In this section, we’re going to observe some slides of onion root tip and whitefish blastula cells undergoing mitosis, so that you can observe mitosis in action in both plant and animal cells. Then we’re going to do an experiment, where we’re going to count the number of cells in each stage of mitosis to determine how long an average cell spends in each stage. By the end of this activity, you should be able to recognize the stages of mitosis in both plant and animal cells.

Are you ready to look through the microscope? Here’s a slide of an onion root tip. This part of the root elongates and grows, so these cells are actively dividing. Here’s a slide of a whitefish blastula. The blastula is a very early stage in embryonic development, so a lot of mitosis must occur in order for the blastula to grow into a baby whitefish.

Let’s take a closer look. Here’s a cell in interphase. Notice that the chromatin is spread throughout the nucleus, and the nucleolus is visible. In prophase, the chromatin has condensed into chromosomes, and the nucleolus has disappeared. In metaphase, the chromosomes are lined up along the metaphase plate. We can also see the mitotic spindle! In anaphase, the sister chromatids are moving toward opposite poles. Finally, in telophase, the spindle apparatus is disassembled, and the nuclear envelopes form.

Now we’re going to do an experiment to determine how much time cells spend in each phase of cell division. Let’s count the number of cells in some onion root tips during each phase, and use this information to answer the question. On this slide, there are 21 cells in interphase, 4 in prophase, 2 in metaphase, and 1 in telophase. Why are most of the cells in interphase? Recall that interphase makes up the majority of the cell cycle. Let’s look at another slide. In this slide, there are 16 cells in interphase, 1 in prophase, and 1 in anaphase.

Go ahead and try characterizing the cells yourself.

Good job!

Good going!

You’ve got it!

Nicely done!

That’s right!

Sorry, that’s not correct.

This cell is in interphase. It has a visible nucleolus, and the chromatin is not condensed.

Sorry, that’s not quite right. This cell is in prophase. The nucleolus is not visible, and the chromosomes are condensed.

Sorry, that isn’t right. This cell is in metaphase. The chromosomes are lined up in the middle of the cell along the metaphase plate.

Sorry, that’s not correct. This cell is in anaphase. The sister chromatids are being pulled to opposite ends of the cell by the spindle fibers.

Sorry, that’s not right. This cell is in telophase. The sister chromatids have already reached opposite ends of the cell.

Now we’ll divide the total number of cells in each stage by the total number of cells counted, and multiply by 100 to get the percentage of cells in each stage. We get 69.6 % cells in interphase, 12.5% in prophase, 8.9% in metaphase, 5.4% in anaphase, and 3.6% in telophase.

It takes about 24 hours, or one-thousand, four-hundred and forty minutes, for an onion root-tip cell to complete the cell cycle. Let’s multiply the percentage of cells in each stage by one-thousand, four-hundred and forty minutes to calculate how long the cell spends in each stage. We find that in a 24-hour period, the cells we observed spend 1000.2 minutes in interphase, 180 minutes in prophase, 128.2 minutes in metaphase, 77.8 minutes in anaphase, and 51.8 minutes in telophase. As you can see, cells spend most of their time in interphase, which is expected since this makes up most of the cell cycle! These numbers are not absolute; they only reflect the slides we observed.

In the lab, biologists count much larger numbers of cells to get accurate calculations for each phase. But our experiment gives you an idea of the relative amount of time that cells spend in interphase vs. mitosis.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education