Molecular Biology - Structure of DNA and RNA

Nucleotides are the monomers of nucleic acids, consisting of a pentose sugar, a phosphate group ($PO_{4}^{3-}$), and a nitrogenous base.

In DNA, the pentose sugar is deoxyribose ($C_{5}H_{10}O_{4}$), whereas in RNA, it is ribose ($C_{5}H_{10}O_{5}$), which has a hydroxyl group ($-OH$) on the $2'$ carbon.

DNA is double-stranded and forms a double helix, while RNA is typically single-stranded. The two strands of DNA are antiparallel, running in opposite $5' \rightarrow 3'$ and $3' \rightarrow 5'$ directions.

Nitrogenous bases are categorized into Purines (Adenine $A$ and Guanine $G$) and Pyrimidines (Cytosine $C$, Thymine $T$, and Uracil $U$). In DNA, $A$ pairs with $T$ and $G$ pairs with $C$. In RNA, $U$ replaces $T$.

Nucleotides are linked covalently by phosphodiester bonds between the $3'$ hydroxyl group of one sugar and the $5'$ phosphate group of the next, creating a sugar-phosphate backbone.

Hydrogen bonding facilitates complementary base pairing: $A$ and $T$ (or $U$) form $2$ hydrogen bonds, while $G$ and $C$ form $3$ hydrogen bonds.