Answers to Review Questions for Exam #3

1. A. Both; B protein Q.  Both proteins should be made on ribosomes attached to the ER because both are integral transmembrane proteins; they must enter the ER and pass through the Golgi to reach the plasma membrane.  Prot. Q should have more than one cytoplasmic domain, since G proteins are usually 7 pass proteins. If there are 7 hydrophobic segments in the membrane, there must be hydrophilic loops (domains) connecting the transmembrane domains. Some loops will be extracellular and some cytoplasmic. 

    C. Protein Q; Both.
G-protein linked receptors can trigger the cAMP/PKA pathway as follows:  ligand binds to receptor à activates G protein à activates adenylyl cyclase à  cAMP à  activates PKA. TK receptors can trigger the IP3 pathway (as well as the ras/MAP kinase pathway) but not the cAMP pathway. Long ans to 6 & 7


2. A. PKC. Phospholipase C hydrolyzes PIP2 à    releases DAG à   activates PKC. (At same time, hydrolysis  of PIP2 releases IP3 à    releases Ca++ from ER which also activates PKC.)

B. CDK. The cyclin associated with the active kinase is unstable. (Note that activation/inactivation cycles of stable proteins are not the same as synthesis/degredation cycles of unstable proteins.)

C. TK (A receptor tyrosine kinase). RME leads to internalization of surface proteins, which can lead to recycling or degradation of the proteins involved.  Many TK's are receptors that are transmembrane proteins; the TK on the inside of the cell is activated by binding of the ligand on the outside. These receptors (plus ligand) can be internalized by RME à    degredation of receptor and/or ligand. This is a way of regulating the response by controlling the level of ligand and/or surface receptor. All the other proteins listed are cytoplasmic or associated with the inside of the plasma membrane; none of the others bind ligand on the outside and are expected to be involved in RME. 

D. CDK. An active CDK/cyclin complex is required for entering either S or M, but different ones are required for G2 à   M and G1 à   S. (GF, ras, & MAPKinase etc. are required for G1 à    S but not for G2 à    M. That's why the "start" point of the cycle is the major control point in cells of higher organisms.)

E. A single type of ligand binding to its receptor can activate TK, PKC & MAPK, because GF's generally activate TK receptors , and TK receptors can activate both the IP3 and the ras pathway.  (This was the intended answer.) A single molecule of ligand usually cannot activate a TK because several individual monomers of TK receptor must bind ligand and polymerize to form active, polymeric receptor.  (This was not what the question was getting at, but was accepted if explained.) Growth factors generally activate TK receptors à   PLC à   PKC; simultaneously TK à   ras pathway à   MAP Kinase. So if you have many molecules of ligand, but only one type of ligand and one type of receptor, you should be able to activate TK, PKC & MAPK. However TK receptors must dimerize to activate, so if you took question to mean what happens if you have one molecule of GF and receptor, answer is different. In that case, a single molecule of ligand might not be able to activate a TK (cause dimerization) and so the only type of receptor that could be activated would be a G protein linked receptor; this would in turn activate either the PKC pathway or the PKA pathway. 


3.
A. Mutation 2. Constitutively active ras à synthesis of cyclin independent of GF's and automatic G1 à S. Therefore there will be loss of control. The other two will block the signaling chain from GF àcyclin, so cell will not grow.

    B. All are proto-oncogenes; none are tumor suppressors. The term "proto-oncogene" refers to the normal gene, not the mutant. All these genes can cause uncontrolled growth if they are constitutively active. 

    C. Only 1 + 2 will cause loss of control.

1 + 2 à   overactive Ras à   same effect as 2 alone. (Oncogene acts after the block to growth.)
2 + 3 à   lack of growth as Ras cannot activate MAP Kinase. (Block to growth acts AFTER oncogene.)
1 + 3 à   no growth; 2 blocks in pathway.

4. A. Embryonic induction and regulative development (2 pts each).

    B. Answers: B-1. Floor Plate; B-2. All neural tube cells other than floor plate AND on the cell surface. (2 pts each ans).
Explanation : Cells near the floor plate become motor neurons. Wherever the floor plate is placed, whether it is the normal position or not, the nearby cells of the neural tube become neurons. This indicates that the floor plate cells produce some signal molecule and all the neural tube cells (except the floor plate itself) are capable of responding to the signal (by becoming motor neurons). Therefore all the neural tube cells (except the floor plate cells) must have the receptor for the signal involved. (4pts). Since the signal molecule is a secreted protein, the receptor should be a surface molecule  -- proteins can not cross membranes by themselves (without a carrier) and bind to internal receptors. (You didn't need to explain this.)

