What's inside a cell?

Biology lecture used for CBS 2012
Useful information
Kit List: 

Kiwi DNA kit, plus another couple of fruits, such as strawberries and bananas
Model DNA
Camcorder/ camera to show experiments live on the projector

Optional extra demos:
Dropsophila (fruit flies)
Protein precipitation (milk, vinegar, plastic glass)
Custard fireballs
Oil and pyrex

Kiwi dna kit includes:
Consumables: Kiwi fruit, salt, washing up liquid, 96% ethanol or 99% isopropanol (*caution*)
Equipment: Water bath (50C), plastic pint glasses, half rolling pin (for mashing), sieve, plastic champagne flutes, ice bucket, bowl (for liquid waste), cloths (it's generally messy), sharp knife (*caution*), chopping board, spoon/fork/cocktail stick/pipette, model of DNA, posters about DNA and proteins
*NB. Alcohol needs to be kept out of reach of children, as does the sharp knife. If possible, keep the knife and any spare ethanol in the box when demonstrating*

Packing Away: 

*Do not leave fruit in the water bath... they might end up there for months if we don't use the experiment again soon and fruit slime is not very nice!*



Breaking open the cells of a kiwi fruit to extract the DNA. But demonstrated to a whole lecture theatre, rather than just one group!

Extra demonstrations (Drosophila and protein precipitation) help extend this talk to cover other areas of cell biology.

Tips for demonstrating:

  • It's difficult for the audience to participate in all of the practical activities, so it's more of an interactive show with lots of questions and answers.
  • Do the preparation work before the audience arrives!
  • Be aware that young children won't know a lot of science. For instance, don't assume they know about cells, they know about humans being animals and that plants and animals have a lot in common. They might not even have reached the stage where they relate the idea of parts of the body having different structures and functions.


You need to have:

  • 96% ethanol/99% isopropanol on ice as it works best when cold. If ice is not available, isopropanol tends to work better at room temperature.
  • Water bath heated to 50C
  • Use masking tape or similar to divide the water bath into pint-glass sized sections so that the glasses don't fall over when you put them in the water bath
  • A kiwi or two peeled and cut into 1 cm chunks
  • A bottle of lysis buffer - contains washing up liquid, salt and water. As a general guide, in a bottle, put about 1/8 volume salt and dissolve in 3/4 bottle of warm water, then top up with washing up liquid and invert a few times to mix. You don't want it to be particularly thick, so err on the side of caution when adding washing up liquid - you can always add more later if it's not working well.

Basic procedure and explanation

The steps of the experiment are below in bold, with an example way of talking through the experiment for younger children. It includes questions and answers (of course if you get the right answer modify your reply). Try and use simple words and up the language as you deal with older kids or adults. You can use this experiment as a basis for talking about scale, with respect to cells and molecules and also as a basis for discussing what DNA does, and how similar our DNA is to kiwis (about 85%).

You may want to have some mashed DNA in lysis buffer incubating, and use that rather than the one the group with you has prepared as it takes a while for the cells to lyse


Do you know what this is? It's a kiwi fruit. And what's a kiwi fruit made out of? (you'll probably get seeds, flesh, skin types suggestions)

What are all living things made out of? If I scratch my nail across this what do I have under my finger nail? Cells. Just as a house is made out of bricks every living thing is made out of cells but they're very small. There are different cells in the flesh to what there are in the seeds or in the skin and in you their are hundreds of types of cell. Different types of cells in your eyes, blood, brain and skin too.

Now what I want you to do for me is mash up some kiwi fruit. Add a lump of Kiwi to a pint glass and get them to mash it with the rolling pin (gently!). You can explain that what they're doing is like demolishing a house, all you've got left is a pile of bricks.

Now what I'm going to add is some washing up liquid. Add enough lysis buffer that there's a thin liquid layer on top of the lumps (maybe 1cm depth total) and put in water bath. You can ask: what does washing up liquid do? (You'll probably get "it makes bubbles" at this stage!) What do you use it for? Cleaning plates. Well what does it take off the plates? The grease and fat. Do you do the washing up? *Teasing to amuse the parents!*

Well next time you're doing the washing up put some water in the frying pan then add some washing up liquid. The lumps of fat floating in the liquid break up into smaller lumps and then smaller lumps and then such small lumps that they disappear into the water. Do you know what that's called?

Do you know what it's called when sugar or salt disappears into water? Being dissolved. So washing up liquid breaks up / dissolves fat.

Do you know what's in a cell? A cell is like a bag full of water but the bag is made out of fat. Inside there's lots of important stuff floating around and the DNA. What do you suppose happens to the cell when you add the washing up liquid?

It breaks up the cell walls.and everything that's inside the cell gets spilled out. So the DNA inside the cell gets spilled out.

What do you know about DNA? Discuss! DNA is like the plans for building us. Just like you need plans for building a hospital. But if you build a hospital it's useless unless you know who to build the doctors, nurses and beds inside them and the doctors and nurses know what to do. So it's not just the plans for building you it's also the instructions for how you should be run. And what we're going to do is we're going to take the DNA from the kiwi fruit.

