DNA extraction with CTAB and chloroform:isoamyl alcohol

Prepare DNA extraction buffer:

100mM Tris-HCl (pH 8), 100mM EDTA (pH 8), 100mM sodium phosphate buffer (pH 8), 1.5M NaCl, 1% CTAB

For a 1000mL (1L) solution of DNA extraction buffer combine:

100mL 1M Tris-HCl (pH 8)

200mL 0.5 M EDTA (pH 8)

100mL 10x sodium phosphate buffer (pH 8)*

300mL 5M NaCl

10g CTAB

*Note: 1M sodium phosphate buffer is made from a mixture of (A) 1M sodium phosphate monobasic dihydrate (NaH₂PO₄ · 2H₂O, 156.01g/mol) and (B) 1M sodium phosphate dibasic (Na₂HPO₄, 141.96g/mol). Solutions should be autoclaved.

For pH 8 solution, combine 68mL of A (1M sodium phosphate monobasic dihydrate) with 932mL of B (1M sodium phosphate dibasic).

Prepare enzyme stock solutions (aliquot as necessary)

Combine 200mg of lysozyme and nuclease free water to a final volume of 2mL (100mg/ml). Store aliquots at -20°C.

Combine 40mg of proteinase-k and nuclease free water to a final volume of 2mL (20mg/ml). Store aliquots at -20°C.

For sterivex filters, remove casing from filters using pliers. Squeeze at the top of the filter and apply force at the seam of the casing. It is recommended to open the sterivex over a sterilized container (e.g. clean, empty pipette box) as the filter may pop out suddenly.

Using sterile razor blades, cut around the top and bottom of the filter (in the white space) and then cut along the filter to divide the filter into 4-6 strips. Depending on the sample type, 1/2 to the full filter can be used for extraction. Place strips in clean 2mL microcentrifuge tube.

For other sample types (e.g. sediment, microbial mat), weight out desired starting sample amount. For 1mL of extraction buffer, recommended mass range: 0.1-0.25g

Add 1mL of DNA extraction buffer to samples in 2mL microcentrifuge tubes.

With samples in buffer, perform 3x freeze thaw cycles. Freeze at -20°C for 20 minutes and thaw at 37°C for 2 minutes.

(Note: in a comparison of -20°C and -80°C freezing temperature, no difference was found in the DNA yield)

Add 20μl of lysozyme (100mg/ml) and incubate at 37°C for 30 minutes with a very gentle rotation (200rpm).

Add 10μl of proteinase-k (20mg/ml) and incubate at 37°C for 30 minutes with a very gentle rotation (200rpm).

(Good stopping point. Can freeze overnight at -80°C)

Add 100μl of filter-sterilized 10% SDS and incubate at 65°C for 2 hours. Invert gently every 25 minutes.

(Can freeze overnight at -80°C. If stopping here, warm again to ~50°C to dissolve SDS before proceeding to next step.)

Add chloroform:isoamyl alcohol (24:1) to fill the microcentrifuge tube until full. Mix thoroughly by inverting and/or vortex briefly.

Centrifuge for 10minutes at 3,200xg and transfer supernatant to a clean tube.

To supernatant (step 11), again add chloroform:isoamyl alcohol (24:1) to fill the microcentrifuge tube. Mix thoroughly by inverting and/or vortex briefly.

Centrifuge for 10minutes at 3,200xg and transfer supernatant to a clean tube.

Precipitate DNA by adding 0.6x volumes of room temperature 100% isopropanol to the supernatant (step 13). Adjust the amount of isopropanol added to each microcentrifuge tube based on the volume of supernatant. Incubate for 2h in the dark at room temperature.

Centrifuge at 20,000xg for 30 minutes and remove supernatant while avoiding DNA pellet. Pellet will be transparent to white in appearance but may be colored for some sulfur-rich sediment samples.

Perform 2x ethanol washes using 70% ethanol. Add 1mL 70% ice-cold ethanol, invert several times, centrifuge at 20,000xg for 10 minutes at 4°C. Remove ethanol (supernatant) and repeat for a total of 2 rinses.

Dry pellet for 15-30 minutes at room temperature

Add 250μl of nuclease free water to pellet. Allow pellet to dissolve by incubating for 1h at 4°C. (Note: reduce elution volume if low yield is expected)

DNA can be stored at -20°C for short term storage or -80°C for long term storage.