Ciencias

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,the two 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 cellreceives one 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 replicatedchromosomes prior 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 thecentromere of 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 cellcluster begin to cluster
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 processand stages of mitosis.
• 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 Page - rtm2
Region
of cell
elongation
Region
of cell
division
Protective
root cap
I. Viewing mitosis in onion root tips.
Why use onion roots forviewing mitosis?
• The roots are easy to grow in large numbers.
• The cells at the tip of the 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 moreeasily observable.
Regions of Onion Root tips
There are three cellular regions near the tip of an onion 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, cellare increasing in size, but not
dividing.
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...