Take the kiwi mush/lysis buffer mix out of the water bath. So this liquid here is full of DNA.

Pour out some of the liquid into a glass or get an assistant to do it. It may be easiest to do this through a sieve. So what we're going to do is get out that DNA but first I want you to tell me what this is.

Get out the alcohol and pass it around the noses present, taking care to keep control of it. What does that smell like? It's not water, it's pure alcohol. I emphasize this as children commonly think anything that is liquid has water in it. Many kids will recognise the smell as antibacterial hand gel!

For about 9+ children, as some motor skill is required, I address them with: Have you ever poured champagne? The things about pouring champagne like pouring coke is that if you pour it down to the bottom of the glass it fills with bubbles. If you don't want to get any bubbles in it you need to pour it down the side of the glass, now that's what I want you to do with the alcohol. If you pour it gently into the glass because the alcohol is lighter than water it will float on the water.

(We use a champagne glass so this works quite well.) Get them to pour out about a centimeter or so if they're old enough, otherwise do it yourself, taking care to pour it down the side of the glass so you don't mix it too much with the lysed kiwi.

Show the glass around. What you can see here are two layers, the green layer is the water with the DNA in and the clear layer is the alcohol layer.

Now do alcohol and water mix? Sure they do. Because if you look at a bottle of whiskey or beer there's only one layer there. So what's happening is the water is moving up into the alcohol layer and the alcohol is moving down into the water layer *wibbly finger demo!* and the two are mixing.

Can you see anything appearing between the two layers? Some stringy white stuff forming? That's the DNA. This sometimes takes a while to appear, so you can use a pipette to pick up some of the bottom layer and release it through the top layer to make the DNA visible.

Possible discussion points:

Now what I'm going to do is try and pick up a single molecule of DNA. (showman mode, on the end of a spoon I fish a little bit out.)
What's the largest number you can think of? Because what i have here on the end of this spoon is a billion billion molecules of DNA. Just as the Kiwi is made up of the bricks we call cells the cells are made up of molecules.

So how big do you think one of these molecules is? It's about a millionth of a millimetre across. But because DNA is an especially long molecule it's a metre long. Now what I want you to do is use your imagination and I'm going to pick up one molecule of DNA *pretend to pick up a very thin strand and pass it to a child to hold and stretch it out to about a metre*

Now this is a molecule of kiwi DNA and if we use your imagination again I can pick up a molecule of your DNA and that's about a metre long too. Now in your right hand we've got a molecule of kiwi DNA, this is the instructions on how to build and run a kiwi and in your left hand we have your DNA which is the instructions on how to build and run you. Now how similar do you think these two are.

How much is the same? (Sweepstake the entire audience) About 85cm is the same (alternatively 85% the same), that's this much. (Mark out 85 cm and you've got around percentages!)

That's because both you and the kiwi are made up of cells and the cells in the kiwi do the same sort of thing as the cells inside of you. They make more cells, they use sugar and oxygen to make energy and use protein and fat. So who do you think is the most similar person in the worl to you? It's your brother or sister, not your mum or your dad. Which is why you've got to look after you're little brother as they're the most similar person in the world to you. You're all but a tenth of milimeter the same as you're brother or sister and you're all but about millimetre the same as anyone else in this room. You're all but 2 cm different from a chimpanzee. Which is why I think we should look after everything in this world as we're really not very different from anything else.

Risk Assessment
Date risk assesment last checked: 
Wed, 30/12/2015
Risk assesment checked by: 
Date risk assesment double checked: 
Thu, 11/02/2016
Risk assesment double-checked by: 
Risk Assessment: 

Kiwi DNA demo

DESCRIPTION NB: This risk assessment is taken from the kiwi DNA experiment
Heat mixture of salt, water, washing up liquid and mashed up DNA (60°C) for 15 mins. Decant some of the liquid into a plastic champagne flute. Drizzle an equal volume of ice-cold ethanol/isopropanol on top. DNA visible at interface. Using model and posters to explain DNA.
  1. Ethanol/isopropanol - Irritant, flammable, and very toxic
  2. Water bath - Hot water can scald
  3. Glassware and plastic containers- If broken can cause cuts
  4. Kiwi/detergent solution Harmful
  5. Plastic knifes, forks, spoons and sharp knife
  6. Wet surfaces - Slip hazard
  7. Small risk of allergic reaction to kiwi fruit in a small minority of people.
  8. Risk of heat element and stirrer overheating if not covered by water.
  9. Small pieces in model may present choking hazard.
  1. Only have a small amount of ethanol/isopropanol out, away from naked flames AND THE PUBLIC. Avoid contact with skin and eyes. Do not ingest. Only allow parents and children to mash kiwi on its own - not once mixed with detergent or ethanol/isopropanol. Ensure eyewash is nearby and that you know the location of it.
  2. Maintain the water bath at 60°C out of reach of children. Ensure has passed PAT test in last 2 years. Ensure cables are taped to the ground/table to reduce risk of person tripping on/pulling the cables.
  3. Dispose safely of any broken glassware immediately. Keep spares out of reach. Check plastic container and ethanol are compatible.
  4. Do not allow children to mash the kiwi once it has been mixed with detergent or ethanol/isopropanol. Keep all mixtures within demonstrator's reach. Avoid contact with eyes or the mouth. Try to work on a surface which is not at eye level. Demonstrator can show children how to mash the kiwi without splashing kiwi everywhere. Know the location of the nearest eyewash
  5. Supervised use only. Keep sharp knives away from children (cut up kiwi fruit before the event and put the knife out of sight e.g. back in the blue crate).
  6. Wipe up any spills. Keep experiments away from electrics.
  7. Demonstrator to check that visitors are not allergic to kiwi fruit before commencing the experiment.
  8. Demonstrator to ensure that element is always covered with water and stirrer is always rotating.
  9. Keep model completely assembled and prevent children taking it apart. Keep the black stopper on top of the stand as this prevents the rest being disassembled.
  1. Wash off skin. Use eyewash to wash out of eyes if trained and confident to do so, and call First Aider. If ingested call First Aider immediately.
  2. Run cold water over affected area for at least 10 mins. Call First Aider
  3. Call First Aider
  4. Use eyewash to wash out of eyes if trained and confident to do so and call First Aider. If ingested call First Aider immediately.
  5. Call First Aider.
  6. Call First Aider.
  7. Call first aider. Rinse skin with clean water.
  8. Turn off electricity at mains. Call first aider if necessary. Allow to cool before using again.
  9. If child ingests small part of model, call first aider immediately.

