Biology Subject 1 - PSL

Biology

Subject 1 (Questions are in bold)

Studying Cellular Neurophysiology often comes down to measuring ion flows.

Why ionic flows through membranes are a key issue in neuronal function?

In the following images, the authors record potentials (in mV) or currents (in nA) in two

synaptically connected neurons (from Llinas et al., 1992).

A, recordings of presynaptic and postsynaptic action potentials.

B, the "pre" electrode imposes a potential waveform that mimics the original action

potential. This elicits a change in membrane potential in the postsynaptic cell. Also shown the

calcium current (I Ca) to be measured in D to F.

C, superimposition of A and B.

D to F: techniques for determining the calcium current (I Ca).

D, like in B, the "pre" electrode imposes an action potential waveform and elicits a

potential in the postsynaptic cell. Also recorded is the current passing through the "pre" electrode

(I total).

E, same than D but in the presence of Cd2+

.

F, difference between D and E.

What kind of proteins is responsible for the action potential? Represent in a simple scheme

their opening and closing during the action potential. Do you know another state apart

from open or closed?

Why measuring calcium in neurons is of particular interest? Do you know different

methods to measure it?

Why there is calcium current in the presynaptic cell?

What happens at the synapse when calcium enters the presynaptic element?

What is the effect of Cd2+

?

What is the change in postsynaptic potential shown in B or D? How does it originate?

Study of cathecolamine release in a rat chromaffin cell.

Vesicle fusion (exocytosis) can be studied by combining two techniques: amperometry and

electrophysiology. Amperometry can detect oxydizable molecules on a carbon fiber.

Electrophysiology can be used to measure the electrical properties of (here) a patch of plasma

membrane.

(a) A schematic drawing showing the cell-attached capacitance recording combined with patch

amperometry (carbon fiber electrode) to study vesicle fusion.

(b) An event revealed by the cell-attached capacitance recording combined with patch

amperometry at a rat chromaffin cell.

(i) Diagram of a vesicle illustrating the processes occurring in the traces

(ii) before, during and after the event.

Trace of the capacitance

Gp is the calculated fusion pore conductance.

The amperometric signal (A) was detected with a carbon fiber electrode inside the patch pipette.

The amperometric signal indicates transmitter release.

Summarize the experiment

Can you imagine a different exocytotic event? Describe it with a scheme (like in (i)) and

traces (like in (ii))

Do you know specific proteins involved in exocytosis?

Capacitance

BIOLOGY

Subject 2

(Questions are in bold)

What is a metalloprotease?

Which are their functions?

How are metalloproteases secreted in the extracellular space? Why?

Cite two metalloprotease families, and a major difference between them.

A) MT1-MMP is a transmembrane metalloprotease. This study investigates its role during

cell migration.

In order to determine the subcellular location of the protein, a specific antibody has been used

onto cultured cells (osteoclasts). Figure 1 shows a cell after double staining with the antibody

anti-MT1-MMP (in A) and phalloidin coupled to a fluorochrome (in B). Phalloidin interacts

with filamentous actin. This cell was migrating; the migration direction is indicated by the

arrow.

fig1

Is the cellular distribution of MT1-MMP homogeneous? Where is it localised?

What kind of adhesion structure happens to be at this place?

Why doing this double staining?

Propose an experience to demonstrate that MT1-MMP is associated with the adhesion

structures that you have cited previously.

B) A cDNA construct MT1-MMP bearing a FLAG motif has been transfected into cells in

suspension. After a certain delay to allow protein expression, cells are seeded onto coverslips

covered with fibronectin. Then cell behaviour is observed at different times (40 min, 3h, 12h)

after seeding. The markers followed are: MT1-MMP-FLAG (with an antibody anti-FLAG)

and a protein from an adhesion complex called paxillin (with a specific antibody), Fig 2.

Indicate with an asterisk on the "overlay" pictures the non-transfected cells.

In fact, the fibronectin used to cover the coverslips has been coupled to FITC (FN-FITC).

During the experiment, this allows the identification of the areas lacking FN: they appear

darker.

What is fibronectin?

Why fibronectin disappears at some places? Propose an explanation.

Compare the size of the zones where fibronectin has disappeared with the size of cells.

