Question

Meiosis - Examining Experimental Data Tre after Induction Aretage Amount of DNA per Cell 24.0 When nutrients are low, cells of the budding yeast, Saccharomyces cerevisiae, exit the mitotic cell cycle and enter meiosis. In this exercise, you will track the DNA content of a population of yeast cells as they progress through meiosis 470 475 0 30 Researchers grew a culture of yeast cells in a nutrient-rich medium and then transferred them to a nutrient-poor medium to induce meiosis. At different times after induction, the DNA content per cell was measured in a sample of the cells, and the average DNA content per cell was recorded in femtograms fg: 1 femtogram 1 x 10 15 gram) 240 95 140 10.0 Their data are shown in the table to the right. 110 120 13.0 Most of the yeast cells in the culture were in G, of the cell cycle before being moved to the nutrient-poor medium to induce meiosis. How many femtograms of DNA are there in a yeast cell in G? Estimate this value from the data in the table. 12.0 Further Reading G. Simcher, Commitment to meiosis: what determines the mode of division in budding yeast? BoEssays 311169-177 (2009). doi 10.1000 bis 200800124 Based on this data how much DNA is present in a gamete of Saccharomyces cerevisiae? Estimate how many femtograms of DNA are present in a cell in G,? What stage happens between G1 and G2 that causes this change to occur? How many femtograms of DNA are present in a cell at the end of meiosis I? What has occurred during meiosis that causes this change in DNA amount? How many femtograms of DNA are present in a cell at the end of meiosis II? What has occurred during meiosis ll that causes this change in DNA amount?

Answer 1

During G1 (growth 1 phase) phase the cell is preparing all proteins, enzymes and other biomolecules required for the cell to enter S phase or synthetic phase.

This experiment starts with all the yeast cells in their G1 phase, the amount of DNA present in one yeast cell during G1 phase is 24fg. ( we can also see that the cell stays in G1 phase for 1 hour).

During S phase or synthetic phase in which the DNA replication occurs, the DNA content of the cell doubles . From the table it is clear that at 5th hour the DNA replicated completely and became 48fg ( double the amount in G1 phase - 24 X2 fg). Then cell enters the next phase -the G2 phase ( G2 phase also has the same amount of DNA as S phase) and stays in G2 phase till 6th hour.

After that the meiosis I division starts . Here the cell divides into two and divides the DNA equally into the two daughter cells. So the meiosis I produces two daughter cells with 24 fg of DNA each. From the table it is clear that meiosis I starts from 7th hour and prolongs till 8th hour.

After meiosis I the two divided daughter cells will enter meiosis II division, thus dividing the DNA equally between their two daughter cell. This results in four daughter cells with 12 fg of DNA each. These are the gametes of the yeast cells.

So based on the data the gametes in Saccharomyces cerevisiae will have 12fg of DNA in each of them.

During G2 phase each yeast cell has 48 fg of DNA. S phase happens in between G1 and G2 phase.

At the end of meiosis 1 there will be 24fg of DNA in each yeast cell. one round of cell divisions was followed by one round of DNA replication to result in this amount of DNA (same as the DNA content of a regular diploid yeast cell) in each cell after meiosis I.

At the end of meiosis II  there will be 12 fg of DNA in each yeast cell. Two rounds cell divisions were followed by one round of DNA replication to result in this amount of DNA ( half the DNA content of a regular diploid yeast cell) in each cell after meiosis II.