Drosophila demo

DESCRIPTION Drosophila fruit flies in plastic tubes + eggs/larvae/pupae in petri dishes. Kids will look at the flies under a light microscope, and at laminated pictures of fly development.
  1. Petri dishes/plastic tubes may be dropped/broken – risk of injury from broken containers.
  2. Electrical equipment - hot and risk of electrocution if faulty or wet.
  3. Petri dishes and plastic tubes may be opened - drosophila are non-harmful insects, but it could cause panic/great excitement/silly attempts to inflict the insects on siblings, which can be a safety risk in a crowded place.
  1. Ensure that the flies are kept in close range. Ask children to be careful when handling tubes/Petri dishes. Use plastic vials (instead of glass) to contain flies if possible.
  2. Switch off microscope between uses if it starts to become hot. Ensure experiment is not near a watery experiment. See separate electrical parts risk assessment.
  3. Secure the Petri dish lids to the petri dishes using tape (stretchy lab tape is fine) so that it is obvious to all that the dishes should not be opened. Demonstrator to keep an eye on all plates.
  1. Wash any small cuts with soapy running water. Compress and elevate large cuts. Call a first aider.
  2. Switch off electric equipment in event of an electric accident, call a first aider, clear area. In the event of a burn, hold area under cold water for at least ten minutes. Call a first aider.
  3. Demonstrator must regain the petri dish/vial and diffuse the situation.

Protein preciptation demo:

DESCRIPTION Mix milk (contains proteins) and vinegar (weak acid) together in plastic container, causing milk proteins to precipitate
  1. Risk of spillage, causing trip hazard.
  2. Risk of spillage on electrical items, causing risk of electrical shock.
  3. Risk of splashing vinegar (an irritant) in eyes, particularly of children.
  1. Use bottles that can be closed when not in use, placed away from edges of surfaces.
  2. Keep liquids away from electrical items.
  3. Vinegar to be poured by adult, eyewash on hand in case of incident.
  1. In the event of spillage, should be wiped clean as soon as possible (cloths to be stored nearby). Call First Aider in event of injury.
  2. Electrical items to be turned off at mains in event of nearby spillage. Call First Aider in event of injury.
  3. Use eyewash to wash out of eyes if trained and confident to do so, and call First Aider.

Oil and pyrex demo:


Putting a pyrex bowl into vegetable oil. Since they both have the same refractive index the pyrex bowl will disappear.


  1. The oil is very slippery, and could cause a nasty slip hazard.
  2. If the pyrex bowls broke they would be sharp.


  1. Be very careful to minimise the chance of this happening, probably use a towel on top of a plastic sheet to contain any small spills if it is done inside. Mop up any spills carefully.
  2. If the Pyrex does break, dispose of it carefully. Be especially careful of any pieces that are in the oil as they will be INVISIBLE, so it is better to pour the oil out rather than fishing for them.


Call first aider in case of injury.

Custard fireballs demo:


Custard fireballs – blowing custard powder through a blow torch


  1. The resulting flame: demonstrator- burns, audience - minor burns
  2. The blow torch: minor burns
  3. Custard powder in air: minor risk of triggering asthma in the demonstrator


  1. Keep anything flammable away from above the experiment. Blow the powder upwards through the flame, not in any less controlled direction.
  2. Arrange blow torch so it is unlikely to damage presenter. Be aware torch will be hot after it goes out.
  3. There should be minimal powder getting to the audience, due to control measures relating to flame. Most of the powder as dust will be ignited anyway. Consider warning audience at start of lecture that there will be smoke and dust.


Call first aider in case of injury.

This experiment contains mains electrical parts, see separate risk assessment.