    C. Answers (simplest case): C-1. (2); C-2. Product made after gastrulation AND floor plate cells AND protein active after gastrulation. (1 pt each answer). 

    Neurulation (part of organogenesis) occurs at a fairly late stage of embryonic development, after gastrulation. (2 pts) The simplest case is that the mRNA and protein from gene ess are made after gastrulation since the protein is not needed until then. There is no indication the gene is expressed earlier. The mRNA here is not likely to be of maternal origin as the protein is used so late in development. Maternal messages control very early events, prior to gastrulation. Also the protein is made only by floor plate cells, and these cells don't specialize until after gastrulation. If gene ess is expressed (& active) after gastrulation , the cells grown at 37 C after gastrulation (expt. 1) will not have normal motor neurons, as the signal made at 37 C will be defective. The cells grown at 37 C before gastrulation ( expt. 2) will be fine -- they will be shifted to 25 C before the protein is made or used. (Reasonable explanation of when protein made and used = 2 pts).

    It is possible that the mRNA and/or protein from gene ess are made early in development, but that they are only used (or activated) later. If this occurs, then mRNA and/or protein could be made in cells other than floor plate but only used in floor plate cells. There are two possible scenarios here:  (a) the mRNA is made in many cell types (or undifferentiated cells) at an early stage and translated after gastrulation only in floor plate cells, OR (b) the protein (& mRNA) is made in other cells at an early stage but the protein is only activated in floor plate cells. In case (a), the gene product (meaning mRNA) is made before gastrulation and is active after gastrulation; in case (b) the gene product (meaning protein) is made before gastrulation and active after. In both cases, the transcription of gene ess is not necessarily restricted to floor plate cells (which aren't formed at the time the mRNA and/or protein is made). If the protein is made before gastrulation, but used afterwards, the cells grown at 37 C before gastrulation will not make motor neurons (expt. 2) and neither will the cells grown at 37 C after gastrulation (expt. 1). In both cases, the protein will be irreversibly denatured before it can be used.

Answers to remaining questions are in bold; explanations are in red. Some incorrect choices (for the answer) have been omitted.

5. An action potential has just occurred in a neuron, and the membrane is hyperpolarized. At this point, the concentration of K+ inside the cell is ____ the concentration of K+ in the fluid that surrounds the cell.

a. higher than b. lower than c. equal to  K+ is always higher inside the cell; just a small fraction of the K+ leaves in the action potential.

6. The refractory period occurs in a neuron because of the time needed for

a. a neurotransmitter to diffuse across the synaptic cleft.
b. the Na/K pump to restore the membrane potential to resting  Na/K pumps not needed to restore potential
c. the voltage-gated Na channels to open 
d. the voltage-gated K channels to open
e. the voltage-gated Na channels to return to the closed state In this state, they're re-set, ready to be activated.

7. The equilibrium potential for an ion refers to

a. the membrane potential that occurs when the ion concentration is equal on both sides of the membrane.
b. the electrical charge that is needed to balance the concentration gradient of that ion across the membrane.
c. the proportion of the membrane potential that is due to that particular ion.
d. the potential that occurs when permeability to Na equals permeability to K.
e. the potential at the peak of the action potential, when the rate of incoming ions equals that of outgoing ions.

8. A cell is made more permeable to sodium ions. As a result, the equilibrium potential for Na should

a. become more positive     b. become more negative      c. not change  E(Na+) depends on the concentration gradient, not membrane permeability

9.(13) The motor neuron from the spinal cord to the left leg is exposed to woorali, and when stimulated, the innervated muscle contracts.  So this neuron must be conducting an action potential, indicating that the following channels are not affected by woorali: (5 points for this reasoning and for including all three types of channels listed below)

The neuron to the right leg must also be conducting an action potential, but the muscle doesn't contract, so the affected channels could be those responsible for conducting the message from the neuron to the muscle, or those found within the muscle itself.  These would be  (8 points for this reasoning and for including all three types of channels listed below)

Claude Bernard actually did another experiment, stimulating the right muscle directly, with electrical current, and so concluded that the muscle itself was not affected by woorali, but it must be the junction between the neuron and muscle.  This was the first hint that there was a special junction between the neuron and muscle, some 30 years before the synapse was discovered.   "Woorali" was the way some European explorers heard the word that we now pronounce "curare". 

