Cell Lab # 7 – Onion Root Mitosis
Problem: How do cells reproduce?
Background Information:
DNA: DNA is the chemical code that is used to build all the proteins that make up your cell. Life
is possible because the DNA molecule can duplicate itself. This makes reproduction, growth, and
thus life possible.
 DNA is contained in units called genes. Each gene contains the DNA to build one
protein – for a cell part or a specific enzyme. This is why DNA is called the genetic
 The genes with the DNA are found on strands called chromosomes.
 The chromosomes are found in the nucleus of the cell.
 The chromosomes are found in pairs because DNA comes from both mother and father.
Humans have 23 pairs of chromosomes: 23 from each parent.
Mitosis (Cell Division): Because DNA can copy itself, a cell can copy itself. This duplication
(reproduction) of a cell is called mitosis. This is how organisms grow (add cells) and reproduce
(split into 2 organisms). We call the original cell the parent cell and the 2 new cells the
daughter cells.
What Happens in Mitosis? - Before mitosis, the nucleus is visible but the chromosomes are not.
During this Interphase, the DNA duplicates in preparation for division. Next the membrane
around the nucleus breaks down, and the chromosomes start to coil. Then the chromosomes line
up in the center of the cell and start to pull apart. The chromosomes group at opposite ends of the
cell, and a membrane forms around each group, making 2 nuclei. The cell membrane and cell
wall (in plants) then form between the two daughter cells, completing mitosis.
Pre-lab Questions:
1. What is the purpose of the DNA molecule?
2. Where is DNA found?
3. Why is mitosis a part of both growth and reproduction?
1. Write out the title and the problem.
2. Answer the pre-lab questions in sentences. (Do not write out the question.)
3. Set up a microscope and set it to the lowest magnification (40X, using 4X objective).
4. View the slide of onion root mitosis in both low magnification (100X) and high
magnification (400X).
5. Diagram a cell in each of the 5 stages above.
6. For each stage, state in a sentence what is happening in this stage.
7. Estimate the percentage of cells in the onion root tip which are undergoing mitosis.
Do this in lower magnification.
8. Answer the post-lab questions in sentences. (Do not write out the question.)
9. Write a conclusion using the R-A-C-E format.
Percentage of cells undergoing mitosis _________
Percentage of cells in Interphase
What is Happening in this Stage
Cell Diagrams:
Diagram of Cell
Post-Lab Questions:
1. Write a definition of mitosis in your own words.
2. Explain why the characteristics of DNA make mitosis possible.
3. Why is DNA considered the key to life?

Cell Lab # 7 – Onion Root Mitosis