Do onion cells spend the same proportions of time in interphase and mitosis as reported in the Integrating Concepts of Biology textbook for a “typical eukaryotic cell cycle”

Instructions: Complete the following pre-lab activity prior to attending lab. You need to print and complete this document and turn it in by the beginning of lab on Thursday.

This week we will be using microscopes and professionally prepared/stained slides to identify the various stages of the cell cycle in onion root tip cells. We will differentiate between interphase (not specifically G₁, S, and G₂), prophase, metaphase, anaphase, and telophase/cytokinesis (lumped together). Our research questions are as follows:

Q₁: Do onion cells spend the same proportions of time in interphase and mitosis as reported in the Integrating Concepts of Biology textbook for a “typical eukaryotic cell cycle” (See Table 3.5 in ICB)?

Q₂: Does each stage of mitosis (i.e. prophase, metaphase, anaphase, and telophase/cytokinesis) require the same amount of time to complete?

It is important for you to understand that you will be using a slide that was prepared by taking a thin slice through an onion root tip (example image on right). All the cells were stained, and therefore killed, at the same time regardless of what stage of the cell cycle they were in. We will operate under the assumption that each cell divides whenever it is ready, and is therefore independent of what the cells around it are doing. Therefore, we can consider the cells seen in the slide to be a representative sample of all cells in the root tip. Under this assumption, we can estimate the proportion of time that cells in an onion root tip spend in a particular stage, based on the relative proportions of cells displaying any given stage at the moment the slide was prepared. For example, let’s assume that the cell cycle takes 24 hours (as it does Table 3.5 in ICB) and you are going to identify the stage of cell cycle for 24 cells on the slide. If 8 cells were in some stage of mitosis while 16 were in interphase, you might conclude that onion root tip cells spend 8 out of 24 hours undergoing mitosis, and 16 out of 24 hours in interphase.

Read the Chi-Square Test document so you understand how we can analyze data to answer the research questions listed above. Then, complete the questions below.

1.     Write at least two hypotheses (at least one per research question) that provides a tentative answer to each question above. Make sure the hypotheses have the necessary characteristics of being testable and falsifiable.

 

 

2.     Write a predictive statement (i.e. and if…then…statement) for each of your hypotheses from Question 1. These should be testable and should utilize information that you obtained from Table 3.5 in ICB, so that you can make a reasonably educated prediction.

 

 

 

 

3.     Describe an experimental design (e.g. a plan for the collection of empirical observations) that could be used to test each of your hypotheses.

 

 

 

 

 

4.     Identify the independent and dependent variables, and potential controls, for your experimental design.

 

 

 

 

5.     What values would you use for your expected values in a Chi-Squared Test of your experimental data collected according to question 3?

 

 

 

 

6.     Visit the page http://www.biology.arizona.edu/cell_bio/activities/cell_cycle/cell_cycle.html to become familiar with the appearance of cells in each stage of the cell cycle. Complete the identification of 36 cells, then create a data table below and record the number of cells in each stage.

 

 

 

 

7.     Use a Chi-Square table to compare your values with expected values based on an assumption that cells spend equal time at each stage listed in the table below (this is probably a bad assumption, but will serve our purposes here). Is your distribution of virtual data significantly different than the expected distribution (i.e. equal time in each stage)? **Hint: If I expected an equal number of emails to arrive on each day of the week, and I got a total of 70 emails during the entire week, I should expect 10 emails to arrive each day. (If only I were so lucky to have that few emails each day!!)