Vector
Bacterial plasmid is the most commonly used vector. Plasmids used in genetic engineering are said to be under relaxed control, their replication is totally independent of chromosomal replication. These plasmids may be present in copies of 10-700 per cell.
The most popular plasmid is pUC18. Under certain culture conditions, plasmids can be induced to replicate to produce multiple copies within a single cell. Bacterial plasmids can not accept DNA strands larger than 5000 base pairs, hence they are restricted to cloning DNA ≤5000 base pairs. Some plasmids can carry DNA segments that are 10Kb long.
Specially developed bacteriophage lambda chromosome can incorporate up to 15-16 kilobases of DNA segment. A central one-third of its genome is normally not required for phage infection and therefore can be replaced by foreign DNA. The chimeric phage DNA can be introduced into the host cells by infecting them with phages.
Cosmids are recombinant vectors that combine features of both plasmids and bacteriophage chromosome. It can accommodate DNA segments up to 50 kilobases.
cos site is segment of DNA, which is 14 base pair long sequence and is located at either ends of phage chromosome.These ends have be separated by
36-51 kilobases of DNA strand. Only those segments of DNA that have two cos sides at either end and are separated by 36-51 kb of DNA are packaged into the phage capsid. If two cos sites are placed 36-51 Kb apart on a functional plasmid vector, it becomes a cosmid. Since cosmids have no phage DNA, upon introduction into a host cell via phage infection, they reproduce as plasmids.
Yeast artificial chromosome (YAC) is a specially constructed yeast chromosome that can incorporate DNA strands up to 1 million base pairs. YACs are liner DNA segments that have all the information required for replication in a yeast cell. Several hundred kb of foreign DNA can be cloned into YACs.
Continuation of Genetic engineering: (part 3) Cloning technique
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