Onion Rot Mitosis

Mitosis in Onion Root Tip Cells
A quick overview of cell division
The genetic information of plants, animals and other eukaryotic organisms resides in several (or many) individual DNA molecules, or chromosomes. For example, each human cell possesses 46 chromosomes, while each cell of an onion possesses 8 chromosomes. All cells must replicate their DNA when dividing. During DNA replication, thetwo strands of the DNA double helix separate, and for each original strand a new complementary strand is produced, yielding two identical DNA molecules. DNA replication yields an identical pair of DNA molecules (called sister chromatids) attached at a region called the centromere. DNA replication in eukaryotes is followed by the process called mitosis which assures that each daughter cell receivesone copy of each of the replicated chromosomes. During the process of mitosis, the chromosomes pass through several stages known as prophase, metaphase, anaphase and telophase. The actual division of the cytoplasm is called cytokinesis and occurs during telophase. During each of the preceding stages, particular events occur that contribute to the orderly distribution of the replicated chromosomesprior to cytokinesis.

The stages of mitosis
Prophase. During prophase, the chromosomes supercoil and the fibers of the spindle apparatus begin to form between centrosomes located at the pole of the cells. The nuclear membrane also disintegrates at this time, freeing the chromosomes into the surrounding cytoplasm. Prometaphase. During prometaphase, some of the fibers attach to the centromereof each pair of sister chromatids and they begin to move toward the center of the cell. Metaphase. At metaphase the chromosomes have come to rest along the center plane of the cell. Anaphase. During anaphase, the centromeres split and the sister chromatids begin to migrate toward the opposite poles of the cell. Telophase. During telophase, the chromosomes at either end of the cell cluster begin tocluster together, which facilitates the formation of a new nuclear membrane. This also is when cytokinesis occurs, leading to two separate cells. One way to identify that telophase has begun is by looking for the formation of the cell plate, the new cell wall forming between the two cells.

The objectives of this lab exercise are for you to:

• Better understand the process and stages ofmitosis. • Prepare your own specimens of onion root in which you can visualize all of the stages of mitosis. • Apply an analytical technique by which the relative length of each stage of mitosis can be estimated.

Root Tip Mitosis

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I. Viewing mitosis in onion root tips.
Why use onion roots for viewing mitosis? • The roots are easy to grow in large numbers. • The cells at the tip ofthe roots are actively dividing, and thus many cells will be in stages of mitosis. • The tips can be prepared in a way that allows them to be flattened on microscopes slide (“squashed”) so that the chromosomes of individual cells can be observed. • The chromosomes can be stained to make them more easily observable.

Regions of Onion Root tips
There are three cellular regions near the tip of anonion root. 1. The root cap contains cells that cover and protect the underlying growth region as the root pushed through the soil. 2. The region of cell division (or meristem) is where cells are actively dividing but not increasing significantly in size. 3. In the region of cell elongation, cell are increasing in size, but not dividing.
Region of cell elongation

Region of cell divisionProtective root cap

Viewing Chromosomes

Chromosomes generally are not visible as distinct entities in nondividing cells, since the DNA is uncoiled, but the process of mitosis is facilitated by supercoiling of the chromosomes into a highly compacted form. Supercoiled chromosomes can be visualized in cells, particularly if they are treated with a DNA-specific stain, such as the Feulgen stain....