Test#3

Bot 305; Summer 2000 Name: KEY

Test #3

Give succinct, precise answers in the space provided after each question. Unless otherwise noted the value of each question answered is 4 points.

1. (a) Give an example of a photoperiodic response in animals.

Ans.: Deer grow antlers in the summer in response to increasing daylength.

(b) What kind of external stimulus would inhibit the flowering of a short-day plant even under short day conditions? State when the stimulus would have to be given in order for it to inhibit the flowering.

Ans.: A red-light pulse given in the middle of the long night would inhibit the flowering

Ans.: A far-red light stimulus would reverse the effects of red light because the photoreceptor for this response is phytochrome, which exhibits R-FR reversibility.

2. (a) In Fig. 1, p. 4, what is the question and what is the answer?

Ans.: The question is whether the photoperiodically induced flowering agent that is produced in the leaf of one plant can be transmitted through grafts to other plants and induce those plants to flower even though they are not photoperiodically induced to flower. Answer is Yes.

(b) What is the regulatory region of a gene and what is the coding region of a gene?

Ans.: The regulatory region of a gene is a sequence that controls the expression of the coding region of that gene., which is the region that is transcribed into mRNA and translated into protein.

(c) How could one manipulate the regulatory region of one gene and the coding region of another gene to get a plant that glows with a circadian rhythm?

Ans.: If one engineers the regulatory region of a gene that is expressed with a circadian rhythm so that it regulates the coding region of a "reporter" gene that encodes a fluorescent protein, then plants transformed with this construct will glow with a circadian rhythm.

3. (a) Briefly state two functions for membrane proteins.

Ans.: They can be receptors; they can be enzymes. [+ 4 other options described in handout].

(b) For each of the functions named in your answer to 1(a), give an example of how a protein with that function participates in some signal transduction chain.

Ans.: The IP3 receptor, when activated by IP3 releases calcium into the cytoplasm, which can activate calcium binding proteins and the enzymes they regulate. When systemin activates its receptor, one step in the signal transduction chain is the activation of a lipase in the membrane which generates fatty acid precursors to jasmonic acid.

Draw the lipid portion of a bilayer, pointing out the hydrophilic and hydrophobic portions of the bilayer.

Ans.:

oooooooo hydrophilic

.............hydrophobic

oooooooo hydrophilic

4. (a) What membrane question was being addressed by the experiment that fused a mouse cell with a human cell?

Ans.: Are membranes fluid, or, more precisely, can membrane proteins diffuse in the plane of the membrane?

(b) In the experiment named in 4 (a) what was component of the mouse and human cell membranes was labeled and how was it labeled?

Ans.: (a) proteins; (b) labeled by fluorescently labeled antibodies to the membrane proteins.

(c) Indicate two changes a plant could make in its membranes to keep them from “freezing” when the temperature drops below the phase transition point for its membranes.

Ans.: Membranes could introduce more double bonds into the fatty acid chains of their phospholipids, and they could add sterols to their membranes.

5. (a) Signal transduction usually begins with the activation of a receptor. State three common kinds of receptor proteins cells have.

Ans.: Receptor-linked ion channels; G-protein linked receptors; Enzyme receptors.

(b) For each kind of receptor you name in your answer to 5 (a) , give an example of that kind of receptor in a specific signal transduction chain and indicate what signal or ligand typically activates that receptor.

Ans.: IP3 activates receptor linked ion channels to release calcium; phytochrome appears to be a G-protein linked receptor; the receptor for epinephrine is a G-protein linked receptor. PLC is a membrane-localized enzyme receptor for activated G-alpha.

(c) What would be a typical protein that would be associated with the part of a receptor that protrudes out from the bilayer into the interior of the cell, and how does this protein promote the function of the receptor?

Ans.: A G-protein. This protein would be activated when the receptor was activated and it would then leave the receptor to activate or inactivate some enzyme important for advancing the transduction process.

6. (a) Name one key product that is produced from the action of PLC on PIP₂? Ans: _IP₃______________.

(b) What is the main signal transduction function of the product named in the answer to 6(a)?

Ans.: It binds to ion channel linked receptors on the ER and vacuolar membranes and induces the release of calcium these intracellular stores.

(c) (i) What protein activates PLC, and (ii) what molecule or chemical that is not a protein has to bind to that protein in order for it to activate PLC?

Ans.: Activated G-alpha activates PLC; GTP has to bind to G-alpha before G-alpha can activate PLC.

7. (a) Why is it important for cells to maintain low calcium concentrations in their cytosol?

Ans.: In order to keep their phosphate ions free. High calcium in cells would precipitate out the free phosphate. Without free phosphate, DNA, RNA and ATP could not be synthesized by cells.

(b) How does calmodulin help cells maintain low calcium concentrations in their cytosol?

Ans.: When the calcium level in the cell rises above some very low level, it binds to calmodulin and activates it. Calcium-activated calmodulin then binds to calcium pumps that pump the calcium out of the cytoplasm.

8. (a) Describe a signal transduction cascade for phytochrome (first 4 steps).

Ans.: Red light converts Pr to Pfr; Pfr activates G-protein; G-protein activates PLC; PLC generates IP3 and DAG from PIP2.

(b) Name a chemical inhibitor that could block the phytochrome signal transduction chain, and state what step in the chain that inhibitor would block.

Ans.: Calcium channel blocker would block increase in calcium level ; calmodulin inhibito rwould block the ability of calcium-activated calmodulin from activating downstraem enzymes. inhibitor that blocks the ability of the G-proten to interact with the receptor.

(c) Give two explanations that help account for how many different environmental stimuli can all use the same kinds of amplification steps for coupling to a response and yet each stimulus leads to its own specific adaptive response .

Ans.: Specificity can arise from the fact that the receptors are distributed in distinct microenvironments in the cell, from the fact that there are different G-proteins for each receptor; from the fact that the cell is specifically "wired" with cytoskeletal elements that allow for linked processes in one part of a cell near an activated receptor to proceed independently of what is going on in other parts of the cell and lead to independent kinds of responses.

9. (a) Provide two concerns often expressed by opponents of the use of genetically modified plants and two counter arguments often offered to address these concerns.

Ans.: Concerns: New allergens can be produced in foods when new proteins are introduced into them

Counter: Agencies are requiring that new GM foods be tested for their potential to induce allergic reactions, and cannot be released until it can be shown that the danger of this in minimal.

Concerns: We don't know what are all the side effects and cannot predict them all confidently.

Counter: Every new technology has risks associated with it, and the public typically allows these risks to be evaluated over time and then, if discovered, minimized with new technologies. There is a clear risk of starvation or malnutrition in countries that import foods, and this risk in some cases may be minimized by the use of GM foods that can be engineered to grow in enviroments that currently do not support crops or to contain nutrients lacking in the native diets.

(b) It would seem that "Truth in Advertising" regulations would require that food coming from genetically modified plants should be labeled as such. Indicate one kind of consideration that makes labeling food from GM plants a complicated and less-than-obvious process.

Ans.: Examples of problems are: how to track which plants are GM and which are not; how to label food that itself contains no foreign proteins, but comes from a plant that expresses foreign protein in non-edible parts.