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The ABCs of DNA — Part 1

drawn image of a double helix of DNA

DNA Double Helix: demonstrating 2 part ‘ladder rungs’ composed of paired DNA bases.


DNA, short for deoxyribonucleic acid, is the blueprint for everything that makes you – you! Every living creature on Earth — and some quasi-living creatures such as viruses and prions – have a DNA blueprint telling it how and when to make which proteins. Proteins are complex polymers that constitute the basic building material of a living creature. Like a blueprint for a building, DNA uses a universal code to transmit its instructions to the ‘construction crew’ in rest of the cell. All living creatures inherit this information from their parent (asexual reproduction) or parents (sexual reproduction.)


James D. Watson, Francis Crick, Maurice Wilkins and the often-forgotten Rosalind Franklin discovered that DNA forms a double helix, or twisted ladder, structure consisting of a pair of long, polymeric molecules. In the picture above, one molecule would be one side of the twisted ladder and half a rung. Hydrogen bonding holds the rungs together. Hydrogen bonding is a weak force when each rung is looked at individually — one that can easily be overcome allowing the ladder to split and giving direct access to the genetic code on one half of the DNA. When taken in whole, however, over the millions of ladder rungs, made up of paired bases, the cumulative strength of the bond is quite strong ensuring that your DNA doesn’t break apart too easily.

a drawing illustrating the chemical formation of a DNA molecule.

The 3 prime (or 3′) end is at the bottom left of the molecule while the 5 prime (5′) end is above it.

DNA Structure

Each molecule of the polymer pair that makes up the double helix ladder has a direction in which the code must be read starting at the 3 prime end of the molecule and running to the 5 prime end. This will be explained later in the series, for now, it’s enough to understand that the two molecules run in opposite directions. This orientation is called anti-parallel. Because the two molecules are anti-parallel and the strength of molecular attraction along the side of the ladder is not equal, the ladder twists clockwise around itself as the two sides try to get closer to each other by running around the invisible axle in the middle much like Sambo and the tigers.

Photo Credit: University of Southern California, San Francisco. Drawing is from Wikipedia

Further information on the discovery of the structure of DNA: James D. Watson, Francis Crick, Maurice Wilkins and Rosalind Franklin

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