Escherichia coli as Experimental Model

   Because of their comparative simplicity, prokaryotic cells(bacteria) are ideal models for studying many fundamental aspects of biochemistry and molecular biology. The most thoroughly studied species of bacteria is E. coli, which is the most favoured organism for investigation of the underlying mechanisms of molecular genetics. Most of the present concepts of molecular biology –
 DNA replication, the genetic code, gene expression and protein synthesis – are derived from studies of E. coli.

 E. coli is useful to molecular biologists because of its relative simplicity and the ease of its propagation and study in the laboratory. For example, the genome of E. coli consists of
approximately 4.6 million base pairs and encodes about 4000 different proteins. While the human genome is more complicated, with nearly 3 billion base pairs and encodes about 100,000  different proteins. The small size of the E.coli genome provides advantages for genetic analysis, and the sequence of the entire E.coli genome has been determined.

Molecular genetic experiments are further facilitated by the rapid growth of E.coli under well-defined laboratory conditions. E. coli can divide every 20-60 minutes, depending on culture conditions and a clonal population of E.coli all cells derived by the division of a single cell of origin – can be isolated as a colony grown on agar – containing a medium. Bacterial colonies contain many cells, and selecting and analysing genetic variants of E.coli strain is straightforward and rapid. This generally contributes to the success
of experiments in molecular genetics.

E.coli can divide rapidly in nutrient mixtures like glucose, salts, amino acids, vitamins and nucleic acid precursors. However, E. coli can also grow in much simpler media consisting of only
salts as a source of nitrogen (such as ammonia) and a source of carbon and energy (such as glucose). But in such simple medium, the bacteria grow a little slowly (a division time of about 40
minutes) because they must synthesise all their own amino acids, nucleotides and other organic compounds.

The ability of E. coli to carry out these biosynthetic reactions in simple defined media has made them extremely useful in elucidating the biochemical pathways involved. Thus, the rapid growth and simple nutritional requirements of E. coli have greatly facilitated fundamental experiments in both molecular biology and biochemistry.
Although bacteria are models for studies of cell properties, they cannot be used to study aspects of cell structure and function that are unique to eukaryotes. Yeasts, the simplest eukaryotes, have several experimental advantages similar to those of E. coli and have provided a model for studies of many aspects of eukaryotic cell biology.

The genome of the most studied yeasts, Saccharomyces cerevisiae, consists of 12 million base pairs of DNA and contains about 6000 genes; and is about 3 times larger than that of E. coli it is much more manageable than the genomes of more complex eukaryotes, such as humans. Yeasts can be readily grown in the laboratory and can be studied by many of the same molecular genetic approaches that have proved successful with E. coli. Although yeasts do not replicate as rapidly as bacteria, they still
divide as frequently as every 2 hours and can quickly be grown as colonies from a single cell. Yeasts can be used for a variety of genetic manipulations similar to those that can be performed using
bacteria. Yeast mutants have been influential in understanding many fundamental processes in eukaryotes, including DNA replication, transcription, RNA processing, protein sorting and
regulation of cell division.