Cell structure and DNA

The structural and fundamental unit of living organisms is the cell, which contains the genetic material (the hereditary information), as well as a wide variety of molecules and structures that allows it to maintain its vital functions. The hereditary information is stored in macromolecules, the most important of which is the DNA (deoxyribonucleic acid). The cell is the smallest unit of an organism that is able to function autonomously. Each cell is spatially delimited by an outer membrane, the cell membrane, which has the function of separating the content of the cell from the external environment and regulates the flow of substances entering and exiting from the cell thanks to its selective permeability. Outside the membrane, the cells of various organisms (bacteria, plants, fungi) are further bounded by a so-called cell wall, which essentially performs a structural function.

Figure 8: Structure of the prokaryotic cell (A, Escherichia coli, a bacterium normally present in the intestinal flora) and of the eukaryotic one (B, Chlamydomonas, a green alga).

Inside the membrane we find the cytoplasm, a fluid substance (with high water content) that contains a variety of molecules and specialized structures (called organelles). There are two types of cells, mainly distinguished by their different degrees of organization complexity. The cells of prokaryotes (from Greek pro-, “before”, and karion, “nucleus”, Fig. 8(A)) are of a smaller size - the diameter is generally comprised between 1 and 5 $\mu$m - and have a very simple internal structure; their genetic material is not separated from the cytoplasm by a membrane, although it predominantly occupies a region of the cytoplasm called nucleoid. The cells of eukaryotic organisms (from the greek eu-,“good”, and karion, “nucleus”, Fig. 8(B)) have a larger size (10 to 50$\mu$m) and their genetic material is enclosed within a membrane, called the nuclear envelope, which delimits the nucleus. The genetic material (the hereditary information) governs the activities of the cell and allows it to transmit its characteristics to offspring.

Biological inheritance, i.e. the process of transmission of individual traits from parents to their offspring, has always been an object of astonishment. However, only in the second half of the nineteenth century, thanks to the studies by Mendel, scientists started to clarify the functioning of hereditary mechanisms. Mendel demonstrated that inherited traits are transmitted as discrete units - called genes - which are distributed according to certain rules from one generation to the next. The organisms' genes are located on the chromosomes, complex structures formed by proteins and a macromolecule, the DNA. This nucleic acid, whose structure (see Fig. 9) was discovered in the 1950s by Watson and Crick (1953), is like a spiral staircase, or a double-stranded helix. The handrails of the staircase are formed by an alternation of sugar molecules (deoxyribose) and phosphate, while the steps are made of four nitrogenous bases: adenine ($A$), cytosine ($C$), guanine ($G$), thymine ($T$). Each step consists of two bases and each base is tied to a unit sugar-phosphate. The base pairs are bound together by hydrogen bonds and, because of their structure, adenine can pair only with thymine and cytosine only with guanine. The combination of a base and a unit sugar-phosphate is called nucleotide. DNA is therefore a chain of nucleotides. The sequence of three nucleotides or triplet, in the DNA molecule, represents the genetic code because each triplet codes for the synthesis of a given amino acid (amino acids are the basic components of proteins).

Figure 9: The double-stranded helical structure of DNA, proposed by Watson and Crick (1953). The steps of the staircase are made of the four nitrogenous bases that form the nucleotides: adenine, cytosine, guanine and thymine. Each step consists of two bases.

The base sequences contained in the DNA provide the information necessary for protein synthesis which takes place within the cell and is at the basis of life functioning. The DNA can self-replicate, thus allowing the transfer of genetic information from one cell to another. During replication, the chromosomes become clearly visible within the nucleoid (prokaryotes) or the nucleus (eukaryotes). Usually the prokaryotes (e.g. bacteria) have a single circular chromosome, while eukaryotes have different chromosomes with a linear structure.

Fundamental to the cell is being able to exchange energy and matter internally and with the external environment. This occurs through various processes of physical and chemical nature, called metabolism. The metabolism is divided into anabolism (which produces complex molecules, useful to the cell, from simpler molecules and requires energy) and catabolism (which involves the degradation of complex molecules into simpler molecules and produces energy). In eukaryotic cells the real engine is constituted by an organelle called mitochondrion. This organelle produces adenosine triphosphate ($ATP$) from sugars and oxygen. $ATP$ is the high-energy compound required by the vast majority of metabolic reactions. The mitochondrion also contains DNA, which is however different from that of the nucleus.