Before the cell progresses from G1 into S phase, it needs to pass through a checkpoint, which prevents the cell cycle from proceeding if certain conditions are not met.
Suggest one reason why a cell might not progress through the checkpoint.
The graph below shows data produced from a flow cytometer. This measures the number of cells that are labelled with DNA bound to a fluorescent dye, as this is proportional to DNA content. The stages of the cell cycle are indicated.
Suggest why, during the S phase, that the amount of DNA per cell is between 2n and 4n.
During the cell cycle, there are various checkpoints the cell meets to determine if there are any errors. If an error cannot be repaired then the cell goes through cell death (apoptosis).
Scientists have developed cancer drugs that can inhibit the cell cycle and cause the cell to carry out apoptosis. Paclitaxel and 5-fluorouracil are two of these cancer drugs.
Paclitaxel binds to spindle microtubules preventing the spindle from performing its function
5-fluorouracil prevents the synthesis of thymine nucleotides
Determine at which stages of the cell cycle these drugs would take effect and inhibit the cell cycle.
In order to identify which phase of the cycle a cell is in, other than mitosis, biochemical analysis is used. Radioactive thymidine is one example of a biochemical used.
Suggest, giving a reason, which phase is readily identified when radioactive thymidine is used.
A team of biologists estimated the number of cells in different phases of the cell cycle in Saccharomyces cerevisiae (brewer's yeast). They took two samples, A and B, from different environmental conditions. One sample came from a nutrient-rich environment, the other from a nutrient-poor environment.
Their results are shown in the table below.
Phase of the cell cycle
Sample A
/ number of cells counted
Sample B
/ number of cells counted
G1
312
451
S
203
294
G2
136
196
Mitosis
27
39
Total
678
980
In sample A, a full cell cycle took 1 hour and 35 minutes, whereas, in sample B, a full cell cycle took 60 minutes.
Calculate the time, in minutes, that the cells in sample A were in S phase during one cycle. Show your working.
The biologists studying the Saccharomyces cerevisiae (brewer's yeast) from part (a) hypothesised that when the yeast was exposed to stressful conditions, the growth rates were low.
Suggest, with a reason, which sample came from the nutrient-rich conditions.
The availability of nutrients is also a key factor in regulating the cell cycle of Schizosaccharomyces pombe (fission yeast). Below is a table containing data scientists collected for two sites which were deficient in nitrogen and phosphate.
Phase of the cell cycle
Deficient in nitrogen
/ number of cells counted
Deficient in phosphate
/ number of cells counted
G1
207
181
S
135
118
G2
90
79
Mitosis
0
0
Total
432
378
Deduce, giving a reason, the point at which a nutrient-poor environment would arrest the cell cycle of the Schizosaccharomyces pombe.
The Saccharomyces cerevisiae (brewer's yeast) nuclei are, on average, 2 µm in diameter, but the DNA molecules packed into them have been measured up to 355 µm in length.
Describe the process that enables the DNA molecules, that comprise the 16 chromosomes of yeast, to be packed into the nuclei.
A student’s research determined that the cell cycle of the growing tissue in a similar root as part (b) was 1,560 minutes in length, and that on average, cells spent 5 hours in the visible stages of mitosis.
Calculate the percentage difference between the data gathered by the student and the mean length of the mitotic stages found by the researcher in part (b).
