Nucleic Acids and Genetic Information

Every strand of DNA and RNA is a polymer made of monomers called nucleotides. Each nucleotide consists of three distinct parts: a phosphate group, a pentose sugar (a 5-carbon sugar), and a nitrogenous base. The phosphate group is attached to the $5'$ carbon of the sugar, while the nitrogenous base is attached to the $1'$ carbon. These units link together via phosphodiester bonds, forming a 'backbone' that provides structural stability. In the cellular environment, the phosphate groups are deprotonated, giving nucleic acids a net negative charge, which is crucial for their interaction with proteins like histones.

While DNA and RNA look similar, their chemical differences are profound. DNA (Deoxyribonucleic Acid) uses the sugar deoxyribose, which lacks an oxygen atom on the $2'$ carbon. RNA (Ribonucleic Acid) uses ribose, which has a hydroxyl group ($-OH$) at the $2'$ position. This single oxygen makes RNA much more chemically reactive and less stable than DNA. Furthermore, while both use Adenine (A), Guanine (G), and Cytosine (C), DNA uses Thymine (T), whereas RNA replaces it with Uracil (U). DNA is typically double-stranded for long-term storage, while RNA is usually single-stranded for short-term signaling and protein synthesis.

The 'rungs' of the DNA ladder are formed by complementary base pairing. This is governed by the size of the bases and their ability to form hydrogen bonds. Bases are divided into Purines (Adenine and Guanine), which have two rings, and Pyrimidines (Cytosine, Thymine, and Uracil), which have one ring. To keep the double helix width constant, a purine must always pair with a pyrimidine. Specifically, Adenine forms two hydrogen bonds with Thymine ($A=T$), while Guanine forms three hydrogen bonds with Cytosine ($G \equiv C$). This 'three-bond' connection makes $G-C$ pairs stronger and harder to separate than $A-T$ pairs.