DNA Extraction and Molecular Blueprint

These bases follow a strict complementary rule through hydrogen bonding:

• Adenine (A) always pairs with Thymine (T) (forming 2 hydrogen bonds).
• Cytosine (C) always pairs with Guanine (G) (forming 3 hydrogen bonds).

This relationship was famously quantified by Erwin Chargaff in 1950. Chargaff’s Rule states that in any given DNA molecule, the amount of Guanine is equal to Cytosine, and the amount of Adenine is equal to Thymine.

Exam Tip: Worked Example If a DNA molecule contains 100 total nucleotides and 21 of them are Adenine (A), how many of the others can we find?

1. Since A pairs with T, there must be 21 Thymine (T) molecules.
2. Total A + T = $21 + 21 = 42$.
3. Remaining nucleotides = $100 - 42 = 58$.
4. Since C must equal G, we divide the remainder by 2: $58 / 2 = 29$. Answer: T = 21, C = 29, G = 29.

The DNA Extraction Lab: Step-by-Step

To see DNA with the naked eye, we must perform an extraction. We can use oral mucosa cells (cheek cells) from our own mouths as our source material.

Step 1: Collecting the Cells

First, prepare a solution of approximately 50ml of water mixed with a teaspoon of salt. Swish this salt water vigorously in your mouth for one minute. This mechanical action helps collect the oral mucosa cells from the lining of your mouth. Spit the liquid into a clean cup, being careful not to swallow it.

Step 2: Breaking the Barriers

The DNA is currently trapped inside the cell nuclei, which are themselves protected by cell membranes. To release the DNA, we add a drop of dishwashing soap to the salty water.

The detergent performs two vital roles:

1. It breaks down the lipid (fatty) cell membranes.
2. It helps disrupt the protein chains (such as histones) that hold the DNA tightly coiled.

The salt helps further by breaking down these protein chains, allowing the DNA to be released into the solution.

Step 3: Ethanol Precipitation

Once the solution is prepared, we use ethanol precipitation to make the DNA visible. Gently pour cold ethanol (>80% concentration) down the side of the glass so that it forms a distinct layer on top of the soapy salt water.

Because DNA is not soluble in cold ethanol, it will "precipitate" (solidify) out of the solution and collect at the interface of the two liquids. After 1–5 minutes, you should see white, cloudy, stringy material forming.

Step 4: Collection and Preservation

You can use a toothpick or a Q-tip to carefully swirl and lift the DNA out of the solution. To keep the DNA from degrading, it is best to store it in a container of pure ethanol in a freezer. If left at room temperature, the DNA will slowly break into smaller pieces.

Safety Warning: During this lab, do not drink the ethanol and do not eat your extracted DNA!

Summary of Discovery

The ability to extract and study DNA was made possible by the work of many scientists. Rosalind Franklin's X-ray crystallography provided the essential data regarding the helical shape and dimensions of DNA. This data allowed Watson and Crick to assemble the famous 3D double-helix model that we study today.