DNA is at the heart of evolution and is the code of life. What do we know about this molecule? How long is DNA? If all the DNA in our body was laid end to end, how far would it reach? What percentage of the DNA in our cells encodes proteins?
Student Learning Goals:
- Students will observe that DNA is very long , very skinny, and packs well into cells.
- Students will learn that only a small fraction of the DNA in our cells encodes the approximately 23,000 genes.
- Students are invited to discuss the usefulness and limitations of this model.
- Students will consider the role of “Dark DNA” in evolution.
The human genome contains about 3 billion nucleotides, about the same number of nucleotides as mice, apes and most other mammals. If you were to unravel the DNA packed into all 23 pairs of chromosomes in just one of the of your cells, how long would that stretch of DNA be? It would be 2 meters long! Two meters multiplied times 10 trillion cells in your body is a length of DNA to go to the sun and back almost 70 times!
In humans, DNA is packed into 23 pairs of chromosomes. In this activity, we ignore the 23 chromosomes, and pretend the DNA is a single, 2 meter (6 foot) long molecule.
Materials Per student:
- 2 meters black yarn
- 2 cm length of sticky-backed labels (in blue to represent “blue genes”)
- (optional: add 5 cm red sticky backed labels to represent “red regulators”)
Tell students to cut the blue labels into 23,000 pieces to represent the 23,000 genes in our genome. SMILE :-). If their scissors are not sharp enough to do this, they may cut the labels into about a dozen pieces to represent the relative percentage of DNA that encoding proteins. These are then stuck onto the string in a distributed fashion.
Optional: Have students add red sticky labels cut into small pieces representing regulators.
This string is mostly black, with a little red and a little blue. The blue region represent genes. Only about 1 percent of the human genome encodes gene. The red represents control genes or regulatory elements that regulate when genes are turned on or off.
Dark DNA and Evolution
The black is very interesting. It does not encode proteins nor regulate the DNA. It represents the so called “dark DNA” or what some used to call “junk” in our genes. We now know that much of this “Dark DNA” is made of “Jumping Genes,” that is, DNA that moves around. This moving DNA causes much of the variation that causes species to evolve over time.
All that extra DNA is like spare parts, just waiting for the cell to tinker with. This “tinkering” is where variety comes from. Darwin had no idea.
You now know more about the basis of biological variation than Darwin. He would be envious.
All Models are Wrong, Some Models are Useful
What are the limits of this model of DNA? What are useful aspects of this model?