The class of organic compounds known as nucleic acids consists of polymers constructed from nucleotides. Among the best-known nucleic acids are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). DNA provides the blueprint of life in living cells whereas RNA allows the translation of the genetic code into proteins, which make up the cellular components of life. Each nucleotide in a nucleic acid consists of a sugar molecule (ribose in RNA and deoxyribose in DNA) to a nitrogenous base and a phosphate group. The phosphate groups allow the nucleotides to be linked together, creating the sugar-phosphate backbone of the nucleic acid while the nitrogenous bases provide the letters of the genetic alphabet. These components of nucleic acids are constructed from five elements: carbon, hydrogen, oxygen, nitrogen, and phosphorous.
As an organic molecule, carbon is a key element of nucleic acids. Carbon atoms are located in the sugar of the nucleic acid backbone and the nitrogenous bases.
Oxygen atoms are found in the nitrogenous bases, sugar, and phosphates of the nucleotides. An important difference between DNA and RNA is the structure of the sugar. Attached to the carbon-oxygen ring structure of ribose are four hydroxyl (OH) groups. In deoxyribose, one hydroxyl group is replaced with hydrogen. This difference in an oxygen atom leads to the term “deoxy” in deoxyribose.
Hydrogen atoms are attached to carbon and oxygen within the sugar and nitrogenous bases of nucleic acids. The polar bonds created by hydrogen-nitrogen bonds in the nitrogenous bases allow hydrogen bonds to form between strands of nucleic acids, which results in the creation of double-stranded DNA, where two strands of DNA are held together by the hydrogen bonds of the base pairs. In DNA these base pairs align with adenine to thymine and guanine to cytosine. This base pairing plays an important role in both the replication and translation of the DNA.
The nitrogen-containing bases of nucleic acids can be divided into pyrimidines and purines. Pyrimidines are single-ring structures with nitrogen located at the first and third positions of the ring. Cytosine and thymine are the pyrimidines in DNA, with uracil substituted for thymine in RNA. Purines are a double-ring structure, in which a pyrimidine ring is joined to a second ring at the fourth and fifth carbon atoms to a ring known as an imidazole ring. This second ring contains additional nitrogen atoms at the seventh and ninth positions. Adenine and guanine are the purine bases found in DNA. Adenine, cytosine, and guanine have an additional amino group (containing nitrogen) attached to the ring structure. These attached amino groups are involved in the hydrogen bonds formed between base pairs of different nucleic acid strands.
Attached to each sugar is a phosphate group composed of phosphorous and oxygen. This phosphate allows the sugar molecules of different nucleotides to be linked together in a polymer chain.