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Biology 121 (Cell Physiology) Lecture
Exam 4
15May2003
Hao Nguyen
Name: ____________________
NOTE: NO ABBREVIATIONS and NO SYMBOLS in any answer.
1. Please, describe how the inactive form of ras (ras-GDP) is activated to perform its function.
NOTE: Only describe the actual activation. DO NOT describe the entire pathway, either before or after
the activation. Be sure to name all specific molecules that are involved and the function of each
molecule during the activation. (9 points)
Sos catalyzes the exchange of GDP on ras-GDP (inactive form) for GTP producing ras-GTP (active form).
Ras-GTP has an intrinsic GTPase activity that allows it to catalyze the hydrolysis of GTP. This intrinsic
activity of ras-GTP has to be activated by GAP (GTPase activating protein). Therefore, GAP activates the
intrinsic activity of ras-GTP to catalyze the hydrolysis of GTP on ras.
2. A hypothetical cell expresses a mutated form of ras, which is constitutively (always) in the inactive form.
Please, describe how this might affect the signal transduction pathway.
NOTE: Be sure to name the specific pathway and all molecules that are directly affected. “The signal
transduction pathway is activated” or “The signal transduction pathway is not activated” ARE
NOT acceptable answers. (9 points)
If ras is always in the inactive form (ras-GDP), it cannot perform its function, which is to bind to raf-1 and
allows it (raf-1) to associate with the plasma membrane. Normally, ras can do this through the hydrolysis of
the GTP that is bound to it when it (ras) is in the active form (ras-GTP). Without ever having ras in the
active form (ras-GTP), the hydrolysis of the GTP cannot occur; therefore, raf-1 cannot be induced to
associate with the plasma membrane. Association with the plasma membrane activates raf-1. Raf-1, when
activated, activates the MAP kinase cascade; which is a part of the MAP kinase signal transduction
pathway. The inability to activate raf-1, in turn, prevents the activation of the MAP kinase signal
transduction pathway; which inhibits the transcriptional regulation of certain genes.
3. a. What is the function of a second messenger? (5 points)
After receiving a signal from a signaling cell or the extracellular matrix by a membrane receptor, a
second messenger transfers the information that exists at the plasma membrane into certain areas of
the target cell by activating certain signal transduction pathways. Ultimately, this information will reach
the nucleoplasm to regulate the function of the cell.
b. Please, name two examples of second messenger. (4 points)
Ca++, cAMP, DAG, IP3
4. What is the function of cyclin? (5 points)
Cyclin is a regulatory protein that can form a complex with different cyclin-dependent protein kinases (Cdk)
to regulate their activity (of these Cdk’s).
5. A hypothetical cell is unable to express functional cyclin B. Please, describe what might happen to cell
division in the absence of cyclin B. Also, why? (9 points)
Cyclin B forms a specific complex with Cdk-1. The Cdk-1/Cyclin B complex regulates the G2/M checkpoint
within the cell cycle, which is the initiation of cell division. This complex phosphorylates lamin, which is a
part of the nuclear scaffold; thereby, causing the breakdown of the nuclear scaffold and nuclear membrane.
The breakdown of the nuclear scaffold and nuclear membrane is a vital activity during cell division. If cyclin
B were not expressed, Cdk-1 (within the Cdk-1/Cyclin B complex) would not be able to induce the
breakdown of the nuclear scaffold. Therefore, cell division cannot occur.
6. a. In general, what is the ultimate goal of the signal transduction pathway? (4 points)
to regulate or control transcription of certain genes
b. Within the nucleoplasm, describe the sequence of events that leads to this ultimate goal.
NOTE: You are welcome to use specific examples to describe these events. (9 points)
A kinase (eg. MAP kinase) or catalytic subunit of a kinase (eg. catalytic subunit of PKA) activates certain
transcriptional factors by phosphorylating them. The phosphorylation activates the transcriptional
factors and allows them to bind to specific consensus sequences (or response elements) within
promoters. The binding of transcriptional factors to consensus sequences, then, is involved in the
regulation of transcription.
7. What is usually the result of the activation of an ion channel-linked receptor? (5 points)
When activated, a ion channel-linked receptor usually undergoes a conformational change leading to the
opening of the channel. Specific ions can than be passively transported through the plasma membrane via
the open channel.
8. a. Please, name the two major families of proteins that regulate the START checkpoint. (6 points)
cyclin and cyclin-dependent protein kinase
b. Please, describe the significance (or the importance) of the START checkpoint. (6 points)
Once a cell enters or passes the START checkpoint, it is programmed to complete its cell cycle
providing that there is sufficient nutrients and no mutations occurred within the cell. This includes going
through the synthesis and Gap 2 stage as well as completing cell division.
9. Of the seven types of cell communications discussed in class:
(Please, give only one answer for each question.)
a. Which does not involve the communication between one cell and another (but different) cell? (4 points)
focal contact (communication) and autocrine system
b. Which does not involve the secretion of signaling molecules (or ligands)? (4 points)
focal contact, gap junction, contact-dependent communication
c.
Which involves communication between cells that are furthest away from each other? (4 points)
endocrine system
d. Which involves direct contact between the two cells? (4 points)
contact-dependent communication and gap junction
e. Which involves only one cell? (4 points)
Autocrine system and focal contact
10. We know that one of the characteristics of a cancer cell is its inability to regulate or control the cell cycle.
Describe, in detail, a scenario where a cell would lose its ability to regulate its own cell cycle.
Hint: Think about how the cell cycle is regulated. Begin with “a ligand is recognized and bound by a
membrane receptor.” Then, describe a mutated or altered function at a particular step. This scenario
should end with an explanation of how the cell cycle is affected. (9 points)
Roughly, this is what I was looking for:
1. a detail description of a certain signal transduction pathway that is activated by a specific type of
membrane receptor
2. a description of a mutation and the altered function (eg. activating or inhibiting)
3. how does this altered function affect the signal transduction pathway?
4. how does the altered signal transduction pathway affect the expression of specific protein or proteins?
5. how does the altered expression of this or these specific protein(s) affect the cell cycle?
6. which area or checkpoint within the cell cycle is affected?
7. can the cell cycle continue or is it halted at this point?
8. THE END