Anyone willing to help my daughter with DNA science project?

My daughter is in grade 10 and just successfully extracted DNA from fruit using the technique described here:

However the above link doesn’t provide a lot of context to answer questions about why this works, why some fruit had more DNA than others, what DNA is other than “stringy stuff”, etc

I told here I knew a few biologists who might be able to help out. If it’s ok with the @moderators, I’m wondering if I some of you might be willing to help answer some of her questions, or point her to some material that would help out (keeping in mind she won’t take an actual biology course until next year). I’d post her questions to this discussion, and relay the answers to her.


Perhaps post her questions here?


Her first question is why the dish washing detergent works to “let out” the DNA, and why it clumps on the salt?

My first question is why different fruits would have different amounts of DNA?


The purpose of dish washing detergent is to break down bits of food, right? It breaks down cell membranes in a similar way - getting in between the lipid molecules in cell membranes and breaking them apart. The salt “gathers up” the DNA because it’s positively charged, while DNA is negatively charged. The salt acts like a magnet.

Some fruits contains more or less flesh, meaning more or fewer actual cells to extract DNA from. Strawberries are nice and fleshy compared to say, a juicy orange. Also, each individual strawberry cell contains quite a lot of DNA, because strawberry plants are “octoploid” - they basically have 8 copies of their genome in each cell. Humans, by comparison, are “diploid” - we have just 2 copies.


Because different plant species have different amounts of DNA per genome. DNA content varies greatly in an apparently random way throughout eukaryotes. Check out this plant genome size database. And a quick web search finds the largest plant genome to be Paris japonica with 149 billion bases (apparently this is the haploid size), while the smallest angiosperm genome, which may be the smallest plant genome, is Genlisea tuberosa with only 61 million (note: that’s million, not billion) bases. That’s a difference in size of over 2000x. (For comparison, the haploid human genome is 3 billion bases.)

Now, why? Apparently, for no reason. Most of the variation is in the quantity of junk DNA. Junk DNA may be mildly deleterious, and there’s some evidence that the large population sizes are correlated with less junk, hence smaller genomes. But that’s the only sense anyone has found in it.


I actually have my non-majors students do a strawberry DNA extraction. @evograd hit the nail right on the head with these answers.


Oh, that reminded me… @cdods, did your daughter use an ice-cold alcohol precipitation step?


Thanks everyone. I’ll get her to read the responses and follow up with other questions. It will likely go in fits and starts.


This is a fascinating thread, @cdods. I had no idea that extracting DNA could involve such a simple process. Peaceful Science is a great venue for all sorts of questions.


Here is a photo I took in the Fall of a really good look at some precipitated DNA. Strawberry mush on the bottom, alcohol on top, precipitated DNA in the middle :slight_smile:

I found a good, simple protocol a while back for the procedure, in case you need one (here).


A fun additional step is to “spool” the DNA. The method in the opening post uses a thin bamboo stick to grab the DNA which would definitely work. Once you pull it out of the solution you can pour some more alcohol over it to remove salts, put it in a small tube, let it air dry to remove the alcohol, then redissolve it in some clean water (Brita filtered water would probably work well). It may take some time to go back into water with some very gentle mixing. In the lab, I often let genomic DNA sit in the fridge overnight. At least for me, it is fun to experience the DNA in all three “forms”: goopy DNA after cell lysis, precipitated DNA, and dissolved DNA. You can always re-extract the DNA after it is dissolved in water.


That’s a good idea, I’ll bet bamboo skewers would work pretty well. I’ve had students use glass rods before, but with limited success.


Well, that’s why DNA is called “the snot of life”.


This could also be a valuable teaching moment. If you can get some cheap 1 ml TB needles at a medical supply store, or off of Amazon, you can draw the goop up and push it back through the needle multiple times, and the goop will slowly disappear. What you are doing is physically shredding the long DNA molecules into small chunks.

I taught at an elementary summer science camp last year and I used a similar method to extract DNA from saliva with 75 kids from ages 6-12. You end up with a lot of protein mixed in there but you can still see the DNA.

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I take a thin glass pipet and melt it into a hook. It works well for looping out DNA.


What a great photo, @cwhenderson! It leads me to wonder:

(1) Do we know who was the first scientist to do some sort of extraction protocol like this? When was that discovery?

(2) What did that scientist think he/she had found? (I’m no biologist but if I had to take a wild guess after seeing it for the first time, I suppose I would think it was tangled peptide chains of some sort. ??)

The fact that DNA copying mechanisms are able to do their work in tiny nuclei containing such octoploid spirals of genetic material amazes me.

I didn’t know that strawberry plants are octoploid. Having come from a farming background, the importance of exploiting polyploidy in developing new crop varieties has always fascinated me. Every time some anti-evolution activist repeats the lame claim that “mutations never lead to increases in information”, polyploidy! is always the first word out of my mouth.

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Friedrich Meischer is credited with first isolating DNA in 1869, which he called “nuclein”, but I don’t think he used the same precipitation procedures used today. I honestly don’t know who first started using ice-cold ethanol for precipitating DNA.

Meischer was working with cellular extracts, and the first assumption would have been a protein mass. However, his extraction techniques suggested to him that it was a novel component in cell nuclei. Subsequent biochemical analysis showed it contained large amounts of phosphorus and no sulfur, also indicating to him that he was working with something new.

Also interesting - Meischer first suspected (in the late 1800s) that the nuclein was involved in heredity, but was convinced otherwise. It took decades of work (Avery/MacLeod/McCarty, Hershey/Chase, and others) in the early to mid 1900s before convincing work was done showing that his initial hunch (about 80 years previously) was correct. Hershey and Chase didn’t publish their work until 1952!

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Wait, what? Salt is neutral. Do you mean the DNA is attracted to the chloride ions?

I was actually simplifying quite a lot. The positively charged sodium ions neutralise the charges on the DNA backbone which stops the DNA from being soluble, meaning it precipitates and clumps together. Perhaps this isn’t best described by the sodium (or salt) being a “magnet”.