The goal of this exercise is to help you learn how to improve a set of instructions by applying the advice provided in Chapter 23.

The Exercise

The following instructions describe the process for extracting DNA. Although the instructions are fairly well written, they could be improved in a number of ways by more closely following some of the guidelines in Chapter 23. You may download the Revision Checklist for Instructions for use with this exercise.

For this exercise:

1. Read the instructions and indicate places where headings might help the reader.
2. Look for any steps that contain too much information and mark them. Should substeps be included here? Should another step be added?
3. Look for places where warnings or cautions might help the reader.
4. Locate steps that might be clearer were they accompanied by an illustration.

Extracting DNA

1. Place the block of tissue into the histocut. Set at 10 micron sections.
2. Cut the tissue until your slices contain the entire piece of tissue.
3. Using the tweezers, place two sections into a 1 ml eppendorf tube. Use a separate tube for each block of issue. Make sure to label the tubes so that you know which tissue you placed in each eppendorf.
4. Heat the tubes at 65 degrees in the dry bath for approximately five minutes to melt the paraffin.
5. Immediately pipet 1 ml of xylene into each tube upon removing from the bath.
6. Vortex each tube for about ten seconds to thoroughly mix the tissue with the xylene. Do this to separate the tissue from the paraffin. Then let the tubes sit for one minute.
7. Place the tubes in the 4 degree centrifuge and spin for 30 minutes.
8. Remove the tubes and decant slowly, in one smooth motion. Once you have lifted the tube DO NOT attempt to drain off any more liquid as you are likely to lose tissue.
9. Add 1 ml ethanol and spin at four degrees for 15 minutes. Decant.
10. Place the tubes with the caps open into the speed vac.
11. Spin until you can no longer see liquid in the tube.
12. Estimate the amount of Tween 20 needed for each tube by the size of the tissue in the block.
13. Pipet the determined amount of Tween 20 into the eppendorf.
14. Add proteinase K to the correct dilution.
15. Place the tubes in a rubber holder and float in a 55 degree bath for two hours.
16. Transfer the supernatant to a clean eppendorf, leaving any sediment behind.