Why are GeneSavvy employees so sweet? Because the backbone of all 3 billion nucleotides in our genomes is mostly made up of sugar! Nucleotides are the basic building blocks of our DNA and RNA. Both DNA and RNA consist of a sugar molecule attached to a phosphate group and a nitrogen-containing base. The primary difference between the structure of DNA and RNA is that DNA contains a deoxyribose sugar molecule and RNA contains a ribose sugar molecule. These sugar molecules only differ in that deoxyribose lacks a hydroxyl group (an oxygen and hydrogen molecule that are linked together) that ribose contains.
So why is DNA called DNA? Well, pioneer genetic researchers were really creative with their naming system:
- De-oxy-ribose = Ribose without a hydroxyl group
- DNA = Deoxyribose Nucleic Acid.
These deoxyribose sugar molecules are linked to each other in long chains by their associated phosphate groups, creating the “backbone” of the DNA structure. Attached to each deoxyribose sugar molecule in the DNA backbone is one of four nitrogen-containing bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). These four bases can be further subdivided into groups of the purine bases (A & G) and the pyrimidine bases (C & T). The important difference between these two groups is that purine bases (A & G) have a 2-ring structure, while pyrimidine bases (C & T) have a 1-ring structure.
Why is this important? Well 2-rings take up more room than 1-ring, so it helps us make sense of why the bases like to “pair” the way they do. Specifically, Adenine (A), a purine base, always pairs with Thymine (T), a pyrimidine base, while Guanine (G), a purine base, always pairs with Cytosine (C), a pyrimidine base. This pairing structure keeps the distance between the two backbones of phosphate-linked deoxyribose sugar molecules roughly equal. So how does a long line of linked sugars create that double helix structure we all now know and love from our emoji keyboard?
It comes down to love and hate…of water. These nitrogenous bases are hydrophobic, meanin they like to avoid as much contact with water-based substances as they can. Unfortunately for them, DNA is located in our cells, and our cells are 70% water! The good news is the sugar-phosphate backbone of DNA is water-loving (hydrophilic)! So, DNA is arranged such that the sugar phosphate backbone is oriented outwards towards the water-based content of our cells, shielding the nitrogenous bases on the inside. The DNA then twists to further eliminate space where that water could seep in to touch the hydrophobic nitrogen-containing bases, creating the double helix structure!