Science Puzzle
How Long Is the DNA in One of Your Cells?
Almost every cell in your body holds a complete copy of your DNA, the instructions for building and running you. The DNA sits in the nucleus, a blob only about 6 thousandths of a millimetre across, far too small to see without a microscope.
If you could pull out all the DNA from just one of your cells and stretch it out straight, how long would it be?
The Answer
About 2 metres, taller than most adults, packed into a nucleus a few thousandths of a millimetre across.
DNA is a long chain of chemical "letters". You have two copies of your genome in each cell, one from each parent, about 6 billion letters in all, and each letter adds about 0.34 millionths of a millimetre to the length. Multiply them and you get roughly 2 metres. It's possible only because the strand is fantastically thin: about 2 millionths of a millimetre wide.
To fit, the DNA is packed in stages. It's wound round tiny protein spools, a little under twice round each, like thread on bobbins, which already shortens it about seven times. The string of spools is coiled, the coils are looped and the loops are folded again, until each chromosome is a compact bundle. The packing isn't just storage: the cell loosens the parts it needs to read and keeps the rest tightly wound.
Every time a cell divides, it has to copy all 2 metres, sort the two copies apart and repack them without tangling. You have tens of trillions of cells, so the DNA in your whole body, laid end to end, would stretch billions of kilometres.
The principle: Packaging DNA. Each cell's 2 metres of DNA is wound round protein spools and folded again and again, so it fits into a nucleus a few thousandths of a millimetre wide.