1.1. 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 Table 1.

Table 1

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.

1.2. 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.

1.3. The availability of nutrients is also a key factor in regulating the cell cycle of Schizosaccharomyces pombe (fission yeast).  Below is Table 2 containing data scientists collected for two sites which were deficient in nitrogen and phosphate. 

Table 2

Deduce, giving a reason, the point at which a nutrient-poor environment would stop the cell cycle of the Schizosaccharomyces pombe.

1.4. 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.