Nucleic acids (NA) contain the genetic information and play a key role in protein biosynthesis. They are formed by the polymerization of units called nucleotides, which consist of a nitrogenous base, an aldopentose, and phosphoric acid. The base can be a pyrimidine: thymine (T), cytosine (C), and uracil (U); or a purine: adenine (A) and guanine (G). The aldopentose is d-ribose in ribonucleic acid (RNA) or d-2-deoxyribose in deoxyribonucleic acid (DNA). Nucleotides form chains, by binding through ester links established by phosphate between the OH group in C5′ of the pentose of one nucleotide and the OH of C3′ of the pentose from another nucleotide. One extreme of the chain has the 5′ of pentose free and is considered the start of the chain. The last nucleotide has the C3′ nonesterified and it is considered the 3′ end of the molecule. DNA, together with proteins, forms the chromatin of the cell nucleus and is also present in mitochondria. Individuals of the same species have equal amount of DNA in each cell (6 pg in humans); female and male gametes contain half of that amount. DNA contains the bases A, G, T, and C, always with A/T and G/C being equal to 1. DNA has a double helix structure with two polynucleotide strands wrapped around the same axis. The DNA helix coils clockwise, both strands are antiparallel, one running in the 5′ to 3′ and the other in the 3′ to 5′ direction. The double helix forms two grooves, one wider (major groove) than the other (minor groove). DNA strands are also complementary, with adenine pairing with thymine and guanine with cytosine. In eukaryotes, chromatin is formed by the association of DNA to basic proteins, predominantly histones. In chromatin, the DNA molecule makes a double turn (DNA superhelix) at regular intervals on a core of eight histone units (nucleosomes). In contrast, bacteria, plasmids, mitochondria, and chloroplasts have circular DNA. RNA is another polynucleotide that differs from DNA because it has a single chain instead of two; it contains ribose instead of deoxyribose and uracil instead of thymine. Different types of RNA exist in cells; including messenger RNA (mRNA), which transmit genetic information from the cell nucleus to the cytoplasm; transfer RNA (tRNA), which carries amino acids to the site of protein synthesis; ribosomal RNA (rRNA), which has a large (60S) and a minor particle (40S) and is involved in amino acid assembling during protein synthesis. Other types of RNA include small nuclear, small cytosolic RNA, microRNA, small silencing RNA, and long noncoding RNA. Viruses are agents of disease in animals and plants, formed by nucleic acids surrounded by a protein coat. The protein cover is the viral capsid, which contains the viral genetic material, DNA or RNA, commonly single stranded.