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Star Republic: Guide for Biologists

Ethonal precipitation of nucleotic acids

1. Choose the proper salt concentration:

Stock solution

Precipitation concentration

Fraction of total volume

Ammonium acetate

7.5 M

2.5 M

1/2*

Lithium chloride

8 M

0.8 M

1/10

Sodium Acetate

3 M pH 5.2

0.3 M

1/10

Sodium Chloride

5 M

0.2 M

1/25

*Lower concentration (1/5-1/10) is sometimes used.

2. Add 2-2.5 volumes of ethanol or 0.7-1 volume of isopropanl.

Example: 0.4 ml DNA solution, add 40 ml 3 M sodium acetate, and 0.8 ml of ethanol.

3. Precipitate at 4 C to -70 C for > 30 minutes.

For genomic DNA or solutions with high salt or contamination, precipitation at room temperature may be desirable.

4. Centrifuge at room temperature or 4 C to pellet DNA (5-30 minutes).

5. Higher speed and longer centrifugation time may be needed for small fragments and low DNA concentration.

6. Add carrier (e.g., 10 mg tRNA) to improve recovery.

7. Add MgCl2 to 100 mM to improve precipitation of small fragments (<100 bp).