Deoxyribonucleic acid, commonly known as DNA, holds the genetic instructions that guide the development, functioning, growth, and reproduction of all known living organisms and many viruses. Often described as the blueprint of life, this information is encoded within long, intricate structures called DNA strands.
Components of a Single DNA Strand
A single DNA strand is a polymer, meaning it is a large molecule made up of repeating smaller units called nucleotides. Each nucleotide consists of three distinct parts: a phosphate group, a five-carbon sugar known as deoxyribose, and a nitrogen-containing base. There are four nitrogenous bases found in DNA: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
These nucleotides link together to form a long chain. The phosphate group of one nucleotide forms a covalent bond with the deoxyribose sugar of the next, creating a continuous sugar-phosphate backbone. This arrangement can be visualized like beads on a string, where the sugar and phosphate form the repeating string, and each nitrogenous base extends from it. The order of these bases along the strand carries the genetic information.
The Double Helix Arrangement
DNA’s structure involves two single strands coiled around each other to form a double helix, resembling a twisted ladder. This arrangement is established by interactions between the nitrogenous bases on opposing strands. Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G). This precise matching is known as complementary base pairing.
These base pairs are held together across the ladder’s rungs by weak chemical attractions called hydrogen bonds. A-T pairs form two hydrogen bonds, while C-G pairs form three, contributing to the stability of the double helix. The sugar-phosphate backbones form the sides of this twisted ladder, with the paired bases forming the internal rungs. A defining feature of this structure is its antiparallel nature, meaning the two strands run in opposite directions. One strand is oriented from its 5′ (five-prime) end to its 3′ (three-prime) end, while the complementary strand runs from its 3′ end to its 5′ end.
The Role of Strands in DNA Replication
During cell division, the entire DNA molecule must be accurately copied so that each new cell receives a complete set of genetic instructions. This process, called DNA replication, begins when the double helix unwinds and the two strands separate, much like unzipping a zipper. An enzyme called helicase facilitates this unwinding by breaking the hydrogen bonds holding the base pairs together.
Once separated, each individual strand serves as a template for the synthesis of a new, complementary strand. Free nucleotides align with their complementary bases on the exposed template strands (A with T, C with G). Another enzyme, DNA polymerase, then moves along the template, adding these new nucleotides and forming covalent bonds to create the new DNA backbone. This results in two identical DNA double helices, each containing one original strand and one newly synthesized strand, a process termed semi-conservative replication.
Template and Coding Strands in Gene Expression
Beyond replication, DNA strands also play distinct roles in gene expression, the process by which genetic information is used to create proteins. When a specific gene needs to be “read” to produce a protein, only one of the two DNA strands acts as a blueprint. This strand is known as the template strand, also referred to as the antisense or non-coding strand.
An enzyme called RNA polymerase binds to the template strand and uses its sequence to synthesize a messenger RNA (mRNA) molecule. The mRNA sequence is complementary to the template strand, with one difference: where the template strand has Adenine, the mRNA will have Uracil (U) instead of Thymine (T). The other DNA strand, not directly used by RNA polymerase, is called the coding strand or sense strand. Its sequence is nearly identical to the resulting mRNA molecule, differing only in that it contains Thymine where mRNA has Uracil.