and the replicated chromosomes are compacted so that they can be segregated without damage during mitosis. We will deal with mitosis in detail in Chapter 14. For the moment we should be aware that although it is primarily about changes to the nucleus, mitosis is accompanied by dramatic changes in the organization of the rest of the cell. A new structure, the mitotic spindle, is assembled specifically to move the chromosomes apart whilst other structures are dismantled so that their components can be divided among the two daughter cells following cell division.
Viruses occupy a unique position between the living and nonliving worlds. On the one hand they are made of the same molecules as living cells. On the other they are incapable of independent existence, being completely dependent on a host cell for reproduction. Almost all living organisms have viruses that infect them. Human viruses include polio, influenza, herpes, rabies, smallpox, chickenpox, HIV, and SARS‐CoV‐2, the causative agent of COVID‐19. Viruses are submicroscopic particles consisting of a core of genetic material enclosed within a protein coat called the capsid. Some have an extra membrane layer called the envelope. Viruses are inert until they enter a host cell, whereupon their genetic material directs the host cell machinery to produce viral protein and viral genetic material. Viruses often insert their genome into that of the host, an ability that is widely made use of in molecular biology research (Chapter 8). Bacterial viruses, bacteriophages, are used by scientists to transfer genes between bacterial strains. As we will see, human viruses are used as vehicles for gene therapy.
Prokaryotic cells are simpler in their organization than eukaryotic cells and are assumed to be more primitive. According to the fossil record, prokaryotic organisms precede, by at least 1.5 billion years, the first eukaryotes that appeared some 2 billion years ago. It seems highly likely that eukaryotes evolved from prokaryotes, and the most likely explanation of this process is the endosymbiotic theory. The basis of this theory is that some eukaryotic organelles originated as free‐living bacteria that were engulfed by larger cells in which they established a mutually beneficial relationship. For example, mitochondria would have originated as free‐living aerobic bacteria and chloroplasts as photosynthetic cyanobacteria. The endosymbiotic theory provides an attractive explanation for the fact that mitochondria and chloroplasts contain their own DNA and ribosomes both of which are more closely related to those of bacteria than to all the other DNA and ribosomes in the same cell. The case for the origin of other eukaryotic organelles is less persuasive. Nevertheless, while it is clearly not perfect, most biologists are now prepared to accept that the endosymbiotic theory provides at least a partial explanation for the evolution of the eukaryotic cell from prokaryotic ancestors.
| Prokaryotes | Eukaryotes | |
|---|---|---|
| Size | Usually 1–2 μm | Usually 5–100 μm |
| Nucleus | Absent | Present |
| DNA | Usually a single circular molecule (= chromosome) | Multiple linear molecules (chromosomes)a |
| Cell division | Simple fission | Mitosis or meiosis |
| Internal membranes | Rare | Complex |
| Ribosomes | 70Sb | 80S (70S in mitochondria and chloroplasts) |
| Cytoskeleton | Rudimentary | Microtubules, microfilaments, intermediate filaments |
| Motility | Rotary motor (drives bacterial flagellum) | Dynein (drives cilia and flagella); kinesin, myosin |
| First appeared | 3.5 × 109 years ago | 2 × 109 years ago |
a The tiny chromosomes of mitochondria and chloroplasts are exceptions; like prokaryotic chromosomes they are often circular.
b The S value, or Svedberg unit, is a sedimentation rate. It is a measure of how fast a molecule moves in a gravitational field, and therefore in an ultracentrifuge.
Example 1.1 Sterilization by Filtration
Because even the smallest cells are larger than 1 μm, harmful bacteria and other organisms can be removed from drinking water by passing it through a filter with holes 200 nm in diameter. These filters can vary in size from huge, such as those used in various commercial processes, to small enough to be easily transportable by backpackers. Filtering drinking water greatly reduces the chances of bringing back an unwanted souvenir from your camping trip!
IN DEPTH 1.1 OUR ANCESTOR, THE ARCHAEON
When we say prokaryote we usually mean bacterium. The prokaryotes we use to make yogurt and kimchi, those which give us diseases, and those that we use in genetic engineering (Chapter 8) are all bacteria. However, from the very origin of life on earth a second group of prokaryotes called the archaea lived alongside bacteria. Archaea are found throughout nature, for example each of our guts contains at least a trillion cells of the archaeon Methanobrevibacter smithii that help to break down complex sugars. It is now thought that the cell that incorporated bacteria to become the ancestor of the eukaryotes was an archaeon related to a present‐day group called the Asgard archaea. Asgard archaea contain several genes and proteins that are otherwise found only in eukaryotes,