**I. Time for Cell Replication Overview**

**Procedure**

1. Observe every cell in one high power field of view and determine
which phase of the cell cycle it is in. This is best done in pairs. The
partner observing the slide calls out the phase of each cell while the
other partner records. Then switch so the recorder becomes the observer
and vice versa. Count at least two full fields of view. If you have not
counted at least 200 cells, then count a third field of view.

2. Record your data in Table 1

**Table 1**

Field 1 | Field 2 | Field 3 | Total | Percent of total cells | Time in each stage | |

Interphase | ||||||

Prophase | ||||||

Metaphase | ||||||

Anaphase | ||||||

Telophase | ||||||

Total cells: |

3. Calculate the percentage of cells in each phase.

Consider that it takes, on average, 24 hours (or 1,440 minutes) for onion root-tip cells to complete the cell cycle. You can calculate the amount of time spent in each phase of the cell cycle from the percent of cells in that stage.

Percent of cells in stage X 1,440 minutes = minutes of cell cycle spent
in stage

**Questions**

1. If your observations had not been restricted to the area of the
root tip that is actively dividing, h6w would your results have been different?

2. Based on the data in Table 3.1, what can you infer about the relative
length of time an onion root-tip cell spends in each stage of cell division?

**II. Crossing Over during Melosis in Sordaria**

**Table 2**

Number of 4:4 asci | Number of crossover asci | Total asci | % showing crossover / 2 | Gene to centromere distance |

**Analysis of Results**

1. Using the data in Table 2, determine the distance between the gene
for spore color and the centromere. Calculate the percent of crossovers
by dividing the number of crossover asci (2:2:2:2 or 2:4:2) by the total
number of asci x 100. To calculate the map distance, divide the percentage
of crossover asci by 2. The percentage of crossover asci is divided by
2 because only half of the spores in each ascus are the result of a crossover
event (Figure 3). Record your results in Table 2.

2. Draw a pair of chromosomes in MI and MII, and show how you would get a 2:4:2 arrangement of ascospores by crossing over. (Hint: refer to Figure 3).