Nucleic acids are fundamental molecules in all known forms of life, playing roles in heredity and cellular function. Primarily deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), they store, transmit, and express genetic information within cells. They are polymers composed of repeating units called nucleotides, each containing a sugar, a phosphate group, and a nitrogenous base.
Blueprint of Life
Deoxyribonucleic acid (DNA) serves as the primary genetic material, holding the instructions for an organism’s development, functioning, and reproduction. It stores hereditary information in the sequence of its four nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). This information is organized into genes, segments of DNA that code for specific proteins or functional RNA molecules.
The DNA molecule typically forms a double helix, resembling a twisted ladder. This structure consists of two polynucleotide strands coiled around each other, with sugar-phosphate backbones forming the outside “rails” and nitrogenous bases pairing in the interior. Adenine always pairs with thymine (A-T), and guanine always pairs with cytosine (G-C) through hydrogen bonds, a principle known as complementary base pairing.
DNA’s double helix structure facilitates its replication, a process where the two strands separate, and each serves as a template to synthesize a new complementary strand. This ensures genetic information is accurately copied and passed from one generation to the next. The compact coiling of DNA around proteins, forming chromatin and chromosomes, allows the vast amount of genetic information to fit within the tiny nucleus of a cell.
Building Blocks of Life
Ribonucleic acid (RNA) plays a central role in protein synthesis. This process, known as gene expression, involves several types of RNA working in concert.
Messenger RNA (mRNA) carries genetic instructions from DNA in the nucleus to ribosomes in the cytoplasm, serving as a direct template for protein assembly. Transfer RNA (tRNA) delivers specific amino acids to the ribosome according to the sequence specified by the mRNA. Each tRNA has a unique three-nucleotide sequence, called an anticodon, that precisely matches a corresponding codon on the mRNA. Ribosomal RNA (rRNA) forms the structural and catalytic core of ribosomes, where amino acids are linked to form polypeptide chains.
Within the ribosome, rRNA facilitates the formation of peptide bonds between incoming amino acids, elongating the growing protein chain. This collaboration among mRNA, tRNA, and rRNA ensures the genetic code is accurately translated into functional proteins.
Orchestrating Cellular Activities
Beyond their direct roles in genetic information storage and protein synthesis, nucleic acids, particularly certain types of RNA, regulate gene expression. These regulatory molecules include non-coding RNAs (ncRNAs), which do not code for proteins but influence how and when genes are turned on or off.
MicroRNAs (miRNAs) are small ncRNAs, typically 18-24 nucleotides long, that regulate gene expression at the post-transcriptional level. They bind to specific messenger RNA (mRNA) molecules, leading to their degradation or blocking their translation into proteins. This mechanism allows miRNAs to fine-tune the amount of protein produced from a given gene.
Long non-coding RNAs (lncRNAs), over 200 nucleotides in length, also play diverse regulatory roles. LncRNAs can influence gene expression through various mechanisms, including modifying chromatin structure, regulating transcription, and affecting mRNA stability. By controlling these processes, ncRNAs ensure cellular functions are coordinated in response to developmental cues and environmental changes.
Powering and Driving Reactions
Nucleic acid derivatives are central to energy transactions and catalytic processes. Adenosine triphosphate (ATP) and guanosine triphosphate (GTP, both nucleoside triphosphates, serve as the primary energy currency of the cell. Energy is released when a phosphate bond in ATP is broken, typically converting it to adenosine diphosphate (ADP). This released energy drives cellular processes like muscle contraction, nerve impulse transmission, and complex molecule synthesis.
While ATP is the most widely used energy carrier, GTP also provides energy for specific cellular activities, such as protein synthesis and signal transduction. Some RNA molecules, known as ribozymes, possess catalytic capabilities, acting like enzymes to speed up biochemical reactions. For example, ribosomal RNA (rRNA) within the ribosome catalyzes peptide bond formation during protein synthesis. Coenzymes like nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD), derived from nucleotides, aid metabolic reactions by transferring electrons and hydrogen atoms.