Figure 1. The building blocks of DNA. Note that when cytosine is linked with deoxyribose it is called deoxycytidine; thymine then becomes deoxythymidine, adenine, deoxyadenosine, and guanine, deoxyguanosine.

Figure 2. The characteristic building blocks of RNA. Compare the formulas of T and U and those of deoxyribose and ribose.

Figure 3. Principle of base pairing. Pairing is achieved by hydrogen bonds. P (phosphate group) is at the 5′-end and OH is at the 3′-end of the DNA strands.

Figure 4. Basic scheme of DNA replication. Several proteins are involved, but only the DNA polymerase is shown here. It can only function in the direction from the 5′-end to the 3′-end. The DNA fragments formed along the lagging strand are called Okazaki fragments. The gaps between them are closed by the enzyme ligase.

Figure 5. The structures of the 20 natural amino acids. Those essential for humans are indicated with an asterisk.

Figure 6. The genetic code. The triplets given are at the messenger RNA level. At the DNA level, U is replaced by T.

Figure 7. Schematic presentation of the molecules making up the cytoplasmic membrane, in bacteria (a) and archaea (b), and the murein of bacteria (c). a) The central molecule is glycerol; it carries a phosphate group esterified with ethanolamine or trimethylethanolamine (choline). Furthermore, glycerol is esterified with two fatty acids (ester bond: R-O-CO-R). The bilayer is formed by interaction of the carbon chains of the fatty acids. b) The central molecule is also glycerol, but it carries one phosphate residue. The bonds with the long chain hydrocarbons are ether bonds (R-O-R). In this case, a biphytanylether is shown. c) Model of a bacterial murein. The polysaccharide chain consists of N-acetyl-muramic acid (M) and N-acetyl-glucosamine (G). Crosslinkage is achieved by peptide bridges. These bridges are the targets of beta-lactam antibiotics.

Figure 8. Adenosine 5′-triphosphate (ATP). The bonds between the phosphate residues are energy-rich bonds.