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Extracting DNA from onions

Did you know that onions have more DNA that you do? In fact, an onion has 12 times more DNA than your average Cambridge professor. But why? It’s a question that has puzzled scientists for 50 years, ever since DNA was discovered. Not all DNA is involved in genetic processes. Quite a lot of it is what is known as “junk DNA”. This DNA is created when mistakes are made in DNA reproduction. Many organisms actually discard junk DNA at a very high rate. This is good, as it results in compact, junk-free genomes. But onions faithfully reproduce all their DNA, even the junk. Why don’t you try extracting some DNA from onions?

DNA history

It was the early 1950’s, and the race was on to discover the structure of the building block of genes - deoxyribose nucleic acid (DNA). The two camps were: Cambridge University graduate student Francis Crick and research fellow James Watson; and Maurice Wilkins and Rosalind Franklin of King’s College, London. Crick and Watson decided to make physical models to narrow down the possibilities of the structure of DNA. The King’s team took an experimental approach, and examined x-ray diffraction images of DNA in order to establish its structure. History has Crick and Watson as having discovered the structure of DNA, but the truth is that shared knowledge from the King’s College team enabled them to make the leap forward.

Doing it!

Onions have a rather large amount of DNA, and here in Kitchen Chaos, we’re going to extract some of it.

Every living thing contains DNA so you can use just about anything, including spinach, kiwi fruit, wheat germ, green split peas, chicken liver or broccoli, if you don’t have any onions.

What you need:

Onions (don’t use Spanish onions)
Salt (non-iodized)
blender or pestle and mortar
Liquid dishwashing detergent
Strainer or coffee filters
Alcohol (isopropyl will do)
Wooden stick

Method:

Place about one-fifth of a diced onion into a cup.
Add about 1 tsp of salt. The salt adds positively charged atoms to the normal negatively charged DNA. This allows the DNA strands to come together and helps them to dissolve.
Blend (or mash) the onions. This breaks down the cell walls and releases the DNA. Try not to introduce too much air to the mash.
Add about 1 tsp of liquid dishwashing detergent and about 40 ml water. This destroys fatty cell membranes and some proteins. If you’re using a pestle and mortar, continue to pound for a further 5 - 10 minutes. If using a blender, give it a really good whiz.
Using a strainer or the coffee filters, strain the solution into a new container. Now very gently layer about 20ml of ice-cold alcohol on top of the mixture by pouring it slowly and gently down the side of the container. Make sure you don’t pour it too quickly - you don’t want the alcohol to mix. The DNA is attracted to the interface between your mixture and the alcohol.
Gently put the wooden stick into the mixture and spin slowly for about a minute. You are winding the DNA onto the stick. It might be a bit hard to see. Now pull the stick slowly through the alcohol, and the DNA should become visible, looking a bit like slimey gloop on the end of the stick.

The gloop is actually DNA, and contains all of the genetic code for making an onion. Now why don’t you have a go extracting DNA from something else?