E. coli vectors

Bacteria are the bests of choice for DNA cloning among them, E.coli occupies a prominent posion since cloning and isolation DNA inserts for structural analysis is the easiest in this best. Therefore, the initial cloning experiments are generally carried out in E.coli. The E.coli strain k 12 is the most commonly used. It has several substains, e.g. c600 HB101 each of which has some specific features important in cloning. For example, the substain RRi hasm in addition to certain other features, the mutation hsdr which inactivates the restriction enzyme endogous to E coli k12, this minimises the degradation of recombinant DNA introduced into it.
Properties of good host: A good host should have the following features, (1) is easy to transform (2) supports the replication of recombinant DNA, (3) is free from elements that interfere with replication of recombinant DNA,(4) lacks active restriction enzymes, e.g. E. coli k12 substrain HB 101, (5) does not have methylases since these enzymes would to useful restriction enzymes, and (6) is deficient in normal recombination function so that the DNA insert is not altered by recombination events.
E. coli supports several types of vectors, some natural some constructed, which can be grouped as follows: (1) plasmids, (2) bacteriophages (3) cosmids, (4) plasmids, and (5) shuttle vectors
Plasmids.
A plasmid is a DNA molecule, other than the bacterial chromosome, that is capable of independent replication and transmission. Plasmid are circular and may exits either independent of or may become intergrated into the bacterial chromosome gernally they are not essential for the host cell except under spefcific environments. There are severaltyupes of bacterial plasmids, but the three widely studied types are F plasmids (2) R plasmids and (3) Col plasmids the protein that kill sensitive E. coli cells, they also carry genes that provide immunity to the particular through conjugation, and as a result spread rapidly among the bacterial cells of a population e.g F plasmids, many R plasmids and some Col plasmids, or nonconjugative ( do not mediate DNA transfer through conjugation) e.g many plasmids and most Col plasmids.
stringent and relaxed replication. Each plasmid is maintained in the bacterial cell at a characteristic copy number mainly due to its replication control system. In this respect the plasmids are of two types (1) single copy and (2) multiple copy plasmids. The replication control of single copy plasmids is the same as that of their bacterial host cells so that they replicate and segregate with the bacterial chromosome this is called stringent replication. In contrast, the replication control of multi copy plasmids is different from that of their bacterial host genome so that they undergo more than one replication for each replication of their host genome this is referred to as relaxed replication.
Modular organization. Plasmids may visualized as constructed from modular DNA segments. A module may be regarded as a DNA segment or sequence performing a specific function each module may contain one or more genes.