Nucleic acids are composed of a string of individual nucleotides to form macromolecules and, as the main component of genes in cell nuclei, are carriers of hereditary information, and they catalyze many biochemical reactions. The individual nucleotides each consist of a phosphate and a nucleic base moiety as well as the pentose ring molecule ribose or deoxyribose. The biochemical effectiveness of nucleic acids is based not only on their chemical composition but also on their secondary structure, their three-dimensional arrangement.
What are nucleic acids?
The building blocks of nucleic acids are individual nucleotides, each composed of a phosphate residue, the monosaccharide ribose or deoxyribose, each with 5 C atoms arranged in a ring, and one of five possible nucleic bases. The five possible nucleic bases are adenine (A), guanine (G), cytosine (C), thymine (T) and uracil (U). Nucleotides containing deoxyribose as a sugar component are strung together to form deoxyribonucleic acids (DNA), and nucleotides containing ribose as a sugar component are assembled to form ribonucleic acids (RNA). Uracil as a nucleic base occurs exclusively in RNA. There, uracil replaces thymine, which is found exclusively in DNA. This means that only 4 different nucleotides are available for the construction of DNA and RNA. In English and international usage, as well as in German technical papers, the abbreviations DNA (deoxyribonucleic acid) instead of DNA and RNA (ribonucleic acid) instead of RNA are usually used. In addition to naturally occurring nucleic acids in the form of DNA or RNA, synthetic nucleic acids are developed in chemistry to act as catalysts for certain chemical processes.
Anatomy and structure
Nucleic acids consist of a concatenation of a huge number of nucleotides. A nucleotide is always composed of the ring-shaped monosugar deoxyribose in the case of DNA or ribose in the case of RNA, plus a phosphate residue and a nucleic base moiety. Ribose and deoxyribose differ only in that in the case of deoxyribose an OH group is transformed into an H ion by reduction, i.e. by the addition of an electron, and thus becomes chemically more stable. Starting from the ring-shaped ribose or deoxyribose, each with 5 C atoms, the nucleic base group in each nucleotide is connected to the same C atom via an N-glycosidic bond. N-glycosidic means that the corresponding C atom of the sugar is linked to the NH2 group of the nucleic base. If the C atom with the glycosidic bond is called No. 1, then – looking clockwise – the C atom with No. 3 is connected to the phosphate group of the next nucleotide via a phosphodiester bond, and the C atom with No. 5 is esterified to its “own” phosphate group. Both nucleic acids, DNA and RNA are each composed of pure nucleotides. This means that the central sugar molecules of DNA nucleotides are always made of deoxyribose and those of RNA are always made of ribose. The nucleotides of a given nucleic acid differ only in the order of the 4 possible nucleic bases in each case. DNA can be thought of as thin ribbons that are coiled within themselves and completed by a complementary counterpart, so that DNA normally exists as a double helix. In this case, the base pairs adenine and thymine and guanine and cytosine are always opposite each other.
Function and tasks
DNA and RNA perform different tasks and functions. While DNA does not perform any functional tasks, RNA intervenes in various metabolic processes. DNA serves as the central storage location of genetic information for each cell. It contains the building instructions of the entire organism and makes them available when needed. The structure of all proteins is stored in the DNA in the form of amino acid sequences. In practical terms, the coded information of the DNA is first “transcribed” via the process of transcription and translated (transcribed) into the corresponding amino acid sequence. All these necessary complex work functions are performed by special ribonucleic acids. The RNA thus assumes the tasks of forming a complementary single strand to the DNA within the cell nucleus and transporting it as ribosomal RNA through the nuclear pores out of the cell nucleus into the cytoplasm to the ribosomes in order to assemble and synthesize specific amino acids into the intended proteins.An important task is performed by the tRNA (transfer RNA), which consists of relatively short chains of about 70 to 95 nucleotides. The tRNA has a cloverleaf-like structure. Its task is to take up the amino acids provided according to the coding by the DNA and to make them available to the ribosomes for protein synthesis. Some tRNAs are specialized for specific amino acids; however, other tRNAs are responsible for multiple amino acids simultaneously.
Diseases
The complex processes associated with cell division, that is, the replication of chromosomes and the translation of the genetic code into amino acid sequences, can result in a range of dysfunctions, with a wide range of possible effects from lethal (not viable) to barely noticeable. In rare exceptional cases, the random malfunctions can also lead to improved adaptation of the individual to environmental conditions and, accordingly, to beneficial effects. During the replication of DNA, spontaneous changes (mutations) may occur in individual genes (gene mutation) or an error may occur in the distribution of chromosomes among cells (genome mutation). A well-known example of a genomic mutation is trisomy 21 – also known as Down syndrome. Unfavorable environmental conditions in the form of a diet low in enzymes, prolonged stressful situations, excessive exposure to UV radiation facilitate damage to DNA, which can lead to a weakening of the immune system and promote the formation of cancer cells. Toxic substances can also affect the diverse function of RNA and lead to significant impairment.
