Experimental embryology and inductive signals

Question 1

What do transcription factors do?

Answer 1

Ensure that the correct genes are transcribed at the right time.

Question 2

How do transcription factors work - what is their mechanism?

Answer 2

They are proteins that work within the nucleus that can bind to the DNA to cause transcription of other genes or repress the transcription of other genes.

Question 3

What are inductive signals?

Answer 3

Extracellular signals that cause cells to follow a different fate. They may cause transcription factors to be produced in a target cell.

Question 4

How can other cells respond to inductive signals?

Answer 4

The cell surface membranes have receptors that can recognise signals in the extracellular matrix.

Question 5

How do inductive signals cause different cells to have varying growth patterns?

Answer 5

The closer to the cell producing the inductive signals, the highest dose they will receive. The further away, the less. The higher the dose the more transcription factors that will be switched on.

Question 6

What is one way to find inductive signals experimentally?

Answer 6

Experimental embryology.

Question 7

What was the basis of the first experiments carried out by Hans Spemann in the early 1900s?

Answer 7

Experiments with newts. He was curious as to what was important about the grey crescent area and the experiments involved putting ligatures around the embryos to restrict where the DNA can go.

Question 8

What were the two experiments did Spemann do?

Answer 8

In one experiment the grey crescent was divided in two and in the other experiment the grey crescent was only on one side.

Question 9

What were the results of Spemann’s experiments?

Answer 9

When the grey crescent was divided in half the newts developed normally, whereas when it was only allowed to go to one side normal development was not observed.

Question 10

What other experiment did Spemann do with newts?

Answer 10

He cut out bits of tissue from an early gastrula stage embryo and moved it to the other side on the ventral region.

Question 11

What was the result of the grafting experiment Spemann did?

Answer 11

The embryos formed were conjoined.

Question 12

How are the dorsal/ventral regions specified in a blastula?

Answer 12

Depending on where the sperm enters.

Question 13

How is left/right symmetry determined?

Answer 13

The first cleavage forms the two blastomeres that depends on the grey crescent.

Question 14

How are the anterior/posterior regions determined?

Answer 14

The first cells that move into the dorsal lip are the ones that will form the anterior structures. The final cells that come in will form the posterior tissue.

Question 15

Define cell fate.

Answer 15

Depending on where a cell is in an embryo it is going to become a certain type of cell in the future.

Question 16

What change in fate does the organiser graft make?

Answer 16

It induces a dorsal fate in cells that would otherwise follow a ventral fate.

Question 17

What would happen to the donor embryo following the organiser graft?

Answer 17

The embryo would have no dorsal signalling centre so would develop without a head and backbone.

Question 18

What role does signal transduction play?

Answer 18

It relays the message from the extracellular signal from the membrane to the nucleus where it activates gene expression - e.g a transcription factor.

Question 19

What is the “generating mutants” method of finding genes responsible for development?

Answer 19

Inject lots of mRNA into a fertilised egg. If enough different types of mRNA is injected it is likely you will form a mutated embryo.

Question 20

How was the mRNA formed to inject into the fertilised eggs?

Answer 20

The gene was put into a plasmid and underwent in vitro transcription. The mRNA can then be injected into the embryo.

Question 21

What is the “rescue mutants” method for finding a gene responsible for development?

Answer 21

UV irradiating eggs, resulting in deformed embryos. mRNA can be used to try and “rescue” the development of the embryo - trying different mRNA can deduce what genes are important in development.

Question 22

What is noggin?

Answer 22

A signalling molecule that can promote dorsal structures.

Question 23

How did noggin affect the mutants in the “rescue mutants” method?

Answer 23

A low amount of noggin reversed the phenotype and a high amount of noggin caused only the head to develop and not the body.

Question 24

What is the basic method to find where the gene is expressed using in situ hybridisation ?

Answer 24

The fixed embryos (dead) can have a probe added that is complementary to the mRNA you’re interested in - this will bind to the mRNA. You can buy antibodies that detect signals in the mRNA probe which will also bind. A substrate can be added to get staining at these areas - this is where the mRNA is expressed.