180

14

The Nature of Living Things

Fig. 14.2 Schematic diagram of the cell cycle. The successive epochs are known as phases. Areas

of the sectors are proportional to the typical duration of each phase, which succeed each other in a

clockwise direction. A newly born cell starts in the so-called G1 phase. When it reaches a certain

size (the molecular nature of the initiating signal is not known, but it is correlated with size), DNA

synthesis begins; that is, the gene is duplicated. Mitosis (see below) takes place in the M phase. See

also Table 14.1

of one cycle can vary by many orders of magnitude: 20 min for E. coli grown in the

laboratory to several years for the bacteria believed to live in deep ocean sediments.

Typically, fully differentiated cells never divide.

The successive steps of the cell cycle appear to be tightly controlled and, if the

control goes awry, damage and subsequent developmental abnormalities such as the

formation of tumours may ensue. Control takes place principally at the checkpoints

(corresponding to the boundaries separating the phases; Fig. 14.2) at which interven-

tion is possible. Proteins called cyclins are synthesized just before each checkpoint

is reached. They activate kinases that, in turn, phosphorylate other proteins (“cyclin-

dependent kinases” (CDK); cf. Sect. 18.7) that carry out the necessary reactions

to enable the cell to pass into the next phase, whereupon the cyclins are abruptly

destroyed.

Apart from duplicating its DNA and dividing, the cell also has to metabolize food

(to provide energy for its other activities, which may include secreting certain sub-

stances, or simply playing a structural rôle) and neutralize external threats such as

viruses, toxins, and changes in temperature. All of these activities, including gene

duplication, require enzymes, and enzymes for translating and modifying the nucleic

acid genetic material, whose fabrication also requires energy. There is also a consid-

erable amount of degradation activity (i.e., proteolysis of enzymes and other proteins

after they have carried out their specific function). ¹⁵ Degradation itself, of course,

requires enzymes to carry it out. In eukaryotes, most proteins are marked for degra-

dation by being covalently bound to one or more copies of the polypeptide ubiquitin.

15 A good example of a protein subjected to degradation is cyclin, which has the regulatory function

mentioned above and whose concentration rises and then falls during mitosis.