In one of the classic experiments that we discussed for cell cycle and that you
ID: 165035 • Letter: I
Question
In one of the classic experiments that we discussed for cell cycle and that you read a paper on, clam eggs were fertilized and allowed to undergo several rounds of a synchronous cell division. At different times during a replication cycle, some of the eggs were collected and proteins were extracted, subjected to SDS-Polyacrylamide Gel Electrophoresis (SDS-PAGE), and labeled peptides then identified by autoradiography (radioactively). The figure below shows the results for part of the gel. Each lane represents a sample collected sequentially during the cell cycle.
Explain your answers to the following questions using the gel above.
A. What common 3D structure found in proteins will most likely be labeled with S35 (radioactive isotope of Sulfur) in this experiment?
B. What is happening to peptides A and B, as indicated by the increase in intensity of their bands during the cell cycle? Why does the intensity of the bands suddenly diminish? [Hint: What do we know now about why there intensity increases and then diminishes?]
C. Are peptides A and B most likely cyclins, Cdks, or cyclin-Cdks?
D. Which of the peptides is most likely coupled to the triggering of S-phase? … to M-phase?
M 1 G ABExplanation / Answer
Answer:
Cyclins are a group of proteins that control the development of cells through the cell cycle by activating cyclin-dependent kinase.
A. S35 will label methionine in proteins leading to the formation of S35-Methionine.
B. The proteins appear to be cyclins which are made during certain times during the cell cycle and their concentration rises and falls.
Cyclin A plays an important role in initiation of replication of DNA during S phase of the cell cyclin, while Cylin B is involved in the M-phase.
The levels of these cyclins varies and increases when required during the cell cycle. They are prone to ubiquitin mediated degradation when not required, so they diminish.
C. Peptides A and B are most probably Cyclins (Cyclin A and B).
D. Cyclin A binds to CDK and during S-phase and are necessary for initiation of DNA replication.
Cyclin B promotes the events in mitosis (M phase).
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