Cells will often delay entry into mitosis when the DNA has not completed replication, or when the cell is not yet big enough to divide. In yeast, many proteins and enzymes are known which control the cell’s ability to detect unreplicated DNA and to determine exactly how big the cell is. This allows for control of the cell cycle.
You wish to study these cell cycle control proteins in detail. You grow yeast cells and then expose them to a chemical mutagen, know to cause mutations in the DNA. You then examine the cells to see if they exhibit any abnormal growth patterns, indicating cell cycle control proteins may have been mutated. Each mutant cell you obtain has only one mutation in one protein.
You expose your cells to a drug which speeds up the cell cycle, but you grow the cells in a nutrient poor medium. This medium normally prevents the cells from growing quickly, although DNA replication still occurs at a normal pace. Most cells take a long time to enter into mitosis. However, you isolate several cells that do not delay entry into mitosis. You call these mutants 1, 2, 3, etc.
You add a drug to your yeast cells which does not allow replication to occur. Most of the yeast cells arrest and do not grow. However, several do enter mitosis. You call these mutants A, B, C, etc.
Upon careful examination of your mutants, you find a few that allow cells to enter mitosis when either the cell is small or when the DNA has not completed replication. These mutants you call by both previous names (e.g. 5C, 9G, etc.).
Using this data, you try to determine where in the cell cycle these mutations are exerting their influence, and hence, where their normal counterparts act.
At what stage in the cell cycle is mutant D most likely defective?
The cell should not be allowed into G2 if all the DNA has not yet been replicated. Therefore, the mutation probably occurred in S phase.