Nucleic acids, essential biomolecules, consist of three primary components: phosphate groups, a five-carbon sugar, and nitrogenous bases. The five-carbon sugar can be ribose in RNA, characterized by a hydroxyl group on the 2' carbon, or deoxyribose in DNA, which lacks this hydroxyl group, having only a hydrogen instead. This absence of the hydroxyl group is the reason for the term "deoxy."
There are five nitrogenous bases found in nucleic acids: adenine, guanine, cytosine, uracil, and thymine. Adenine and guanine are classified as purines, while cytosine, uracil, and thymine are pyrimidines. A helpful mnemonic for remembering purines is "pure as gold" (Adenine and Guanine), and for pyrimidines, "CUT" (Cytosine, Uracil, Thymine). Notably, uracil is exclusive to RNA, while thymine is found only in DNA, where they substitute for each other.
The chemical distinction between uracil and thymine is significant. Cytosine can be converted to uracil through deamination, especially when exposed to UV light, which can lead to mutations. However, thymine's methyl group allows cells to recognize that uracil is not a normal base in DNA, thus preventing potential errors in genetic information.
Nucleic acids are polymers made up of nucleotides, which are composed of a phosphate group, a five-carbon sugar, and a nitrogenous base. If a nucleotide lacks a phosphate group, it is referred to as a nucleoside. To synthesize nucleic acids, nucleotide triphosphates are utilized, as the high-energy bonds between the phosphate groups provide the necessary energy for polymerization reactions. DNA polymerase, for instance, cleaves these bonds to facilitate the synthesis of DNA.
In nucleotides, the carbon atoms of the sugar are numbered with a prime notation (1', 2', 3', 4', 5') to distinguish them from the carbon atoms in the nitrogenous bases. The first carbon in the sugar is derived from the base, which is why this unique numbering system is employed. Understanding the numbering of both the sugar and the bases is crucial, as purines and pyrimidines have distinct numbering systems that must be memorized.
Additionally, the phosphate groups in nucleotide triphosphates are designated as alpha, beta, and gamma, rather than simply numbered. This naming convention is important for accurately discussing and understanding the structures and functions of nucleic acids.