Nucleic acids, which include deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are polymers. These compounds are fundamental to life, serving as the storage and expression system for genetic information within every cell. Like other large biological molecules, such as proteins and carbohydrates, nucleic acids are constructed from many smaller, repeating chemical subunits. Understanding the structure of these polymers begins with identifying the single molecular unit that links together to form the long chain.
Identifying the Building Block
The fundamental molecular unit that serves as the building block for nucleic acids is the nucleotide. Nucleotides are monomers, the single, repeating molecular components that assemble into the much larger nucleic acid polymers. The specific sequence and arrangement of these nucleotide monomers encode the entire genetic blueprint for an organism. Each nucleotide is joined end-to-end to create the long, linear strands of DNA and RNA, and this monomer unit determines the characteristics of the resulting nucleic acid.
Components of a Nucleotide
Every nucleotide is composed of three chemically distinct parts joined together: a five-carbon sugar, a phosphate group, and a nitrogenous base. The sugar component is a pentose, a molecule with five carbon atoms, which forms the structural core of the nucleotide. The specific sugar present differentiates the two main types of nucleic acid; DNA contains deoxyribose, while RNA contains ribose.
The difference between these two sugars lies in a single oxygen atom at the 2′ carbon position. Ribose has a hydroxyl (-OH) group, while deoxyribose has only a hydrogen atom, hence the name “deoxy,” meaning lacking oxygen. This distinction contributes to DNA’s greater stability, making it suitable for long-term genetic storage.
The phosphate group, derived from phosphoric acid, is attached to the 5′ carbon of the sugar molecule. This group gives nucleic acids their acidic nature and imparts a negative electrical charge to the entire polymer chain. The final component is the nitrogenous base, which is attached to the 1′ carbon of the sugar.
The nitrogenous bases are categorized into two classes based on their ring structure: purines and pyrimidines. Purines (Adenine (A) and Guanine (G)) have a double-ring structure. Pyrimidines (Cytosine (C), Thymine (T), and Uracil (U)) are smaller with a single ring. Thymine is found exclusively in DNA, while Uracil replaces it in RNA.
How Nucleotides Form Nucleic Acids
Nucleotides join together in a process called polymerization to create the long chains of a nucleic acid. This linkage occurs through a condensation reaction, where a molecule of water is removed to form a stable covalent bond between two adjacent monomers. The resulting covalent bond, known as a phosphodiester bond, links the nucleotides in a chain.
A phosphodiester bond forms between the phosphate group attached to the 5′ carbon of one nucleotide and the hydroxyl group (-OH) found on the 3′ carbon of the sugar of the next nucleotide. This repeating pattern of sugar and phosphate connections forms a strong, continuous structural element known as the sugar-phosphate backbone. The nitrogenous bases extend inward from this backbone like rungs on a ladder.
Because the nucleotides are always joined in this specific 5′-to-3′ orientation, the resulting nucleic acid strand has directionality, or polarity. One end of the chain is characterized by a free phosphate group attached to a 5′ carbon, known as the 5′ end. The opposite end has a free hydroxyl group on a 3′ carbon, which is called the 3′ end. This polarity is fundamental, as all processes that read or synthesize the genetic code, such as DNA replication and transcription, must proceed in the 5′ to 3′ direction.