Multiple choice: 3 points each, no explanations

10. The all-or-none principle of nerve action states that

c. The action potential always reaches a certain potential, or doesn’t occur at all

11. A neuron and its attached muscle is placed in a fluid in which Ca++ has been replaced by a different divalent cation (X++). This will make it harder to        

  1. generate an action potential when the neuron is stimulated with electrical current

b. get neurotransmitter release when the neuron is stimulated with electrical current. This is the only response that depends on extracellular Ca++.

c. get muscle contraction when the muscle is stimulated with electrical current. ( It will be harder to stimulate the muscle by way of the neuron, but if the muscle is stimulated directly with electrical current, bypassing the synapse, then extracellular Ca++ is not needed, and the intracellular Ca++  in the SR is not directly affected by this manipulation.)

12. When the sarcomeres in a muscle are at their shortest possible length, you would expect to find the t-tubules are

c. depolarized (Depolarization of the t-tubules is the stimulus for Ca++ release from SR;  Ca++ binds to troponin, allowing sarcomere shortening.)

13. Myoglobin is a protein that binds oxygen. Based on the spelling of the word, you could assume that myoglobin stores oxygen in

a. mitochondria       b. glands       c. muscle       d. the CNS       e. Schwann cells and oligodendrocytes

14. When HIV infects the brain, it can lead to the death of brain neurons as a result of paracrine signals released from

a. Glial cells  Paracrine signals are local signals, so it must be some cell that is in the brain.  The other neurons and ganglia listed here are in the peripheral nervous system.

15. Prior to secretion of ______ an action potential develops in the cell that secretes it.

a. Thyroid hormone      b. TSH      c. TRH  This is a neuroendocrine secretion, released from a neuron after an action potential, in a manner similar to neurotransmitter release.

16. When we eat other animal species, we generally eat their skeletal muscles, though sometimes we eat other organs, such as the brain. Which food would be higher in protein? Higher in fat? Explain. Your answer should indicate the specific types and locations of the protein and fat in the brain and muscle cells. (8)

Muscle is virtually wall-to-wall protein (1)  chock full of actin(1), myosin,(1)  as well as smaller amounts of troponin, tropomyosin in the myofibrils inside the cells (1)

Brains are pretty fatty  (1) since neurons are coated with myelin  (1) which consists of multiple layers of phospholipid bilayers, with relatively little protein, since there's no transport occurring in these regions.   (1)  This is outside the neuron, surrounding the axons, as a result of activity of oligodendrocytes.  (1) 

17. "Not poppy, nor mandragora, nor all the drowsy syrups in the world shall ever medicine thee to that sweet sleep which thou ou’dst yesterday", said Iago, suggesting that there’s no drug that would ever again let Othello get a good night’s sleep. The poppy and mandrake plants have long been known to contain compounds that could put a person to sleep, but only recently have scientists discovered the receptors that are stimulated by these compounds, called opiates. There are 3 such receptors. When stimulated, mu receptors leads to opening of K+ channels, lambda receptors leads to inhibition of adenyl cyclase, and kappa receptors leads to closing of Ca++ channels.

    A. Stimulation of which receptor is most likely to cause an IPSP? Explain briefly. (4)

Mu.  Opens K+ channels (1) K+ flows out of cells, down its concentration gradient, bringing positive charge out of the cell (2) and the cell becomes more negative inside, meaning that it's been inhibited, or taken further from threshold by an Inhibitory PostSynaptic Potential.  (1) 

    B. After opiates are used repeatedly, the postsynaptic cell often requires a greater than normal stimulus to respond. This is most likely due to (down-regulation of opiate receptors) (up-regulation of opiate receptors) (lengthening of the relative refractory period) (lengthening of the absolute refractory period) . (2)   Down-regulation implies fewer receptors, ie, requires greater-than-normal stimulus to get same response.

    C. The opiate receptor at C is likely to be (mu) (lambda) (kappa). (2)   Closing of Ca++ channels would prevent neurotransmitter release from B, so fewer action potentials would occur on the postsynaptic membrane of cell C..

    D. The finding that humans have receptors for a plant compound suggests that there may be some molecule normally produced by humans which is the natural ligand for this receptor. The natural ligands are called endorphins, and are cleaved from POMC, the pre-prohormone for ACTH. Endorphins must be synthesized in the (dendrites) (cell body) (axon) (axon terminal) of the neuron that releases them. (2)  ACTH, and its pre-prohormone, are peptides; Peptides are synthesized on ER in the cell body, and transported down the axon along microtubules.