Figure 1. The eukaryotic and the prokaryotic cell. (a) The eukaryotic cell contains a nucleus (center) surrounded by a membrane (with pores), a vacuole (light blue), the endoplasmatic reticulum (green), the Golgi-apparatus (purple), mitochondria (yellow/orange), ribosomes (black dots) and cytoplasm. The cell is surrounded by a cytoplasmic membrane and a cell wall. Diameter of the yeast cell depicted: 10 µm. (b) The prokaryotic cell contains a circular, coiled-up chromosome; ribosomes; and cytoplasm. The cell is also surrounded by a cytoplasmic membrane and a cell wall. A flagellum is depicted on the left (not present in all bacteria). Bacterial cells have an average length of 1 µm. (c) The cytoplasmic membrane consists of a phospholipid bilayer. In living organisms the membrane is charged, negative inside and positive outside. Proteins (red) are inserted into the membrane. (Watercolor and gouache: Anne Kemmling, Goettingen, Germany.)

Figure 2. The three principal processes for the transmission and utilization of genetic information. (a) Replication: The enzyme helicase separates the double strand into its two single strands. The exposed bases (letters) are then subject to base pairing, and the second, complementary strands are formed by the action of DNA polymerase. (b) Transcription: RNA polymerase widens the double helix and transcribes the base sequence of one DNA strand into the complementary sequence of an RNA strand, messenger RNA (mRNA). The DNA strand subject to transcription is called the codogenic strand. (c) Translation: As soon as mRNA becomes available, it binds to ribosomes (blue) and protein synthesis is initiated. The ribosomes closer to the RNA polymerase in Figure 2 have been “working” for a longer period of time, so “their” amino acid chains are longer. Each of the tRNAs is linked to a specific amino acid. They recognize their “turn” in protein synthesis with the help of an anticodon, which is complementary to the codon on the mRNA. (Diagrams: Anne Kemmling, Goettingen, Germany.)