Chapter 7 – Complementary Experimental Tools  271

DNA (called λ DNA), which is used in many in vitro investigations, including single-​molecule

experiments of optical and magnetic tweezers (see Chapter 6). Another model of bacterium-​

infecting virus includes bacteriophage Mu (also called Mu phage), which has generated sig­

nificant insight into relatively large transposable sections of DNA called “transposons” that

undergo a natural splicing out from their original location in the genetic code and relocated

en masse in a different location.

KEY POINT 7.2

“Microbiology” is the study of living organisms whose length scale is around ~10⁻⁶ m,

which includes mainly not only bacteria but also viruses that infect bacteria as well as

eukaryotic cells such as yeast. These cells are normally classed as being “unicellular,”

though in fact for much of their lifetime, they exist in colonies with either cells of their

own type or with different species. However, since microbiology research can perform

experiments on single cells in a highly controlled way without the added complication

of a multicellular heterogeneous tissue environment, this has significantly increased

our knowledge of biochemistry, genetics, cell biology, and even developmental biology

in the life sciences in general.

7.3.2  MODEL UNICELLULAR EUKARYOTES OR “SIMPLE” MULTICELLULAR

EUKARYOTES

Unlike prokaryotes, eukaryotes possess a distinct nucleus, as well as other subcellular

organelles. This added compartmentalization of biological function can complicate experi­

mental investigations (though note that even prokaryotes have distinct areas of local architec­

ture in their cells so should not be perceived as a simple “living test tube”). Model eukaryotes

for the study of cellular effects possess relatively few genes and also are ideally easy to culti­

vate in the laboratory with a reasonably short cell division time, allowing cell cultures to be

prepared quickly. In this regard, three organisms have emerged as model organisms. One

includes the single-​celled eukaryotic protozoan parasite of the Trypanosoma genus that

causes African sleeping sickness, specifically a species called Trypanosoma brucei, which

has emerged as a model cell to study the synthesis of lipids. A more widely used eukaryote

model cell organism is yeast, especially the species called Saccharomyces cerevisiae also

known as budding yeast or baker’s yeast. This has been used in multiple light micros­

copy investigations, for example, involving placing a fluorescent tag on specific proteins

in the cell to perform superresolution microscopy (see Chapter 4). The third very popular

model eukaryote unicellular organism is Chlamydomonas reinhardtii (often shortened to

“C. reinhardtii“) . This is a green alga and has been used extensively to study photosynthesis

and cell motility.

Dictyostelium discoideum is a more complex multicellular eukaryote, also known as slime

mold. It has been used as a model organism in studies involving cell-​to-​cell communication

and cell differentiation (i.e., how eukaryote cells in multicellular organisms commit to being

different specific cell types). It has also been used to investigate the effects of programmed

cell death or apoptosis (see the following text).

More complex eukaryotic cells are those that would normally reside in tissues, and many

biomedical investigations benefit from model human cells to perform investigations into

human disease. The main problem with using more complex cells from animals is that they

normally undergo the natural process of programmed cell death, called apoptosis, as part of

their cell cycle. This means that it is impossible to study such cells over multiple generations

and also technically challenging to grow a cell culture sample. To overcome this, immortalized

cells are used, which have been modified to overcome apoptosis.

An immortal cell derived from a multicellular organism is one that under normal

circumstances would not proliferate indefinitely but, due to being genetically modified, is

no longer limited by the Hayflick limit. This is a limit to future cell division set either by