Within A-Level Biology, there are required practicals which help you demonstrate skills which cannot be shown in exam papers e.g. safe handling of apparatus and gathering/analysing your own data.
For Required Practical 2, you must be able to:
- Prepare a root tip cell squash
- Use an optical microscope
- Calculate the mitotic index
- Calculate the actual cell size
Important:
When carrying out this practical, you must wear safety goggles, a lab coat and gloves.
How to prepare a root tip cell squash
- Add 1M hydrochloric acid to a boiling tube - it must be enough to cover the root top! Place this tube in a water bath until it reaches 60°
- Cut 1cm from the tip of a growing root, using a scalpel
- Add the root tip to the hydrochloric acid in the boiling tube and leave it for 5 minutes
- Using tweezers, remove the root top from the tube and rinse it well with cold water. Leave it to dry on a paper towel.
- Place the dry root tip on a miscroscope slide, then cut 2mm off and discard the rest of the root
- Use a mounted needle to break the root tip
- Add a few drops of stain
- Place a cover slip over the cells, add a piece of folded filter paper on top - push down on the filter paper to squash the tissue. Be careful and do not move the cover slip sideways, as otherwise the chromosomes will be damaged
How to use an optical microscope
- Clip the pre-prepared microscope slide onto the stage
- Use the lowest-powered objective lens
- Use the coarse adjustment knob to bring the stage near to the objective lens
- Look down into the eyepiece and then use the coarse adjustment knob to move the stage away from the lens until the image starts to come into focus
- Adjust the fine adjustment knob to get a clearer image
- Sometimes, just using the adjustment knobs will not be enough and then you can use a higher-power objective lens. Then you will need to refocus the microscope
How do you calculate the mitotic index?
[katex] Mitotic space Index = frac {number space of space cells space with space visible space chromosomes}{total space number space of space cells space observed}[/katex]