In this chapter we will introduce the main properties of the eukaryotic cell, starting from its characterization in terms of its organelles, such as the nucleus, and its structural components, such as the cytoskeleton and the membrane (see Section 1.1). In Section 1.2, we discuss in detail how DNA is organized, and we introduce different chromatin structures such as B-DNA and Z-DNA and their condensation into chromosomes. Section 1.3 discusses how DNA is replicated so that the genetic information it encodes can be passed over to daughter cells. We also explain how DNA is repaired when damage due to external perturbation occurs. Next, in Section 1.4, we explain how the genetic information encoded in the DNA is transcribed into RNA and then translated into proteins, a process that has been termed the “central dogma” of molecular biology. Cells are surrounded by the plasma membrane formed by lipid bilayers, which also enclose the organelles and have a key role in intracellular and extracellular transport. This issue is illustrated in Section 1.5. Section 1.6 discusses the regulation of gene expression in the cell, and in particular that performed by miRNAs, a set of small RNA molecules. Finally, Section 1.7 illustrates the process of cell division and Section 1.8 discusses cell death and cell senescence.

Architecture of the Eukaryotic Cell

A cell is a small organized machine where DNA stores information, RNAs translate the message in protein language and proteins are the effectors. The ingredients needed to control the behavior of a cell are a mixture of biochemical and physical factors. Before discussing in detail how the machine functions, we first describe its general architecture. Cells in the human body can differ widely in terms of shape, size and function, but some general features are common to all cell types and are illustrated in Figure 1.1.

Eukaryotic cells are enclosed by the plasma membrane, a semi-permeable membrane in the form of a lipid bilayer that we will discuss in detail in Section 1.5. In contrast to bacterial cells, eukaryotic cells contain a set of membrane bound structures, known as organelles, that perform specialized functions.