Nucleic Acids: Information Storage

Every nucleic acid is a polymer built from monomers called nucleotides. Think of a nucleotide as a single 'lego brick' in the tower of life. Each brick consists of three parts: a pentose sugar (5-carbon), a phosphate group, and a nitrogenous base. In DNA, the sugar is deoxyribose ($C_5H_{10}O_4$), while in RNA, it is ribose ($C_5H_{10}O_5$). These nucleotides link together via phosphodiester bonds, forming a 'sugar-phosphate backbone.' This backbone is directional, running from the $5'$ end (phosphate side) to the $3'$ end (hydroxyl side). This directionality is crucial for how cells read genetic 'recipes' during protein synthesis.

While both are nucleic acids, DNA and RNA have distinct roles and structures. DNA (Deoxyribonucleic Acid) is the long-term storage vault. It is double-stranded, forming an antiparallel double helix where one strand runs $5' \rightarrow 3'$ and the other $3' \rightarrow 5'$. It uses the bases Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). Conversely, RNA (Ribonucleic Acid) is the short-term messenger. It is usually single-stranded and replaces Thymine with Uracil (U). Because RNA has an extra oxygen atom on its ribose sugar, it is more chemically reactive and less stable than DNA, making it perfect for temporary tasks.

How does a string of chemicals turn into a human? The secret lies in the sequence. The four nitrogenous bases act like a four-letter alphabet. In the same way that the letters 'A-C-T' mean something different than 'C-A-T', the order of bases in a gene determines the order of amino acids in a protein. This is the Central Dogma of biology: DNA is transcribed into RNA, which is then translated into proteins. Because the DNA double helix is held together by relatively weak hydrogen bonds, the strands can be 'unzipped' easily by enzymes to be read or copied without breaking the strong covalent backbone.