Nucleic acids are fundamental biological macromolecules that play a central part in the storage and expression of genetic information. These large polymers are constructed from smaller, repeating units called nucleotides, each composed of a phosphate group, a five-carbon sugar, and a nitrogenous base. The two primary categories of these molecules, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), function together in a complex, highly regulated system that governs cellular life.
The Role of DNA in Genetic Storage
Deoxyribonucleic acid (DNA) functions as the long-term repository for the hereditary instructions of an organism. Its structure, known as the double helix, consists of two long strands of nucleotides coiled around a central axis. This stability is attributed, in part, to the deoxyribose sugar in its backbone, which is less reactive than the ribose sugar found in its counterpart, RNA.
The two strands of the DNA helix are held together by hydrogen bonds that form between specific nitrogenous bases: adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). This complementary base pairing ensures that the genetic code can be copied precisely before a cell divides, a process called replication. The long polymer is tightly coiled and packaged around proteins into structures called chromosomes, allowing the vast amount of information to fit within the confines of the cell nucleus.
The precise sequence of these base pairs contains the instructions, or “genes,” necessary for the construction and maintenance of the entire organism. Because the two strands run in opposite directions, referred to as antiparallel, the entire structure is robust and can be reliably passed down from parent cell to daughter cell.
Carrying and Translating Genetic Messages
Ribonucleic acid (RNA) molecules are responsible for transforming the instructions stored in DNA into functional proteins, a process known as gene expression. Unlike DNA, RNA is a single-stranded molecule and contains the sugar ribose instead of deoxyribose, along with the base uracil (U) in place of thymine (T). This structural difference makes RNA less stable and more temporary.
The expression process begins with messenger RNA (mRNA), which is transcribed from a segment of the DNA template and carries the genetic code out of the nucleus to the protein-building machinery. This mRNA molecule is read in sequential blocks of three nucleotides, called codons, each of which specifies a particular amino acid. The length and sequence of the mRNA ultimately determine the amino acid sequence of the resulting protein.
Translation of the mRNA code occurs on the ribosome. Ribosomal RNA (rRNA) provides the structural framework for the ribosome and harbors the active sites where the polymerization of amino acids takes place.
Transfer RNA (tRNA) molecules act as the decoder, translating the nucleotide sequence of the mRNA into a chain of amino acids. Each tRNA is folded into a compact structure and carries a specific amino acid at one end. At the other end, it possesses a three-nucleotide sequence, the anticodon, which pairs precisely with a complementary codon on the mRNA strand. The tRNA delivers its amino acid cargo to the ribosome, where the rRNA catalyzes the formation of a peptide bond, linking the new amino acid to the growing protein chain.
Specialized Roles in Gene Control
Beyond acting as direct messengers and structural components, nucleic acids regulate the flow of genetic information. A large portion of the genome is transcribed into non-coding RNA (ncRNA), molecules that do not provide instructions for making proteins but instead influence the expression of other genes. These regulatory RNAs ensure that the correct amount of protein is produced at the appropriate time in the cell’s life.
MicroRNA (miRNA) and small interfering RNA (siRNA) are examples of these small regulatory molecules. These short RNAs function in a mechanism known as RNA interference, which often results in the silencing of gene expression after transcription has occurred. They associate with enzyme complexes and use their sequence to locate target messenger RNA molecules.
Once bound to a target mRNA, the miRNA or siRNA can interfere with the translation process or promote the degradation of the mRNA molecule entirely. A single miRNA can regulate the expression of multiple different genes, acting as a fine-tuning switch for complex cellular pathways.
Nucleic Acids as Catalysts and Energy Movers
Nucleic acids also participate directly in chemical reactions within the cell. Certain RNA molecules, known as ribozymes, possess catalytic activity. The peptidyl transferase activity of the ribosome, which forms the peptide bonds that link amino acids together, is carried out by an rRNA component acting as a ribozyme.
Adenosine triphosphate (ATP) is a modified nucleotide that acts as the primary energy currency of the cell. ATP stores chemical energy in the bonds between its three phosphate groups, and when the terminal phosphate bond is broken, a burst of energy is released to power cellular activities.