1 You are a studying the progression of cells through the cell cycle. You are pa
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1 You are a studying the progression of cells through the cell cycle. You are particularly interested in the cyclins E and A, both of which contribute sequentially to the same cell cycle transition. In this case one cyclin initiates the transition and the other completes the process. In these images you see cell extracts from Hela Cells. A) shows the levels of various proteins, while B) shows the amount of DNA in the cells, more DNA shifts this graph to the right, less DNA shifts this graph to the left. Hrs after mitosis Ciz1 Cyclin E Cyclin A 1.2 1.0 0.8 0.6 (4pts) Based on the DNA graphs in B what stages of the cell cycle are the cells undergoing between 0 and 16 hrs. Justify your answer 12 H 0.2 10 Time (Hr) 15 20 Cyclin ACyclin E Ciz1 b. (2pts) Based on the DNA graph and the Western for cyclins A and E what stage of the cell cycle do cyclins A and E generally regulate? (4pts) Given your answer in b, describe what types of proteins or activities are likely to be regulated by these cyclins. Where in the cell would you expect them to be most active? c. d. (3pts) Which cyclin initiates the cell cycle and which one completes the process? How did you know?Explanation / Answer
1) a: Based on graph B we can assume different stages of cell cycle based on the DNA content per cell. at '0' hr the DNA content graph oriented centre, it represents the cell has 2C content of DNA and it is in G1 stage. at '6th ' hr and up to '9th' hr DNA content not much varied but slightly shifted to its left in the graph, this indicated that the cells are in G1-S transition, so cells undergo active transcription and translation, and other processes which leads them to transition into S phase. So the DNA content per cell may be slightly shifted to left. At '12th' hr the DNA content graph widened, it suggests the duplication of DNA has started which suggest the cell is in early S-phase. At '16th' hr there are almost two peaks are appearing in the DNA content per cell, it indicates that the DNA is duplicated and increased the amount to its double, and the stage of cell cycle must be S phase or S-G2 transition state
b) According to western blot, cyclin E is appearing from 9th hr in cell cycle, the sudden rise up to 12th hr and sudden downfall during 16th hr. Because the DNA content at 12hr is increased that means DNA duplicating, which occurs in S phase of cell cycle. The cyclin E also predominantly appearing at this stage only. This indicates that the cyclin E need to proceed the cell into S phase of cell cycle. It must be regulating G1-S phase transition.
Cyclin A is faintly appearing during first 3 hrs but predominant after 12 hrs of the cell cycle, after 12hrs the DNA is duplicated this means the cyclin A is important in S phase of cell cycle. it is necessary at S to G2 phase transition.
c) Cyclin E is important for the cell to start its S phase, it might prepare the cell for DNA replication, the enzymes involved in the DNA replication must be synthesised and the replication of DNA will be started, expression of other proteins might be regulated,
Cyclin A is important for the cell in S phase that means supports the cell to progress from S phase to G2 phase, It must be preparing the cell to divide by initiating the processes like the formation of spindle fibres, and nuclear membrane disintegration disappearing of some of the cyto-organals etc.
d) Cyclin D initiate the cell cycle and cyclin B exit the cell from the cell cycle. Based on the estimated quantities of all cyclins throughout the cell cycle, the importance of them in various stages of the cell cycle is estimated.
e) At 16th hr of cell cycle the cyclin E is disappearing from the cell. In general, cyclins are proteins which are synthesised and destroyed during various stages of cell cycle. Likewise, the cyclin E must be directed to the ubiquitin-mediated proteasome destruction which degrades cyclin E.
f) Cyclins may be produced from free ribosomes, as they must be soluble proteins, which acts to regulate gene expression at various stages of cell cycle.
proteins which are produced on bond ribosomes generally secreted out or may also be targeted to the membranes.
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