Mitosis

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Stages of Mitosis

In mitosis, chromosomes go through interphase, prophase, metaphase, anaphase and telophase in order to produce genetically identical cells.

Interphase

The cell prepares to divide.
DNA is replicated by semi-conservative replication. There is now two copies of every chromosome.
The organelles are also replicated.
More ATP is produced to be used in cell division.

Prophase

The nuclear envelope and the nucleolus break down. Chromosomes are left floating in the cytoplasm.
The chromosomes coil more tightly and become shorter and fatter. They can be seen under a light microscope.
Small protein bundles called centrioles move to opposite poles of the cell.
Microtubules form the mitotic spindle between the centrioles.

Metaphase

The chromosomes line up along the mid-line of the cell.
In metaphase, the chromosomes are maximally condensed.
They are attached to the spindle by the centromere.

Anaphase

The chromosomes break into two chromatids. The sister chromatids separate at the centromere.
The spindles contract and pull the chromatids to each pole of the cell.

Telophase

The chromatids reach the opposite poles and begin to decondense (unravel), becoming chromosomes again.
Nuclear envelopes form around the chromosomes so there are now two nuclei.
The cytoplasm splits and two daughter cells are formed. The daughter cells are identical to the original cell and to each other.
The cell cycle starts again.

Way to remember the stages:

I (interphase).
Picked (prophase).
My (metaphase).
Apples (anaphase).
Today (telophase).

Preparation of Stained Squashes of Cells from Root Tips

Only a few cells are able to continue dividing in a multicellular organism. In plants, the growing tips of roots and shoots contain meristem tissue that can divide by mitosis for growth.

1) Sample preparation

Wear gloves and use forceps to handle the tips.
Root tips must be sprouting (actively growing).
Place into 5 M hydrochloric acid.
After 5 minutes, rinse the tips in cold water in a watch glass.

2) Cut the root tips

Using a sharp scalpel, cut root tips that are 2 mm long.
Place a root tip onto a microscope slide. Ensure the slide is clean to reduce the chances of artefacts.

3) Staining

Carefully add 2-3 drops of stain and leave for two minutes.
Use a mounted needle to spread out the root tips into a thin layer.
Place a coverslip over the top of the tips.

4) Squashing

Squash down by applying force to the cover slip. This could be with the flat end of a pencil, or the slide could be covered with a paper towel and pressed.
Force must be vertical or the cover slip may break and cause injury.

5) Viewing the sample

Place the slide on the microscope stage using the lowest power lens.
Focus the lens on the sample using first the coarse control and then the fine control.
Move the slide to see the range of cells. The cells closer to the tip will be those more actively dividing.
On a lens power of 400x, it should be possible to clearly see the chromosomes in the dividing cells.