Information of life. Christmas Lectures 2008

Transcription

1 Modern computer processors have been improving rapidly since they were first developed in the 1940s. Computer systems have advanced in two main ways: how fast they can run calculations and how much information they can store in a small space. DNA (deoxyribonucleic acid) is the chemical that stores the information of life in all our body s cells, and now researchers are looking into ways of building DNA computers! DNA is an astounding chemical: it stores information in every cell of our body and inside the cells of all life on Earth. This information, stored as genes, describes how to build all the different bits and pieces that make up our bodies and tells our cells how to divide themselves so that we can grow and, if we get hurt, we can repair our bodies. The shape of a molecule of DNA is known as a double-helix it s like a tall ladder, with lots of rungs joining the upright poles, and twisted around into a spiral (see diagram). Here you ll learn about DNA and how it actually saves the information for life (it s nothing like the memory inside your mobile phone). But first let s look at an experiment you can do to actually see the DNA extracted from cells. All you need is strawberries and washing-up liquid and a few bits of kitchen equipment. You could ask your parents or teacher to help. 1

2 EXTRACTING DNA The third step of this experiment, filtering the mushed-up strawberries, can take a little while, so you might want to leave it while you get on with other things. To extract DNA from strawberries you will need: a handful of strawberries 1 /2 teaspoon of salt 1 tablespoon of washing-up liquid (detergent) 1 /4 glass of water (the exact amount isn t important) some rubbing alcohol (from a pharmacist or supermarket) a small clear plastic bag (e.g. a re-sealable sandwich bag) an empty narrow glass coffee filter paper or kitchen roll a funnel a thin skewer or cocktail stick. What to do 1. Before you start this experiment, put the bottle of rubbing alcohol in your freezer you need it ice cold (don t worry, it won t freeze). Add the salt and washing-up liquid to the glass of water and stir it to mix and dissolve it properly. This is your DNA extraction solution. 2. Carefully pull the green tops off the strawberries and put the fruit inside the plastic bag. Squeeze out the air and seal the plastic bag (helps keep the mess inside). Now mash the strawberries as much as you can with your fingers. Pour the DNA extraction solution (the salty soapy water) into the plastic bag and mash it into the strawberries to get the juice nice and frothy! 3. Put the filter paper (or kitchen roll) into the funnel and pour the strawberry mush through it. Collect the filtered juice in the empty glass, just like in the photo. This can take a little while to drip through so be patient! Throw away the filter paper and strawberry pulp. If you don t have any filter paper or a funnel you can do without these by carefully pouring the strawberry juice out of the plastic bag. Try to keep the strawberry pulp in the bag and pour only the juice into the empty glass. It doesn t matter much if a bit of the pulp gets out though. 2

3 4. Take the ice cold rubbing alcohol out of the freezer. Hold the glass with the strawberry juice and DNA extraction solution and tilt it to one side. Now slowly and carefully, pour the rubbing alcohol down the side of the glass into the strawberry juice. Pour about the same amount of alcohol as you have soapy strawberry juice. It s important that the alcohol and strawberry juice don t mix together. Notice that the layer of clear alcohol sits on top of the cloudy juice. Carefully place the glass on the table and leave it undisturbed for a few minutes. 5. When you return to the glass there should be some white stringy stuff floating around in the alcohol layer. Try to fish this out using a skewer or cocktail stick and put it into some clean rubbing alcohol in a small glass jar (an old spice jar is ideal). This white stringy stuff is actually strawberry DNA the molecule that stores the information of life in all cells. This DNA won t be particularly pure, as it will have a lot of cell proteins attached to it. If you want, you can squirt some protein removal contact lens solution into the DNA extraction solution. The contact lens solution contains enzymes, called proteases, which break down proteins. Pineapple also naturally contains lots of proteases so you could use that instead of strawberries if you want to experiment a little more. Why not try other kinds of fruit (kiwis or bananas also work well)? And experiment with how much detergent and salt you put in the solution to see if you can improve how much DNA you can extract. 3

4 How it works The strawberry DNA is held inside the plant cells. So first we mash up the strawberries and try to break open the strong cell walls as much as possible. To get inside the cells to the DNA, we need to dissolve away the cell membranes, which are made from fatty molecules. The DNA extraction solution contains washing-up liquid, which is a kind of detergent. Detergents are very good at dissolving away fatty molecules, just like when you re washing the greasy dinner plates after a tasty roast lunch. The DNA extraction fluid also contains salt, which helps to dissolve the DNA into the solution. The next step is to filter out the left-over strawberry pulp this is mostly cellulose (just like the fibrous bits in celery) and would get in the way of extracting the DNA. The final stage is to pour the rubbing alcohol on top of the strawberry juice. DNA does not dissolve very well in the cold, concentrated alcohol, so when the DNA molecules drift up from the strawberry juice into the alcohol, they can no longer stay dissolved. Lots of DNA molecules tangle and clump together these are the white strands you can see in the alcohol layer. MORE ABOUT DNA Hopefully you were able to extract enough DNA from the strawberries to be able to see clumped-up strands of it. All living things on planet Earth use DNA to store their important information. We used strawberries because it s easy to get their DNA out and it s much safer than trying to extract the DNA from your own body! Biologists have perfected ways of extracting the DNA from any plant or animal cell (and all the microscopic organisms we can t see), using much more expensive equipment than plastic sandwich bags and washing-up liquid! How does a chemical actually store information? The rungs of the DNA ladder shown on the first page are made up of four different kinds of chemical, nicknamed A, G, T and C. These are the letters of the genetic alphabet, and reading along the length of a DNA molecule the order of the letters spells out important information. The DNA inside every single one of our cells has over three billion of these genetic letters. If you stretched out the DNA inside a single cell it would be taller than you, so it is coiled up tightly to fit inside such a tiny cell! Biologists have worked out that the amount of information stored in the DNA in each of our cells is roughly 750 megabytes that s slightly more than a music album on a compact disc. And an adult human body is built up of over 100 million million cells. If you were to save the total DNA information contained inside a human body on to CDs, you d need a pile that stretched almost all the way to the sun! 4

5 Because even a short length of DNA can store a lot of information and other molecules can be used to copy or rewrite sections of DNA, computer scientists are looking into ways of using DNA molecules to perform calculations. DNA computing is still very new, but simple DNA computers have been used to show how they can solve certain problems. DNA is one of the oldest ways of storing information, billions of years old, yet it may also form the future of computing. 5