DNA, the Code of Life, Grade 12: Structure, Replication and the Banana Extraction

You can pull DNA out of a banana with washing-up liquid, salt, a coffee filter and a bottle of surgical spirit. It is a formally assessed practical and it is worth six marks.

Six. It is the smallest assessed practical in Grade 12 Life Sciences, which is exactly why it gets skipped, and it is the one every learner remembers for the rest of their lives.

This page gives the extraction with the three things that make it fail, DNA structure and replication, transcription and translation, and two places where the printed explanation attaches a word to the wrong thing.

Where this fits in the curriculum

Subject Life Sciences
Grade 12
Term 1
Topic Topic 1, DNA, the code of life
Status Carries a formally assessed practical task, the banana DNA extraction
Marks 6 for the assessed extraction, 8 for the base-pairing activity, 6 for the DNA and RNA comparison. Our worksheet is 70

The practical: extract DNA from a banana, 6 marks

Everything on this list is either a kitchen item or something you already own.

Item What it does
Half a banana, mashed The source. Soft fruit breaks down without a blender
Washing-up liquid Breaks open the cell and nuclear membranes, which are made of lipid
Table salt Makes the DNA clump together so you can see it
Meat tenderiser, or fresh pineapple juice Enzymes that strip the proteins off the DNA. Pineapple works because it contains bromelain
Filter funnel and filter paper Separates the pulp from the liquid. A coffee filter also works
Test tube or a tall glass You need depth to see the DNA rise
Ice-cold ethanol or surgical spirit The step the whole practical turns on. DNA will not come out of solution in warm alcohol
A toothpick or glass rod To lift the DNA out once it appears

Method

  1. Mash half a banana with about 100 ml of water until there are no lumps
  2. Add a teaspoon of salt and a teaspoon of washing-up liquid. Stir gently for several minutes. Do not whisk it into a foam, which is the commonest mistake
  3. Add the meat tenderiser or a spoon of fresh pineapple juice and leave it for five minutes
  4. Filter the mixture into a clean test tube. You want a clear-ish liquid, not pulp
  5. Tilt the tube and pour ice-cold alcohol slowly down the side so it forms a layer on top. Do not mix the two
  6. Watch the boundary between the two layers. White stringy threads appear within a minute and rise into the alcohol
  7. Lift the threads out on a toothpick

The three things that make it fail

What happens What caused it
Nothing appears at the boundary The alcohol was not cold enough. It must come straight from a freezer, or at least an hour in a fridge. This is the single commonest failure
A thick foam fills the tube The washing-up liquid was stirred too hard. Stir slowly, and give it time rather than energy
The filtrate is thick pulp Not enough water, or the filter was overloaded. Filter in two batches
Threads appear then vanish The layers were mixed. Pour the alcohol down the side of a tilted tube

Why bananas. A banana is soft enough to mash without equipment and it is triploid, with three sets of chromosomes rather than two, so there is more DNA per cell than in most things you could use.

What the learner has to explain afterwards

The six marks are for the explanation, not for the extraction. Every step maps to a reason, and the marks go to learners who can say which.

Step The reason
Washing-up liquid Dissolves the lipid membranes of the cell and the nucleus, releasing the contents
Salt Neutralises the charge on the DNA so the strands stick together instead of repelling
Meat tenderiser or pineapple Protease enzymes digest the histone proteins that DNA is wound around
Filtering Removes cell walls and pulp, leaving the dissolved contents
Ice-cold alcohol DNA is insoluble in alcohol, so it comes out of solution and becomes visible. Cold makes it less soluble still

What DNA is, and where it is

Where most of it is In the nucleus, wound tightly around proteins called histones to form chromosomes
A gene A section of DNA along a chromosome that carries the code for a characteristic
Extranuclear DNA DNA outside the nucleus: in the mitochondria of plant and animal cells, and the chloroplasts of plant cells
Mitochondrial DNA Passed on only by the mother. Everyone gets their mtDNA from their mother, who got hers from her mother, and so on back

That is why mitochondrial DNA is used to trace ancestry over very long periods. It travels down one line only, so it is not shuffled by meiosis every generation the way nuclear DNA is.

Replication, and why it has to be exact

DNA replication is semi-conservative: each new double helix is one original strand and one new strand.

Step What happens
1 The double helix unwinds and the two strands separate
2 Free nucleotides in the nucleus pair with the exposed bases: A with T, C with G
3 Enzymes join the new nucleotides into a continuous strand
4 Two identical double helices, each with one old strand and one new one

Replication happens during interphase, before division begins. In a cell cycle of about 24 hours, interphase is about 22 of them, and the whole of mitosis plus cytokinesis takes roughly two.

A word attached to the wrong thing, twice

What is printed What is correct
New strands are built "from free molecules of deoxyribose, phosphate and nitrogen bases that are present in the cell" DNA is assembled from free nucleotides, which are already a sugar, a phosphate and a base joined together. The cell does not stock loose sugars, phosphates and bases and bolt them on one at a time at the growing strand
In transcription, the new nucleotides "form pairs with the free nucleotides on the DNA strand" The word "free" is on the wrong side. The free nucleotides are the incoming RNA ones. The nucleotides on the DNA strand are not free: they are bonded into the backbone, which is the entire reason the strand can act as a template

Neither error changes the answer to an exam question, and both will confuse a learner who is trying to picture the process. The fix in each case is one word.

From DNA to protein

Stage Where What happens
Transcription In the nucleus A section of DNA unwinds, mRNA is built against one strand, and the mRNA leaves for the ribosomes
Translation At the ribosome The mRNA code is read three bases at a time, and tRNA brings the matching amino acid for each triplet

In RNA, uracil replaces thymine. So where DNA would pair A with T, the mRNA carries A with U. That substitution is the single most asked detail in this topic.

DNA RNA
Strands Two, a double helix One
Sugar Deoxyribose Ribose
Bases A, T, C, G A, U, C, G
Where Mostly the nucleus Nucleus and cytoplasm

Safety

  • Surgical spirit and ethanol are flammable. No flames anywhere in the room, and no hotplates
  • Nobody tastes anything, including the banana. Once it has had washing-up liquid in it, it is laboratory material
  • Meat tenderiser is an irritant as a powder. Do not let learners breathe the dust. Fresh pineapple juice avoids the problem entirely
  • Wash hands afterwards, and wipe the bench down

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

  • Learner worksheet, 70 marks, covering DNA structure and location, replication, the DNA and RNA comparison, transcription and translation, and the full extraction with its explanations
  • Marking memorandum, with the reason behind every step of the extraction and the five answers that look right and score nothing

What you need to buy

Almost nothing, and we would rather say so. The extraction is a kitchen practical: banana, salt, washing-up liquid, surgical spirit from a pharmacy, and a coffee filter.

Two things make it work better in a laboratory. A borosilicate filter funnel at R30 filters faster and cleaner than a coffee filter in a mug, and borosilicate test tubes at R5 each give you the depth you need to see the layer form. A class set of both costs less than R200.

Everything else for this topic is in laboratory glassware.

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