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2.4  Cell Processes

(cells minus their native DNA genetic material) were also used in generating the first self-​

replicating artificial cell (Gibson et al., 2010). The longest cells known are nerve cells, which

in some animals can be several meters in length.

The way that the number, or ultimately the concentration, of each type of protein molecule

in a cell is controlled is through dynamic fine-​tuning of the rate of production of proteins

and the rate at which they are removed, or degraded, from the cell. There are mechanisms to

controllably degrade proteins in cells, for example, eukaryotes have a mechanism of tagging

proteins (with another protein called “ubiquitin”), leading to their being ultimately captured

inside subcellular organelles and subsequently degraded by the action of the so-​called proteo­

lytic enzymes, with other similar mechanisms existing for prokaryotes but with the absence

of dedicated subcellular organelles. However, the most control that is imparted by cells for

regulating the equilibrium concentration of cellular proteins is through the direct regulation

of the rate at which they are manufactured by the cell from the genes. The fine-​tuning of the

rate of production of proteins in a cell is done through a process called gene regulation, and

to understand how this is achieved, we must explore the concept of the central dogma of

molecular biology.

2.4.1  CENTRAL DOGMA OF MOLECULAR BIOLOGY

For reasons that arguably are more metaphysical than scientific, the process, which is

considered by many expert biologists to be the most important of all biology, which governs

how the DNA genetic code is ultimately read out and transformed into different proteins, is

referred to as a central dogma as opposed to a law. Either way, the process itself is ubiqui­

tous across all domains of life, and essential, summarized in its simplest form in Figure 2.7. In

essence, the following applies:

1 The genetic code of each cell is encapsulated in its DNA, into a series of genes.

2 Genes can be transcribed by molecular machinery to generate molecules of mRNA.

3 mRNA molecules can be translated by other molecular machinery involving the

binding of molecules of tRNA to the mRNA to generate peptides and proteins.

This is an enormous simplification of what is a very complex process requiring the efficient

coordination of multiple different molecular machine components. The principal flow

of information from the genes incorporated into DNA molecules to the rest of the organism

is through the route DNA → mRNA → protein. The proteins that are then generated can

FIGURE 2.7  Central dogma of molecular biology, (a) Schematic of the flow of information

between nucleic acids and proteins. (b) Structure of tRNA. (c) Interaction of tRNA with ribosome

during peptide manufacture.