Nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), carry the genetic instructions for all living organisms. These molecules provide the code that dictates an organism’s traits and directs the synthesis of proteins that perform cellular functions. This information is stored within their molecular structure, dictating everything from eye color to how cells operate.
The Building Blocks of Nucleic Acids
The units that make up nucleic acids are called nucleotides. Each nucleotide consists of three components: a phosphate group, a five-carbon sugar, and a nitrogenous base. The sugar molecule is central, with the base attached to one carbon atom and the phosphate group to another.
The type of sugar distinguishes DNA from RNA. In DNA, the sugar is deoxyribose, while in RNA, it is ribose. The only difference is at the second carbon of the sugar ring, where ribose has a hydroxyl (-OH) group and deoxyribose has only a hydrogen atom.
The nitrogenous base is where genetic information is stored. There are five main bases, categorized as purines or pyrimidines. The purines, adenine (A) and guanine (G), have a two-ring structure, while the pyrimidines, cytosine (C), thymine (T), and uracil (U), have a single-ring structure. DNA contains A, G, C, and T, while RNA uses uracil (U) instead of thymine.
Primary Structure and the Sugar-Phosphate Backbone
The primary structure of a nucleic acid is the specific sequence of its nucleotides, linked together into a long chain called a polynucleotide. The connection is a phosphodiester bond, which forms between the phosphate group of one nucleotide and the sugar of the next.
This repeated linking creates a continuous sugar-phosphate backbone, with the nitrogenous bases extending from it. The specific order of these bases along the backbone constitutes the genetic code.
The two ends of a single nucleic acid strand are distinct. One end terminates with a phosphate group on the 5′ carbon of the sugar, known as the 5′ end. The other end has a free hydroxyl group on the 3′ carbon of the sugar and is called the 3′ end.
The DNA Double Helix
DNA is structured as a double helix, where two polynucleotide chains coil around each other. The strands are connected but run in opposite directions, a property known as antiparallel, meaning the 5′ end of one strand aligns with the 3′ end of the other.
The sugar-phosphate backbones form the outer structure of the helix. The two strands are held together by hydrogen bonds between the nitrogenous bases on the interior. This pairing is specific: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).
An adenine-thymine pair has two hydrogen bonds, while a guanine-cytosine pair has three. This precise pairing allows for accurate replication, as the sequence of bases on one strand dictates the sequence on the other.
The double helix forms major and minor grooves that allow proteins to interact with the DNA. These proteins can read the base sequence within the grooves without unwinding the helix to access the genetic information.
RNA’s Diverse Structures
Unlike the stable double helix of DNA, RNA is a single-stranded molecule, which gives it greater structural flexibility. The RNA strand can fold back on itself to create localized regions of base pairing. This self-pairing allows RNA to form a variety of complex three-dimensional shapes.
This structural versatility enables the diverse functions of RNA. For instance, messenger RNA (mRNA) carries genetic information from DNA to the cell’s protein-making machinery. Transfer RNA (tRNA) has a specific structure to transport amino acids, and ribosomal RNA (rRNA) is a component of ribosomes.
The base pairing rules in RNA are similar to DNA’s, but adenine pairs with uracil (U) instead of thymine. These A-U and G-C pairs can form within the same strand, leading to structures like hairpin loops. These short double-stranded regions, interspersed with single-stranded loops, create the molecule’s overall shape.
Higher-Level Organization
A DNA molecule can be very long; the human genome, for instance, contains approximately 3 billion base pairs. To fit this genetic material inside a cell’s microscopic nucleus, the DNA must be highly compacted using specialized proteins.
The first level of organization involves proteins called histones. The DNA double helix wraps around a core of histone proteins to form a structure called a nucleosome. This “beads on a string” formation is the initial stage of DNA compaction.
These nucleosomes are then coiled and folded into a more compact structure called chromatin. During cell division, chromatin undergoes further condensation to form the highly condensed structures known as chromosomes. This packaging allows the DNA to fit within the cell and helps regulate which genes are active.