Bacterial cells, such as E. coli, are surrounded by a fluid plasma membrane and enclosed by a rigid cell wall that protects the cell from damage. You will recall that there is no nuclear envelope in prokaryotic cells, but rather a “nucleoid region” with a single chromosome that is continuous (circular), not linear, as in eukaryotes. All of the genes required for basic survival and reproduction are found in the main chromosome. In addition to the main chromosomes, some DNA is found in a number of plasmids--small rings of DNA. Plasmids carry only a small number of genes for specialized functions, including resistance to specific drugs.

The genetic code in bacteria is DNA composed of the same nucleotides as in eukaryotes, indicating the universality of the genetic code. Regardless of the source or species, all DNA has the same four nucleotides: adenine, thymine, cytosine, and guanine. For this reason, a eukaryotic gene--such as one for producing human insulin--can be inserted into a plasmid, and when taken up by a prokaryotic bacterium, can then be expressed by the bacterium. This process is called transformation, and occurs when a bacterium takes in foreign DNA and expresses the foreign gene. Transformation can occur naturally when a bacterium is exposed to foreign DNA, as described in the famous experiments of Frederick Griffith using mice exposed to pathogenic and non-pathogenic strains of Streptococcus pneumoniae.
In this investigation, E. coli cells are exposed to plasmids containing a gene for resistance to ampicillin. If the susceptible E. coli bacteria incorporate the foreign DNA, they will become ampicillin resistant.

Which of the following statements are true? Select all that apply.
a)Transformation occurs when foreign genes are introduced into a host cell.
b)Bacterial cells have several linear chromosomes.
c)Plasmids can be engineered to carry foreign DNA.
d)Transformation follows the uptake of foreign proteins into a cell.
e)Plasmid DNA contains genes essential for normal function