Deoxyribonucleic acid (DNA), less commonly referred to as deoxyribose nucleic acid, was discovered in the 1800s by a Swiss biochemist named Miescher. In 1953, James Watson, Francis Crick, and Rosalind Franklin used breakthrough imaging techniques, specifically X-ray crystallography, to determine that DNA is double-stranded and, famously, helical in structure. Now, almost 60 years later, we understand the macromolecule’s structure, even though its secrets are far from being known.
The Chemical Structure of DNA
As the name implies, DNA is a nucleic acid, a macromolecule made of monomeric nucleotides (nitrogen bases) in a chain with a sugar phosphate backbone. The sugar in DNA is deoxyribose. The deoxyribose rings and phosphoric acid of the backbone are connected by covalent bonds and alternate. DNA consists of two chains, or strands, bound in parallel to one another. Each strand is oriented opposite to the other according to the location of the oxygen and hydroxyl groups on phosphoric acid, called the five prime (5′) and three prime (3′) ends. The nitrogen bases bind to the sugar, in this case deoxyribose, and are known as nucleotides in this configuration.
The Physical Structure of DNA
The size of DNA is on the nanometer scale (approximately 2.2-2.6 nm wide), but it is usually measured in Angstroms (approximately 22 Angstroms wide). The nucleotides project into the center of the molecule, binding to complementary bases on the other strand via hydrogen bonds, and giving the molecule its ladder-like physical appearance. There are four DNA bases, called nucleotides when they are attached to deoxyribose: adenine (A), guanine (G), cytosine (C), and thymine (T). These four include two purines (C and T) and two pyrimidines (A and G). Adenine and thymine bind to one another, and cytosine and guanine bind to one another. Knowing the sequence of one strand automatically reveals the sequence of the other strand due to complementarity.
The physical helical structure of DNA is formed because of the chemical properties of the two parallel, though opposite and complementary, DNA strands as they interact. The DNA molecules present within a cell vary in length and quantity depending on the species it represents.The strands are replicated (copied) and passed on during cell division to serve as the instructions for cellular functions.
The structure of DNA has been described as a twisting ladder, with each rung being made of a nucleotide pair. This structure leaves grooves where enzymes and proteins can bind the DNA molecule. There are two types of grooves: minor (small) and major (large). The direction that the helix twists is right-handed in humans and most organisms, known as B-DNA. Other variations include Z-DNA and A-DNA, each slightly different in the chemical configuration of the backbone, either in the direction of the twist or the size of the grooves. Some chemical modifications of B-DNA are thought to produce these configurations, but the natural context is not widely known.
The Purpose and Effect of the Structure
The purpose of DNA is to carry the information required for an organism to function, often called the “blueprint of life”. Sequences of DNA, called genes, ultimately encode proteins that carry out these functions. The four nucleotide sequence is a simple, yet complex, method of coding this information. The seemingly random occurrence of A, T, G, and C is transferred to a similar complementary sequence of single-stranded nucleic acid, specifically RNA, which is read in triplets to string together amino acids, the building blocks of proteins.
The two-stranded structure of DNA helps retain the fidelity of the information contained within its nucleotide sequence by retaining an original copy, though complementary, to compare to newly replicated strands. This allows for the repair of DNA damage and correction of polymerase errors, which can lead to diseases, such as cancer, if not corrected.
The structure is also ideal for allowing enzymes to act on the DNA to influence its functions, which is required for replication and transcription. The non-covalent bonds between nucleotides allow for relatively easy separation of the strands for replication and gene expression. Exceptions to this are DNA methylation and supercoiling that result in the DNA becoming more compact and easily stored in the cell nucleus, which is also a benefit of the helical structure. This compact formation is usually not seen when cells are dividing (e.g., embryonic growth) or when genes present in that portion of DNA are turned on (i.e. actively being transcribed).