• they are loaded into a protein complex named RISC, which uses the siRNA sequence to find mRNAs that are complementary to the siRNA and causes their degradation

Question 41

Why is it important that short RNAs are used in RNA interference?

Answer 41

such oligo can be synthesised chemically

Question 42

How has RNA interference been useful?

Answer 42

this has allowed the creation of siRNA libraries that can target every gene in the human and of course also C. elegans genomes.

this is a great tool to find genes that are involved in the process of choice

Question 43

What is the application of RNA interference?

Answer 43

3 drugs on the market that are directly based on RNAi.
- aim to downregulate genetic diseases caused by overactivity of a gene

Question 44

What are the 3 drugs based on RNAi?

Answer 44

• hereditary transthyretin-mediated amyloidosis
• acute hepatic porphyria
• hyperoxaluria type 1