DNA

The molecule is a "polynucleotide," a long-chain polymer made of repeating units called nucleotides. Each unit consists of a phosphate group, a deoxyribose sugar, and one of four nitrogenous bases. These units are linked by covalent phosphodiester bonds, forming an alternating sugar-phosphate "backbone." Crucially, the two strands run in opposite directions—one 5' to 3' and the other 3' to 5'—a configuration known as "antiparallel" that is essential for how the molecule is read and replicated.

The resulting double helix isn't a smooth cylinder; it features two distinct "grooves" known as the major and minor grooves. Because the strands are not symmetrical, the major groove is wider (2.2 nm) than the minor groove (1.2 nm). This physical gap is vital for life: the major groove provides enough space for transcription factors and other proteins to "read" the chemical signatures of the bases inside without unzipping the entire molecule.

The actual "data" of life is stored in the sequence of four bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). These are divided into two shapes: double-ringed purines (A and G) and single-ringed pyrimidines (C and T). According to strict base-pairing rules, A always bonds with T, and C always bonds with G. This complementarity is the magic of DNA—it means each strand contains a perfect "negative" of the other, allowing information to be copied with near-perfect fidelity during cell division.

To turn this code into a living organism, the cell uses a two-step translation process. First, DNA is transcribed into RNA, a similar but more transient molecule. In this process, Thymine is replaced by Uracil (U). These RNA "messages" are then translated into specific sequences of amino acids to build proteins. While the backbone is the physical frame, it is the linear order of these four letters that dictates every biological trait, from the color of a flower to the complexity of the human brain.

The scale of DNA is staggering. In a single human cell, the diploid genome contains over 6 billion base pairs. If you were to straighten out the DNA from just one cell, it would be approximately two meters long. To fit this into a microscopic nucleus, DNA is wrapped around proteins called histones to form chromatin, which then folds into the dense structures we know as chromosomes.

Not all DNA lives in the same place. In eukaryotes (like humans, plants, and fungi), the majority of DNA is protected inside the cell nucleus, but small "satellite" genomes exist in the mitochondria and chloroplasts. Prokaryotes (like bacteria), however, lack a nucleus entirely and store their DNA in a circular chromosome within the cytoplasm. This difference in storage highlights two distinct evolutionary paths: one focused on massive, protected complexity and the other on streamlined efficiency.

The two strands of the double helix are held together by hydrogen bonds, which are significantly weaker than the covalent bonds forming the backbone. This is a functional feature, not a flaw: it allows the DNA to "zip" and "unzip" like a coat fastener. The strength of this bond depends on the specific sequence; C-G pairs have three hydrogen bonds, while A-T pairs only have two. Consequently, DNA with high "GC-content" is more stable and harder to "melt" into single strands.

Beyond simple zippering, DNA is dynamic. It can undergo "supercoiling," where the molecule twists like a phone cord to become more compact or to regulate access to its code. Furthermore, DNA is not static after it is built; it can be modified by the addition of methyl groups to "non-canonical" bases. These modifications act as a molecular volume knob, turning genes up or down without changing the underlying code—a field of study known as epigenetics that explains how the same genome can produce hundreds of different cell types.