Extracting DNA from Cells

Castle Under Siege

Castle Under Siege

Introduction: Nucleic acids were first isolated from animal tissues in the late 1860s (at the time, we had no idea what they did), but tremendous technological advances have occurred since then. Scientists using PCR enzymes can now extract usable DNA from scraps of hair, from stuffed museum specimens of extinct species, from saliva dried on the stamp of a mailed letter, or even fossilized insects millions of years old.

Question: What does a castle under siege have to do with extracting DNA from a cell? Quite a lot, actually. To conquer the kingdom, we need to cross the moat, penetrate the castle walls (cell wall and cell membrane), break through the inner defences of the keep (the nuclear envelope), break down the door to the throne room, and separate the king from his palace guards (the chromosomes). We must use enough force to get inside, but not so much that we accidentally kill the king; we need to take him alive. As you will see, getting the DNA unwound from the histone proteins that surround it, out of the nucleus of the cell, and out of the cytoplasm of the cell, undamaged and away from all of the rubble of the cell, is a similar trick.

How can extracted DNA be used?

  1. Forensics: DNA samples (from saliva, skin, blood, hair, semen, or bone) found at a crime scene can be used to link a suspect to the crime. Samples collected before these techniques were available have been re-analysed to compare DNA with suspects, sometimes allowing for the closure of cases that have remained unsolved for many years.
  2. Exoneration: DNA evidence has been used to free people accused of crimes they did not commit. The Innocence Project is an organization involved with this work.
  3. Identity: DNA extracted from human remains, such as soldiers declared "missing in action," can be used to confirm identity by comparing it to the DNA of living relatives. A body found buried under a parking lot in England was recently confirmed to belong to King Richard III.
  4. Kinship: Comparing the DNA of two living people can determine whether they are closely related. This can be useful, for example, when there are questions of paternity, or when an adopted child is attempting to identify a biological parent.
  5. Gene Therapy: Unmutated DNA can be introduced into the stem cells of a person with a genetic mutation, replacing the defective genes with normal ones. While this technique has not been perfected, gene therapy shows promise for permanently correcting inherited abnormalities. New techniques of gene editing, such as CRISPR, look very promising for the treatment of genetic conditions.

Extract DNA from yourself (see below), from a strawberry, (or banana, or kiwi using same process) or from an onion. Watch out for those fruits; they are full of chemicals! ;-)

Gross Science | Anna Rothschild

Transcript: Extract Your Own DNA

Posted: March 1, 2012

NOVA Presents: D.I.Y. Science

How to Extract Your Own DNA


(Image of bottled water, clear dish soap, food coloring, table salt, and 70% isopropyl alcohol)

And… you!

Step 1: Mix 500 ml drinking water with 1 tbsp salt

Step 2: Stir until salt is dissolved. Then transfer 3 tbsp of salt water into a clear cup.

Step 3: Gargle the salt water for 1 minute.

Step 4: Spit the water back into the cup. Now your cheek cells are suspended in the salt water.

Step 5: Gently stir the salt water with one drop of soap. (Avoid bubbles as much as possible).

Note: Soap breaks down the cell membranes, releasing the DNA.

Step 6: In a separate cup, mix 100 ml isopropyl alcohol and 3 drops of food coloring.

Step 7: Tilt the salt water cup and gently pour the alcohol so that it forms a layer on top (about 2 cm thick).

Step 8: Wait about 2.5 minutes. You should see white clumps and strings forming.

(Image of strings and clumps of DNA)

That's your DNA!