4.21: Nucleic Acids and Nucleotides
Nucleic acids are the most important macromolecules for the continuity of life. They carry the cell's genetic blueprint and have instructions for its functioning. The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
Deoxyribonucleic Acid (DNA)
DNA is the genetic material in all living organisms, ranging from single-celled bacteria to multicellular mammals. It is in the nucleus of eukaryotes and the organelles such as chloroplasts and mitochondria. In prokaryotes, the DNA is not enclosed in a membranous envelope.
The cell's entire genetic content is its genome, and its study is called genomics. In eukaryotic cells, DNA forms a complex with histone proteins to form chromatin, which is packaged to form the chromosomes. A chromosome can contain tens of thousands of genes, many of which encode proteins. The remaining genes code for RNA products. DNA controls all cellular activities by turning the genes "on" or "off."
Ribonucleic Acid (RNA)
The other type of nucleic acid, RNA, is primarily involved in protein synthesis. The DNA molecules remain in the nucleus but use an intermediary to communicate with the rest of the cell. This intermediary is the messenger RNA (mRNA). Other types of RNA—such as ribosomal RNA, transfer RNA, and microRNA—are involved in protein synthesis and regulation.
Nucleotides
DNA and RNA consist of monomers called nucleotides. Three components comprise each nucleotide: a nitrogenous base, a pentose (five-carbon) sugar, and a phosphate group. Each nitrogenous base in a nucleotide is attached to a sugar molecule, which is attached to one or more phosphate groups. Nitrogenous bases are organic molecules that contain an amino group with a potential to bind an extra hydrogen, thus decreasing the hydrogen ion concentration in their environment and causing it to shift to basic on the pH scale. Each nucleotide in DNA contains one of four possible nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Adenine and guanine are classified as purines. The purine's primary structure is two carbon-nitrogen rings. Cytosine, thymine, and uracil (U) classified as pyrimidines have a single carbon-nitrogen ring as their primary structure. Each of these basic carbon-nitrogen rings has different functional groups attached to it. In molecular biology shorthand, we know the nitrogenous bases by their symbols A, T, G, C, and U. DNA contains A, T, G, and C, whereas RNA contains A, U, G, and C.
This text is adapted from Openstax, Biology 2e, Chapter 3.5: Nucleic Acids
