Onion Root Tips Lab

(Based on the following: )

Background:In this lab, you are going to determine the relative amount of time spent in different phases of the cell cycle. Growth in an organism is carefully controlled by regulating the cell cycle. In plants, the roots continue to grow as they search for water and nutrients. These regions of growth are good for studying the cell cycle because at any given time, you can find cells that are undergoing mitosis.
In order to examine cells in the tip of an onion root, a thin slice of the root is placed onto a microscope slide and stained so the chromosomes will be visible. The cells you'll be looking at in this activity were photographed with a light microsope and then digitized. /
Although slicing the onion root captures many cells in different phases of the cell cycle, keep in mind that the cell cycle is a continuous process. Scientists have divided the process into 5 phases, each characterized by important events, but these divisions are still arbitrary. / (An animal cell during interphase)

The life cycle of the cell is typically divided into 5 major phases. The phases are listed below, along with the major events that occur during each phase. The cells are from an animal, not a plant.

/ Interphase. The cell is engaged in metabolic activity and performing its duty as part of a tissue. The DNA duplicates during interphase to prepare for mitosis (the next four phases that lead up to and include nuclear division). Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible.
/ Prophase. Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nuclear membrane dissolves, marking the beginning of prometaphase. Proteins attach to the centromeres creating the kinetochores. Microtubules attach at the kinetochores and the chromosomes begin moving.
/ Metaphase. Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome.
/ Anaphase. The paired chromosomes separate at the kinetochores and move to opposite sides of the cell. Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules.
/ Telophase. New membranes form around the daughter nuclei while the chromosomes disperse and are no longer visible under the light microscope. Cytokinesis or the partitioning of the cell may also begin during this stage.

In this activity, you will be presented with cells from the tip of an onion root. You will classify each cell based on what phase it is in. At the end you will count up the cells found in each phase and use those numbers to predict how much time a dividing cell spends in each phase. You can base your calculation on a total cell cycle of 12 hours (720 minutes).

Prelab Assignment: Complete the following sections of the lab on the lab report pages: Purpose, Hypothesis,

Safety, Materials, Procedure, set up the data table, figure 1, table 2, figure 2.

Prelab Preparation: You need to cut out each of the 36 cells on the next page BEFORE you come to class to do the lab. You do not need to cut out “on the lines”; cutting them out in rough squares is fine. Put the cells in a ziplock baggie or envelope and seal it well.

When you get to class, you will look carefully at the chromosomes and other cell structures to determine what stage of the cell cycle is shown. Then, glue each cell in the correct column in Table 1. You should find 20 interphase, 10 prophase, 3 metaphase, 2 anaphase, and 1 telophase.

During class (or you may have to finish at home), perform the following procedure now that your prelab is complete.

Data: Arrange the cut out cells into their appropriate column in the data table. You should find 20 in interphase, 10 prophase, 3 metaphase, 2 anaphase, and 1 telophase. Notice that the table is labeled with an appropriate title and number. Once you have the cells in their correct column and have the correct number for each, glue or tape your cells down.You MUST be careful not to use too much glue or it will leak out and glue your pages together!

Next, look at the prepared slide of an allium root tip and make an appropriate scientific drawing in the space provided. Don’t forget to title it above with the name and below with the magnification, use high power (400x). Then using proper labeling techniques, make appropriate labels according to the instructions.

Analysis: Complete the analysis section in your lab report. Determine the percent of cells in each stage and the relative amount of time spent in each phase as indicated in the table. See your lab tips sheet for help. You MUST SHOW YOUR WORK in the Percent of Cells and Time in Each Phase boxes.

Next, create a pie chart showing the relative amount of time spent in each stage. Use a ruler! See your Lab Tips sheet for help.

Then, answer the analysis questions—see lab report document.

Conclusion: In your own words, in paragraph form, write a conclusion for the lab in your lab report document.

  • Restate the purpose in past tense
  • Briefly explain the procedure in past tense
  • Discuss your observations (describe In your OWN words, describe what happens during interphase, prophase, metaphase, anaphase, and telophase. Be sure to include a written description based on what you were supposed to see, as well as a description of what it actually looked like when you were looking through the microscope.
  • DISCUSS ALL results. REFERENCE YOUR DATA. (As shown in Table 1…; As seen in Figure 1…) and use your analysis to EXPLAIN WHAT THE RESULTS MEAN (According to Table 2…; Figure 2 shows…) DISCUSS EVERY SINGLE PIECE OF INFORMATION THAT IS THERE. THIS WILL TAKE A LOT OF WRITING. Write in present tense. No pronouns. Here’s a few hints to help you think about what you should be getting out of the data and analysis.
  • Explain what your observations imply.
  • Were there more cells in a particular phase?
  • Why is this?
  • Explain how scientists could use this information (Hint: Why is it important to understand and study the cell cycle? What could abnormalities in data indicate?)

Mitosis Onion Root Tip Lab Tips Sheet

The organism you are making observations of in this lab is an onion of the genus Allium. The pictures you cut out and identify the phase of the cell in the cell cycle are from the tip of the root at the end of the onion that was in the ground. The cells are in a special type of tissue found in plants called meristem. Meristematic tissue is like embryonic tissue in that it is made of rapidly dividing, small, immature cells that are not specialized into any particular type of tissue yet. This makes meristematic tissue perfect for this lab.

However the pictures at the beginning of the lab that look like they are of round cells are NOT onion cells. They are not even plant cells. These cells are generic animal cells (those model cells that get drawn in textbooks as examples, but no cell like that actually exists). This is important because mitosis in animal cells and plant cells is different. The key difference you need to observe occurs during telophase/cytokinesis. The animal cells develop the cleavage furrow that looks like a pinching in of the membrane, or a fold like in a peach, due to the constriction of a ring of protein around the equator of the cell that eventually cuts it in two. Plant cells have to reform the cell wall; therefore, the plant cell lays down a row of vesicles from the Golgi that forms the cell plate and eventually the new cell wall.

The WORDS with those pictures that describe each phase, ARE GOOD for helping you determine the different phases the cells are in. Below is a little more help:

The Procedure:

INTERPHASE – these cells either have the nucleolus visible, OR no white space visible because the DNA (chromatin) has not condensed enough to leave space between the separate chromosomes

or

PROPHASE – the nucleolus is never visible (it has disassembled) and white space is visible between the condensing chromosomes

METAPHASE – these are the easiest cells to recognize. The condensed chromosomes are lined up across the middle of the cell

ANAPHASE – the cells are moving toward the other side of the cell. When they are not yet at the poles and/or if there is no cell plate forming yet, it is anaphase

TELOPHASE – the chromosomes are at the poles and the cell plate is forming (or a cleavage furrow if it’s an animal cell)

The Calculations Table: Remember that the row of boxes in the table should add up to the number in the total column.

The Pie Chart: There are 360 degrees in a circle. There are 720 minutes in the cell cycle for Allium. Use the mathematical relationship between these two values to make your table accurate. You will probably need to use a protractor, too.

Remember: No pronouns, correct verb tense, use a ruler where needed, spelling and grammar are important and do count. Scientific drawings are accurate sketches of the entire field of view, titled, include the magnification, have all labels down the right hand side and a ruler is used to draw straight, horizontal lines that do not cross each other