Does it suggest something to you?

NB: images at different times correspond to different fields

MT1-MMP-FLAG Paxillin Figure 2

C) In another set of experiments, cell motility has been measured in three different situations:

Control (DMSO), in the presence of BB94 (an inhibitor of MT1-MMP), and after transfection

with the construct MT1-MMP-FLAG (fig3).

Fig 3

Comment the results of this figure.

Considering the results obtained in figures 1, 2 and 3, propose a recapitulative scheme.

MT1-MMP-FLAG

Ecole normale superieure Selection Internationale 2009 Biologie Sujet proposé par Régis Ferrière. English version. ECOLOGY The test includes 8 questions (independent of one another) and three figures. Question 1 (a) What is an ecological disturbance? List five examples of ecological disturbance. Compare and contrast their effects. For each type of disturbance, compare and contrast the consequences of high-frequency and low frequency disturbances and high and low severity of disturbances. (b) How would you design an experiment to test the hypothesis that increasing species richness increases a community’s productivity and ability to resist disturbance and recover from disturbance? Question 2 Summarize the life history attributes of early successional versus late successional species. Why are these attributes considered adaptations? Question 3 Describe the continental pattern in species diversity across latitude that exists for many taxonomic groups (an example is shown in Figure 1). State and discuss one classical hypothesis to explain this pattern. Question 4 State three hypotheses that have been proposed to explain the low level of herbivory in terrestrial plant communities. Are these hypotheses mutually exclusive? Explain why or why not. Question 5 (a) How does competition, predation, and mutualism each affect the size of interacting populations and their long-term evolution? Illustrate your answer with give specific examples. (b) Explain what a coevolutionary arms race is and give an example. (c) Give an example of how competition can evolve into commensalism, and how a mutualistic relationship can evolve into a parasitic one. For each case explain what the selection pressures are. Question 6 Some insects harvest nectar by chewing through the wall of the structure that holds the nectar. As a result, they obtain a nectar reward, but pollination does not occur. Suppose that you observed a certain bee species obtaining nectar in this way from a particular orchid species. How might you expect the characteristics of the orchid population to change over the short term in response to this bee behavior? And over long term ?

Question 7 In ocean intertidal zones where blue mussels occur, crabs are potential predators (Figure 2a). Figure 2b shows observational data on the strength of attachment of mussels to their substrate (rocks) and on mussels’ shell thickness. The data were gathered from two types of sites: low predation sites where crabs are rare, and high predation sites where crabs are abundant. What is shown by the data? What hypothesis can you propose to explain the pattern? How would you set an experiment to test your hypothesis? Question 8 Beavers are well known for selectively cutting riparian trees like cottonwoods to build dams and thus shape the habitat to suit their life cycle (Figure 3a). Figures 3b and 3c show the results of two experiments: the concentration of salicortin produced by artificially cut and resprouted trees versus control trees (Figure 3b), and the survival time of beetles that usually feed on these trees, when exposed to predation risk by ants (Figure 3c). Describe and comment these data. Explain why such results have led to view beavers as “ecosystem engineers”. (End of Test. See Figures on next two pages.)

Figure 1. Caption translation/Traduction des légendes : Equator = Equateur. Near poles = Vers les pôles. Latitude (degrees north or south) = Latitude (degrés Nord ou Sud). Number of vascular plant species per 10,000 km2 = Nombre d’espèces de plantes vasculaires par unité de surface (10000 km2). Figure 2. Caption translation/Traduction des légendes : Type site = Type de site. Low predation = Faible prédation. High predation = Forte prédation. Attachment strength (Newtons, N) = Force d’accrochage (Newtons, N). Shell thickness (mm) = Epaisseur de la coquille (mm).

Figure 3. Caption translation/Traduction des legendes : Control trees = Arbres contrôles. Cut and resprouted trees = Arbres coupés et régénérés. Salicortin concentration (mg/g dry mass) = Concentration en salicortine (mg/g matière sèche). Larvae from control trees = Larves prélevées sur arbres contrôles. Larvae from cut and resprouted trees = Larves prélevées sur arbres coupés et régénérés. Larval survival time (sec) = Temps de survie des larves (secondes).